Boston Geology
SOUTH BAY RESEARCH NOTES & RESOURCES:
The Boston Basin
Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
Patrick J. Barosh & David Woodhouse, Geology of the Boston Basin, Civil Engineering Practice (2011/2012).
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Barosh, 2016
Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
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Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
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Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
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Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
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Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
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Barosh & Woodhouse, Geology of the Boston Basin (2011/2012)
A paleomagnetic study of the Late Precambrian Roxbury formation, consisting of extrusives and clastic sediments, has yielded a steeply downward directed and pre-folding characteristic magnetization (D/I = 219°/71°, α95 = 5°, k = 311, paleopole at 13°N, 267°E, N = 4 sites), which indicates a significantly higher paleolatitude (55°) than would be expected if the Boston basin were part of the equatorial North American craton in the latest Precambrian and earliest Paleozoic. This characteristic magnetization reveals dual polarities and is further supported by a positive conglomerate test. A ubiquitous post-folding late Paleozoic overprint is present in nine sites (D/I = 183°/14°, α95 = 8°, k = 42), with a paleopole at 41°S, 285°E. The pre-folding magnetization resides in hematite, which is inferred to have formed during early oxidation of the rocks; the high stability of this hematite may have prevented its magnetization from being reset during the late Paleozoic chemical event responsible for the magnetic overprint. The Boston basin has a marked geological similarity to the Avalon basement terranes in Nova Scotia and Newfoundland, as well as the Armorican Massif in France, and the high paleolatitudes observed for all these terranes suggest a common paleogeographical affinity; a likely paleolocation is near the northwestern margin of Gondwana which was located at the southpole in the latest Precambrian and Early Cambrian.
Eocambrian paleomagnetism of the Boston Basin: Evidence for displaced terrane, December 1986
the basin structure of the Boston Basin is true only in a general sense. If we were to draw a perimeter around the known outcrops of the sedimentary and volcanic rocks — the Boston Basin — we would find that for only about half its length are the lowermost, or basal, rocks in contact with older rocks. The rest is faulted. Moreover, within the basin itself, the centripetal structure is lost, and instead we find a series of long fault blocks, some of which are structurally high and some structurally low in a seemingly disorganized way.
Kaye, C. A., Boston Basin Restudied, USGS B2-1 (1984).
Eocambrian paleomagnetism of the Boston Basin: Evidence for displaced terrane, December 1986
the basin structure of the Boston Basin is true only in a general sense. If we were to draw a perimeter around the known outcrops of the sedimentary and volcanic rocks — the Boston Basin — we would find that for only about half its length are the lowermost, or basal, rocks in contact with older rocks. The rest is faulted. Moreover, within the basin itself, the centripetal structure is lost, and instead we find a series of long fault blocks, some of which are structurally high and some structurally low in a seemingly disorganized way.
Kaye, C. A., Boston Basin Restudied, USGS B2-1 (1984).
After the terrane-making period had passed away, owing to the rising of the land above the sea, there came a second advance of the glaciers, which had clung to the higher hills, and had not passed entirely away from the land. This second advance did not cover the land with ice ; it only caused local glaciers to pour down the valleys.
The Neponset, the Charles, and the Mystic valleys were filled by these river-like streams, which seem never to have attained as far seaward as the peninsula of Boston.
This second advance of the ice seems to have been very temporary in its action, not having endured long enough to bring about any great changes. At about the time of its retreat, the last considerable change of line along these shores seems to have taken place. This movement was a subsidence of the land twenty feet or more below the former high-tide mark.
The mud brought down by these streams, consisting in part of clay and in part of decomposed vegetable matter, derived from land and water plants, coats the sandy bottoms or under-water terraces.
In this mud, even at considerable depths, eel-grass and some sea-weeds take root, and their stems make a dense jungle. In this grass more mud is gathered, and kept from the scouring action of the tide by being bound together by the roots and cemented by the organic matter.
This mass slowly rises until it is bare at low-tide. Then our marsh-grasses creep in, and in their interlaced foliage the waste brought in by the tide is retained, and helps to raise the level of the swamp higher. The streams from the land bring out a certain amount of mud, which at high-tide is spread in a thin
sheet over the surface of the low plain.
Some devious channels are kept open by the strong scouring action of the tide, but the swamp rapidly gains a level but little lower than high-tide. Except when there is some chance deposit of mud or sand from the bluffs along its edges, these swamps are never lifted above high-tide mark, for the forces that build them work only below that level.
Their effect upon the harbor of Boston has been disadvantageous. They have diminished the area of storage for the tide-water above the town, and thereby enfeebled the scouring power of the tidal currents.
Winsor, J., The Memorial History Of Boston (1882).
The Neponset, the Charles, and the Mystic valleys were filled by these river-like streams, which seem never to have attained as far seaward as the peninsula of Boston.
This second advance of the ice seems to have been very temporary in its action, not having endured long enough to bring about any great changes. At about the time of its retreat, the last considerable change of line along these shores seems to have taken place. This movement was a subsidence of the land twenty feet or more below the former high-tide mark.
The mud brought down by these streams, consisting in part of clay and in part of decomposed vegetable matter, derived from land and water plants, coats the sandy bottoms or under-water terraces.
In this mud, even at considerable depths, eel-grass and some sea-weeds take root, and their stems make a dense jungle. In this grass more mud is gathered, and kept from the scouring action of the tide by being bound together by the roots and cemented by the organic matter.
This mass slowly rises until it is bare at low-tide. Then our marsh-grasses creep in, and in their interlaced foliage the waste brought in by the tide is retained, and helps to raise the level of the swamp higher. The streams from the land bring out a certain amount of mud, which at high-tide is spread in a thin
sheet over the surface of the low plain.
Some devious channels are kept open by the strong scouring action of the tide, but the swamp rapidly gains a level but little lower than high-tide. Except when there is some chance deposit of mud or sand from the bluffs along its edges, these swamps are never lifted above high-tide mark, for the forces that build them work only below that level.
Their effect upon the harbor of Boston has been disadvantageous. They have diminished the area of storage for the tide-water above the town, and thereby enfeebled the scouring power of the tidal currents.
Winsor, J., The Memorial History Of Boston (1882).
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Composite magnetic map at flight altitude of 152 m, grid interval=160 m.
Aeromagnetic Map of the Boston South Quadrangle, GP-677, USGS/MA (1969)
Magnetic Data over Gulf of Maine and Adjacent Land Areas, USGS Magnetic Anomaly Map, 1990
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Magnetic Data over Gulf of Maine and Adjacent Land Areas, USGS Magnetic Anomaly Map, 1990
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Magnetic Data over Gulf of Maine and Adjacent Land Areas, USGS Magnetic Anomaly Map, 1990
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Cambridge Agrillite
Cambridge silica contents lie with few exceptions between 60 and 70 weight percent SiO2 (table 2), consistent with reference compostions in figure 6A (data re-calculated volatile-free from Clarke, 1924; El Wakeel and Riley, 1961; Hirst, 1962; Gromet and others, 1984). By contrast, Cambridge compositions are commonly richer than reference muds in Al2O3 and K2O, so that points plot in a broad band that sweeps into the lower left corner of figure 6A. Illite compositions from bentonitic sediments (table 2–16 in Weaver, 1989, calculated volatile-free) likewise fall into that area, in line with the previous suggestion (Thompson and Bowing, 2000) that Cambridge deposits include a component of volcanic ash. Terrigenous detritus is the obvious candidate for the other end member of this mixture.
MASSACHUSETTS HYDROGEOLOGIC INFORMATION MATRIX SIXTH EDITION (2013), 6th Edition Revised and Updated.
The Cambridge slate, named from Cambridge, where it has been encountered in many excavations, consists of perhaps 3,500 feet of slate, shale, argillite, and some interbedded sandstone, and at or near the top about 40 feet of greenish and yellowish quartzite. Beds here and there are composed of reworked tuff. The formation is of rather uniform lithologic character, and appears to have been deposited in a body of fresh water, possibly a lake at the margin of the ice.
GEOLOGY OF MASSACHUSETTS AND RHODE ISLAND, https://pubs.usgs.gov/bul/0597/report.pdf
GEOLOGY OF MASSACHUSETTS AND RHODE ISLAND, https://pubs.usgs.gov/bul/0597/report.pdf
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"It appears that the fundamental basis for what historically is the most deeply entrenched, stratigraphically derived interpretation of the depositional history and setting of the Boston Basin (ie alluvial fan/braided stream) emerged largely devoid of critical evaluation and discussion."
Boston Bay Group probably originated as a glacially influenced submarine slope/fan/apron deposited at unknown water depths, conceivable within an evolving transtensional basin, possibly a failed rift, associated with or perhaps just pre-dating, the opening of Iapetus and the break-up of a supercontinent.
Cambridge Argillite:
- Thickness: min 7,600 ft and may exceed 18,000 ft
- Lithology: Fine grained, mostly argillaceous (quartz-sericite-chlorite), some silt stone and tuff, typically 90 percent argillite, 10 percent feldspathic sandstone, slightly calcareous
- Bedding: rhythmic banding comprising about half of formation; beds generally .5-3 inches, pinch and swell in beds 0.25 inches thick
- Other: color mostly gray to the north, sixty percent reddish to purplish and fourth percent gray or greenish to the south
The historic stratigraphy of the Boston Basin is ambiguous, untestable, and impractical.
Socci & Smith, Stratigraphic implications of facies within the Boston Basin, Geology of the composite Avalon Terrane of southern New England (1990).
Geology of the Boston Basin (2011/2012)
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There can be little doubt that the peninsula of Boston has a foundation of argillaceous slate. This is, indeed, the only rock that has ever been found there in place. And from the occurrence of argillaceous slate in South Boston, and in Charlestown, with a northerly dip in both places, it would be very surprising if anv other rock should be found in Boston ; unless it were an intruding mass of trap rock. But this slate on the peninsula is buried deep by clay, gravel, and sand ; although, from the quantity of diluvium found there above the tertiary beds, 1 have been led to color the peninsula as a diluvial deposite.
Kaye, USGS
Berman, Germaine, Ladd; Characterization of the engineering properties of Boston blue clay for the MIT campus, Prepared for Department of Physical Plant, Massachusetts Institute of Technology; Massachusetts Institute of Technology. Department of Civil and Environmental Engineering (Sept. 1993). https://dl.tufts.edu/concern/pdfs/zp38wr66w
"The pre-Cambrian age of the argillite appeared contradicted by the presumed age of the Mattapan Volcanic Complex that is at the base of the sequence. The Mattapan volcanic rock in the basin, rhyolite in the Blue Hills and some rhyolite in the Lynn Volcanic Complex to the north were considered correlative (Kaye, 1984a). The rhyolite ash flows (welded tuffs), breccia and flows in the Blue Hills that were included in the Mattapan by Chute (1966 & 1969) are involved with the Quincy, which intrudes Cambrian strata. However, these volcanic rocks were separated by Kaktins (1976), who found them to be a much younger unit (described below as the Blue Hills Rhyolite). This rhyolite and the Lynn volcanic rock are associated with the Quincy and Cape Ann granites, respectively, which have the same Late Ordovician age (Zartman & Marvin, 1971; Dennen, 1991a), which is about 100 million years younger than the Cambridge Argillite. Radiometric dating has produced mixed results and is not always consistent· with the field relations. The chronological data have become more confused and contradictory as the process of rock dating becomes easier and practitioners multiply. Thompson and Grunow . (2004), Hatch (1991), and Kaye and Zartman (1980) describe the controversy and chronological challenges of the Boston Bay Group and Naylor (1976), who was an expert in the field, emphasized that fossil dating is better in the region. Dating is a difficult task when samples are disturbed by metamorphism or when using detrital zircons. Zircons, and other minerals commonly used for dating, formed with the inclosing igneous rock and the radioactive decay date determines the rock's age. Ratios of various elements are used, such as rubidium/ strontium (Rb /Sr) and lead and argon ratios. However, these ratios can be reset or partially reset by later heating and, therefore, would not give the original age of the rock There have been numerous times of heating in the region that have affected the rock Also the different elements used in dating give somewhat different results. Unlike zircons formed in an igneous rock, detrital zircons are ones carried in with the sediment that later formed sedimentary rock and their age gives a maximum age for the rock A major problem is the sampling of the wrong rock by someone unfamiliar with the geology (usually, an error that tends to favor a younger, fresher looking rock). Invalid interpretations of the laboratory data may thus stem from various problems, including insufficient sample control, misunderstanding the local geologic relations, laboratory error and the general limitations of the methods. Radiometric ages are far from precise in southern New England and experience demonstrates that they are commonly· only suggestions - but are still useful ones within their limits."
BAROSH & WOODHOUSE, Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE (2011/2012)
Mineralogy throughout the Cambridge sequence is dominated by felty intergrowths of fine silt- to clay-sized white mica and chlorite, accompanied by varying amounts of fragmental quartz and plagioclase, along with phyllosilicate-rich pods interpreted as altered volcanic or argillaceous rock fragments. Such grains typically measure 0.2 to 0.5 mm (coarse silt) but locally reach 1 mm in the fine sand range. Accessory pyrite appears macroscopically, and zircon can be seen in thin section. Electron microprobe analysis reveals chamositic chlorite with Fe/FeMg between 0.60 and 0.75 and K-deficient white mica consistent with illite; plagioclase is uniformly pure albite. Sub-microscopic accessory minerals identified by microprobing include ubiquitous apatite, monazite, rutile and other sulfides (arsenopyrite, galena, chalcopyrite, sphalerite), and in three samples, titanite. Sub-millimeter thick opaque laminae are commonly composed of framboidal pyrite, rutile and/or organic material, but backscatter microprobe images in one sample show a preponderance of Fe-rich chlorite.
In contrast to their mineralogical uniformity, Cambridge deposits show considerable textural variability as illustrated in figure 4. The finest-grained layers are mainly composed of chlorite and illite with 10 modal percent fragments of quartz and plagioclase (fig. 4A). The coarsest layers contain 20 to 25 modal percent quartz plagioclase varying from rounded to angular in shape. The distinctly grainy appearance of such samples is enhanced by an equal abundance of altered lithic fragments in the form of irregular pods that are typically difficult to distinguish from surrounding phyllosilicate-rich matrix (fig. 4B). Many samples contain alternations between coarser and finer laminae across indistinct contacts, with some laminae measuring only a few crystal diameters (fig. 4C). In fault zones or in hinge areas of folds, silty beds may be truncated and translated parallel to cleavage (S1 in fig. 4D) or completely transposed (fig. 4E). Grading is also common in Cambridge samples as illustrated in figure 4F where coarsest quartz grains reach into the fine sand range (0.15 mm)
A 300’ drill core at Shaft 9 of the City Tunnel Extension (fig. 2) contains numerous samples showing textures consistent with volcanic origin. Several of these are illustrated in figure 5. The rock at depth 28.5 ft was sampled from a 4 ft (1.2 m) thick layer of mafic porphyry containing abundant 0.5 mm sericitized plagioclase laths aligned in a phyllosilicate-rich matrix (fig. 5A). The turbid brownish color of the matrix in plane light is consistent with considerable Fe-rich chlorite. In contact with the host crystal-poor argillite, an altered glassy rind retains vestiges of a tubular, possibly scoriaceous texture (fig. 5B). The irregular interpenetrating contact between argillite and a thiner porphyry sampled lower in the core suggest interaction between lava and soft sediment (fig. 5C). Possible accretionary lapilli or pumice clasts in the form of flattened crystal-free phyllosilicate domains tapering into the matrix (fig. 5D) are present at several depths, and embayed quartz appears in a crystal-rich layer at - 165.62 ft (fig. 5E).
Whole-rock geochemistry provides the most convenient means of characterizing Cambridge compositions and comparing them across the Boston Basin. Available data include whole-rock major and trace element analyses listed in table 2. Penetrative cleavage observed in 15 of 45 analyzed samples (symbols with bold outline in figs. 3 and 6A) raise questions about mobility of CaO, Na2O and K2O and whether measured concentrations of these reliably reflect depositional compositions. However, in figure 6A adapted from a diagram developed by Garrels and Mackenzie (1971, p. 213) to represent the full chemical range of “average” modern sediments, cleaved Cambridge samples span the same compositional range as the majority of samples that lack cleavage. Compositions for reference clays from the original diagram as well as the North American shale composite (NASC of Gromet and others, 1984) lie on the same trajectory, suggesting that strain-related alkali mobility (Wintsch and others, 1991, for example) has not significantly altered the compositions of these rocks
Cambridge silica contents lie with few exceptions between 60 and 70 weight percent SiO2 (table 2), consistent with reference compostions in figure 6A (data re-calculated volatile-free from Clarke, 1924; El Wakeel and Riley, 1961; Hirst, 1962; Gromet and others, 1984). By contrast, Cambridge compositions are commonly richer than reference muds in Al2O3 and K2O, so that points plot in a broad band that sweeps into the lower left corner of figure 6A. Illite compositions from bentonitic sediments (table 2–16 in Weaver, 1989, calculated volatile-free) likewise fall into that area, in line with the previous suggestion (Thompson and Bowing, 2000) that Cambridge deposits include a component of volcanic ash. Terrigenous detritus is the obvious candidate for the other end member of this mixture
The geochemical composition of upper continental crust which supplies terrigenous detritus to marine basins (overview in Taylor and McLennan, 1985) is typically approximated using rare earth element [REE] concentrations in fine grained terrestrial deposits, but major element data are also available in some cases. Included in figure 6A are ratios calculated from New Zealand loess sampled from the Banks Peninsula of New Zealand’s South Island (Taylor and others, 1983). These cluster on the right edge of the diagram at the end of a proposed mixing line that terminates among illite compositions on the lower left. The NASC average shows higher calcium and potassium than most argillite samples, but plots towards the terrigenous end of the Cambridge spectrum. Seafloor deposits of terrestrial derivation documented in settings including the Nankai accretionary prism of southwest Japan (ODP Site 808; Underwood and Pickering, 1996) and the Argo Abyssal Plain adjacent to northwest Australia (ODP Site 765; Plank and Ludden, 1992; Plank and Langmuir, 1998) also plot on the right side of figure 6A.
The presence of terrigenous components in Cambridge argillite is also consistent with REE patterns obtained from six samples from the CTE, MDT and NMRT (table 3). All of these show light REE enrichment, a mild Eu anomaly and a flat heavy REE distribution quite similar to values both from the New Zealand loess and the two ODP sites (fig. 6B). The gray field in figure 6B encompassing patterns for average continental crust based on the NASC as well as the Post-Archean average Australian Shale (PAAS of Nance and Taylor, 1976) and European shale composite (ES of Haskin and Haskin, 1966) also lies within the hatched Cambridge envelope. A more enriched REE pattern was obtained from core boring113-N30 28.5 from CTE Shaft 9. This sample contains abundant plagioclase laths of probable volcanic origin (fig. 5A), and its REE distribution falls within the range shown by alkali basalt from Hawaii’s Kohala Volcano (Spengler and Garcia, 1988; horizontally ruled field in fig. 6B).
Thompson, Margaret D. and James L. Crowley. “Avalonian arc-to-platform transition in southeastern New England: U-Pb geochronology and stratigraphy of Ediacaran Cambridge “argillite,” Boston Basin, Massachusetts, USA.” American Journal of Science 320 (2020): 405 - 449.
The most prominent feature of the CSP records is a sequence of closely spaced, parallel echoes which may total over one hundred feet thick. This sequence has been identified as the Boston Blue Clay from the character of the echoes, their vertical and horizontal extent in the harbor, surficial cores, and correlation with boring data in and around the harbor. The origin of the blue clay, and in particular the problem of whether it is a marine or fresh water deposit, is a disputed subject. Phipps (1964,pp.17 and 18) summarizes the arguments and describes the blue clays: They have a blue-grey to slightly greenishgrey color when unoxidized; oxidized they become yellow-brown. The fresh clays are tough and plastic but contain extremely fine quartz. Layers of sand and/or silt, varying in thickness from less than one inch to more than one foot, occur in the clays and appear to be rhythmically deposited, but on too coarse a scale to be classified as varves. The silty or sandy layers probably are responsible for the apparent layering seen in the records. As yet, no attempts have yielded clay cores of sufficient length to permit correlation with CSP records to identify reflecting interfaces. The blue clay appears to be deposited on an uneven surface of either glacial till or bedrock. It is not now possible from the CSP records alone to differentiate till from either bedrock or bedrock overlain with a thin deposit of till. In the President Roads area, this surface probably represents till, but this conclusion is based upon indirect evidence. In his thesis, Phipps (1964) attempts to differentiate between rock and till surfaces by the quality of the reflection on the CSP records, a till interface being less distinct. While such a correlation may indeed be accurate for Boston Harbor, a smooth till surface might give a more distinct echo than a rough rock surface. On June 10,1965, Mr. Vincent J. Murph"y of Weston Geophysical Engineers, Inc. cooperated with us in conducting a seismic refraction study over a peak in this bedrock or till surface. The results so far have not yielded a definite determination of the nature of the interface. However, results of the CSP studies may possibly be used to refine the refraction data to greater accuracy. Perhaps the most impressive fact revealed by Figures 5 and 6 is the extremely rapid scale of variation in this part of the harbor. A surface defined by an interface in the clay is very complex. This result indicates the need for conducting closely-spaced surveys when studying layering. Large scale trends of layering in the blue clay appears to be controlled by the topography of the underlying bedrock or till surface. Phipps (1964) -suggests that this phenomenon results from differential compaction in the clays. No evidence of slumping or faulting was found. The upper surface of the clay does not follow the layering in general. Erosion of at least twenty feet of the clay must have occurred.
CONTINUOUS SEISMIC PROFILING OF THE PRESIDENT ROADS AREA, BOSTON HARBOR, MASSACHUSETTS by JOHN A. YULES S.B., Massachusetts Institute of Technology (1963) SUBMITTED IN PARTIAL FULFILLLENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February, 1966
A vei:y thin deltaic deposit, which is present to the south beneath the Back Bay, is described as 1.2 meters (4 feet) of coarse sand and several centimeters (a few inches) of overlying clay beneath a thin upper till (Judson, 1949). It overlies the Cambridge Argillite, which lies at a depth of between elevation -34 and -43 meters (-112 and -142 feet) MSL below the John Hancock site. The thin blue clay appeared to grade into the till, which is bluish with a high clay content. This deposit appears to be a thinned remnant of the deltaic sand and clay smeared by the overriding till. The relatively limited area of the thick portion of the deposit, plus the recognition of. deltaic foreset and bottomset beds (Kaye, 1976a), indicate that the sand and gravel, clay, and peat were part of a delta complex before being overridden and pushed up into Beacon Hill. The present distribution of the sediment suggests that a large delta was centered in the Charles River Basin and adjacent Cambridge and that this delta was fed from the north by a subglacial river along the ancient Mystic River Valley, which followed the Aberjona-Fresh Pond Buried Valley (west of the present Mystic Valley) and then the Malden Buried Valley (east of the present Mystic River). It may have been similar to the many subsequent well-preserved deltas along coastal Maine that built out into the marine clay equivalent of the "Boston Blue Clay" and the large one on the northeast side of the center of Concord (Koteff, 1964b). Kaye (1961) also felt that the clay beneath the edge of Beacon Hill was marine, but at least some clay seen by Woodhouse appears to be layered bottomset .or lake deposit. Perhaps after the toe of a delta built southeastward onto the Shawmut Peninsula it was overlapped by clay as marine waters flooded the still depressed crust while the ice retreated farther north. The next ice readvance of the Boston Substage tore into the delta and pushed it up to form Tri.mountain, of which Beacon Hill forms a remnant, and the overriding glacier left a capping of till. The deposit is . probably Early Wisconsin, but its exact age is yet to be determined.
BAROSH & WOODHOUSE, Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE (2011/2012)
Argillite - This is perhaps the most common rock type in the basin. It consists of silt-size particles of quartz, feldspar, seritic, chlorite and kaolinite. Darker argillite contains more sericite and chlorite while the lighter argillite contains more kaolinite (Kaye, 1967). The argillite is typically gray, but purple, purplish brown, tan, and green colors also occur. Kaye (1984) describes some mineralogical variations of argillite which include calcareous argillite interbedded with normal argillite, sideritic argillite, gypsiferous and dolomitic argillite, red argillite, and black argillite. The argillite is typically hard and well indurated, more consolidated than shale but not fissile like shale. According to Kaye (1979), fresh rock tends to break across bedding planes. surfaces (Rahm, 19 62) . Bedding is typically laminated, consisting of alternating 0.1- to 0.2-inch-thick light and dark colored layers. Individual beds generally range in thickness from less than 1/16-inch to 4 inches and can be up to 5 feet thick. The individual beds maintain a rather uniform thickness for many feet or tens of feet (Billings and Tierney, 1964) . Grain size can vary locally to sandy or silty. Sedimentary structures such as slump folds, ripple marks and cross beds are common in this unit. Severe alteration of the argillite (known as kaolinization), which results in a soft, whitish rock or even clay, occurs in random areas of the Boston Basin. Thin-section study shows that the normal minerals of the argillite have been replaced by sericite and kaolinite during the alteration process. 16
Kaolinization is probably the result of thermo-alteration of the argillite, with an igneous intrusion acting as the catalyst
(Kaye, 1967)
CONTRACT DOCUMENTS FOR APPENDIX A SUBSURFACE EXPLORATION GEOTECHNICAL INTERPRETIVE REPORT INTER-ISLAND TUNNEL, CONTRACT PACKAGE NO. 151 MASSACHUSETTS WATER RESOURCES AUTHORITY MWRA CONTRACT NO. 5541 EPA NO. C 259713-18
BAROSH & WOODHOUSE, Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE (2011/2012)
Mineralogy throughout the Cambridge sequence is dominated by felty intergrowths of fine silt- to clay-sized white mica and chlorite, accompanied by varying amounts of fragmental quartz and plagioclase, along with phyllosilicate-rich pods interpreted as altered volcanic or argillaceous rock fragments. Such grains typically measure 0.2 to 0.5 mm (coarse silt) but locally reach 1 mm in the fine sand range. Accessory pyrite appears macroscopically, and zircon can be seen in thin section. Electron microprobe analysis reveals chamositic chlorite with Fe/FeMg between 0.60 and 0.75 and K-deficient white mica consistent with illite; plagioclase is uniformly pure albite. Sub-microscopic accessory minerals identified by microprobing include ubiquitous apatite, monazite, rutile and other sulfides (arsenopyrite, galena, chalcopyrite, sphalerite), and in three samples, titanite. Sub-millimeter thick opaque laminae are commonly composed of framboidal pyrite, rutile and/or organic material, but backscatter microprobe images in one sample show a preponderance of Fe-rich chlorite.
In contrast to their mineralogical uniformity, Cambridge deposits show considerable textural variability as illustrated in figure 4. The finest-grained layers are mainly composed of chlorite and illite with 10 modal percent fragments of quartz and plagioclase (fig. 4A). The coarsest layers contain 20 to 25 modal percent quartz plagioclase varying from rounded to angular in shape. The distinctly grainy appearance of such samples is enhanced by an equal abundance of altered lithic fragments in the form of irregular pods that are typically difficult to distinguish from surrounding phyllosilicate-rich matrix (fig. 4B). Many samples contain alternations between coarser and finer laminae across indistinct contacts, with some laminae measuring only a few crystal diameters (fig. 4C). In fault zones or in hinge areas of folds, silty beds may be truncated and translated parallel to cleavage (S1 in fig. 4D) or completely transposed (fig. 4E). Grading is also common in Cambridge samples as illustrated in figure 4F where coarsest quartz grains reach into the fine sand range (0.15 mm)
A 300’ drill core at Shaft 9 of the City Tunnel Extension (fig. 2) contains numerous samples showing textures consistent with volcanic origin. Several of these are illustrated in figure 5. The rock at depth 28.5 ft was sampled from a 4 ft (1.2 m) thick layer of mafic porphyry containing abundant 0.5 mm sericitized plagioclase laths aligned in a phyllosilicate-rich matrix (fig. 5A). The turbid brownish color of the matrix in plane light is consistent with considerable Fe-rich chlorite. In contact with the host crystal-poor argillite, an altered glassy rind retains vestiges of a tubular, possibly scoriaceous texture (fig. 5B). The irregular interpenetrating contact between argillite and a thiner porphyry sampled lower in the core suggest interaction between lava and soft sediment (fig. 5C). Possible accretionary lapilli or pumice clasts in the form of flattened crystal-free phyllosilicate domains tapering into the matrix (fig. 5D) are present at several depths, and embayed quartz appears in a crystal-rich layer at - 165.62 ft (fig. 5E).
Whole-rock geochemistry provides the most convenient means of characterizing Cambridge compositions and comparing them across the Boston Basin. Available data include whole-rock major and trace element analyses listed in table 2. Penetrative cleavage observed in 15 of 45 analyzed samples (symbols with bold outline in figs. 3 and 6A) raise questions about mobility of CaO, Na2O and K2O and whether measured concentrations of these reliably reflect depositional compositions. However, in figure 6A adapted from a diagram developed by Garrels and Mackenzie (1971, p. 213) to represent the full chemical range of “average” modern sediments, cleaved Cambridge samples span the same compositional range as the majority of samples that lack cleavage. Compositions for reference clays from the original diagram as well as the North American shale composite (NASC of Gromet and others, 1984) lie on the same trajectory, suggesting that strain-related alkali mobility (Wintsch and others, 1991, for example) has not significantly altered the compositions of these rocks
Cambridge silica contents lie with few exceptions between 60 and 70 weight percent SiO2 (table 2), consistent with reference compostions in figure 6A (data re-calculated volatile-free from Clarke, 1924; El Wakeel and Riley, 1961; Hirst, 1962; Gromet and others, 1984). By contrast, Cambridge compositions are commonly richer than reference muds in Al2O3 and K2O, so that points plot in a broad band that sweeps into the lower left corner of figure 6A. Illite compositions from bentonitic sediments (table 2–16 in Weaver, 1989, calculated volatile-free) likewise fall into that area, in line with the previous suggestion (Thompson and Bowing, 2000) that Cambridge deposits include a component of volcanic ash. Terrigenous detritus is the obvious candidate for the other end member of this mixture
The geochemical composition of upper continental crust which supplies terrigenous detritus to marine basins (overview in Taylor and McLennan, 1985) is typically approximated using rare earth element [REE] concentrations in fine grained terrestrial deposits, but major element data are also available in some cases. Included in figure 6A are ratios calculated from New Zealand loess sampled from the Banks Peninsula of New Zealand’s South Island (Taylor and others, 1983). These cluster on the right edge of the diagram at the end of a proposed mixing line that terminates among illite compositions on the lower left. The NASC average shows higher calcium and potassium than most argillite samples, but plots towards the terrigenous end of the Cambridge spectrum. Seafloor deposits of terrestrial derivation documented in settings including the Nankai accretionary prism of southwest Japan (ODP Site 808; Underwood and Pickering, 1996) and the Argo Abyssal Plain adjacent to northwest Australia (ODP Site 765; Plank and Ludden, 1992; Plank and Langmuir, 1998) also plot on the right side of figure 6A.
The presence of terrigenous components in Cambridge argillite is also consistent with REE patterns obtained from six samples from the CTE, MDT and NMRT (table 3). All of these show light REE enrichment, a mild Eu anomaly and a flat heavy REE distribution quite similar to values both from the New Zealand loess and the two ODP sites (fig. 6B). The gray field in figure 6B encompassing patterns for average continental crust based on the NASC as well as the Post-Archean average Australian Shale (PAAS of Nance and Taylor, 1976) and European shale composite (ES of Haskin and Haskin, 1966) also lies within the hatched Cambridge envelope. A more enriched REE pattern was obtained from core boring113-N30 28.5 from CTE Shaft 9. This sample contains abundant plagioclase laths of probable volcanic origin (fig. 5A), and its REE distribution falls within the range shown by alkali basalt from Hawaii’s Kohala Volcano (Spengler and Garcia, 1988; horizontally ruled field in fig. 6B).
Thompson, Margaret D. and James L. Crowley. “Avalonian arc-to-platform transition in southeastern New England: U-Pb geochronology and stratigraphy of Ediacaran Cambridge “argillite,” Boston Basin, Massachusetts, USA.” American Journal of Science 320 (2020): 405 - 449.
The most prominent feature of the CSP records is a sequence of closely spaced, parallel echoes which may total over one hundred feet thick. This sequence has been identified as the Boston Blue Clay from the character of the echoes, their vertical and horizontal extent in the harbor, surficial cores, and correlation with boring data in and around the harbor. The origin of the blue clay, and in particular the problem of whether it is a marine or fresh water deposit, is a disputed subject. Phipps (1964,pp.17 and 18) summarizes the arguments and describes the blue clays: They have a blue-grey to slightly greenishgrey color when unoxidized; oxidized they become yellow-brown. The fresh clays are tough and plastic but contain extremely fine quartz. Layers of sand and/or silt, varying in thickness from less than one inch to more than one foot, occur in the clays and appear to be rhythmically deposited, but on too coarse a scale to be classified as varves. The silty or sandy layers probably are responsible for the apparent layering seen in the records. As yet, no attempts have yielded clay cores of sufficient length to permit correlation with CSP records to identify reflecting interfaces. The blue clay appears to be deposited on an uneven surface of either glacial till or bedrock. It is not now possible from the CSP records alone to differentiate till from either bedrock or bedrock overlain with a thin deposit of till. In the President Roads area, this surface probably represents till, but this conclusion is based upon indirect evidence. In his thesis, Phipps (1964) attempts to differentiate between rock and till surfaces by the quality of the reflection on the CSP records, a till interface being less distinct. While such a correlation may indeed be accurate for Boston Harbor, a smooth till surface might give a more distinct echo than a rough rock surface. On June 10,1965, Mr. Vincent J. Murph"y of Weston Geophysical Engineers, Inc. cooperated with us in conducting a seismic refraction study over a peak in this bedrock or till surface. The results so far have not yielded a definite determination of the nature of the interface. However, results of the CSP studies may possibly be used to refine the refraction data to greater accuracy. Perhaps the most impressive fact revealed by Figures 5 and 6 is the extremely rapid scale of variation in this part of the harbor. A surface defined by an interface in the clay is very complex. This result indicates the need for conducting closely-spaced surveys when studying layering. Large scale trends of layering in the blue clay appears to be controlled by the topography of the underlying bedrock or till surface. Phipps (1964) -suggests that this phenomenon results from differential compaction in the clays. No evidence of slumping or faulting was found. The upper surface of the clay does not follow the layering in general. Erosion of at least twenty feet of the clay must have occurred.
CONTINUOUS SEISMIC PROFILING OF THE PRESIDENT ROADS AREA, BOSTON HARBOR, MASSACHUSETTS by JOHN A. YULES S.B., Massachusetts Institute of Technology (1963) SUBMITTED IN PARTIAL FULFILLLENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February, 1966
A vei:y thin deltaic deposit, which is present to the south beneath the Back Bay, is described as 1.2 meters (4 feet) of coarse sand and several centimeters (a few inches) of overlying clay beneath a thin upper till (Judson, 1949). It overlies the Cambridge Argillite, which lies at a depth of between elevation -34 and -43 meters (-112 and -142 feet) MSL below the John Hancock site. The thin blue clay appeared to grade into the till, which is bluish with a high clay content. This deposit appears to be a thinned remnant of the deltaic sand and clay smeared by the overriding till. The relatively limited area of the thick portion of the deposit, plus the recognition of. deltaic foreset and bottomset beds (Kaye, 1976a), indicate that the sand and gravel, clay, and peat were part of a delta complex before being overridden and pushed up into Beacon Hill. The present distribution of the sediment suggests that a large delta was centered in the Charles River Basin and adjacent Cambridge and that this delta was fed from the north by a subglacial river along the ancient Mystic River Valley, which followed the Aberjona-Fresh Pond Buried Valley (west of the present Mystic Valley) and then the Malden Buried Valley (east of the present Mystic River). It may have been similar to the many subsequent well-preserved deltas along coastal Maine that built out into the marine clay equivalent of the "Boston Blue Clay" and the large one on the northeast side of the center of Concord (Koteff, 1964b). Kaye (1961) also felt that the clay beneath the edge of Beacon Hill was marine, but at least some clay seen by Woodhouse appears to be layered bottomset .or lake deposit. Perhaps after the toe of a delta built southeastward onto the Shawmut Peninsula it was overlapped by clay as marine waters flooded the still depressed crust while the ice retreated farther north. The next ice readvance of the Boston Substage tore into the delta and pushed it up to form Tri.mountain, of which Beacon Hill forms a remnant, and the overriding glacier left a capping of till. The deposit is . probably Early Wisconsin, but its exact age is yet to be determined.
BAROSH & WOODHOUSE, Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE (2011/2012)
Argillite - This is perhaps the most common rock type in the basin. It consists of silt-size particles of quartz, feldspar, seritic, chlorite and kaolinite. Darker argillite contains more sericite and chlorite while the lighter argillite contains more kaolinite (Kaye, 1967). The argillite is typically gray, but purple, purplish brown, tan, and green colors also occur. Kaye (1984) describes some mineralogical variations of argillite which include calcareous argillite interbedded with normal argillite, sideritic argillite, gypsiferous and dolomitic argillite, red argillite, and black argillite. The argillite is typically hard and well indurated, more consolidated than shale but not fissile like shale. According to Kaye (1979), fresh rock tends to break across bedding planes. surfaces (Rahm, 19 62) . Bedding is typically laminated, consisting of alternating 0.1- to 0.2-inch-thick light and dark colored layers. Individual beds generally range in thickness from less than 1/16-inch to 4 inches and can be up to 5 feet thick. The individual beds maintain a rather uniform thickness for many feet or tens of feet (Billings and Tierney, 1964) . Grain size can vary locally to sandy or silty. Sedimentary structures such as slump folds, ripple marks and cross beds are common in this unit. Severe alteration of the argillite (known as kaolinization), which results in a soft, whitish rock or even clay, occurs in random areas of the Boston Basin. Thin-section study shows that the normal minerals of the argillite have been replaced by sericite and kaolinite during the alteration process. 16
Kaolinization is probably the result of thermo-alteration of the argillite, with an igneous intrusion acting as the catalyst
(Kaye, 1967)
CONTRACT DOCUMENTS FOR APPENDIX A SUBSURFACE EXPLORATION GEOTECHNICAL INTERPRETIVE REPORT INTER-ISLAND TUNNEL, CONTRACT PACKAGE NO. 151 MASSACHUSETTS WATER RESOURCES AUTHORITY MWRA CONTRACT NO. 5541 EPA NO. C 259713-18
The occasional glacial gravel to boulder clasts in the clay are apparently iceberg dropstones, perhaps rafted when the ice front was close to Boston (see Figure 3-97).
Barosh & Woodhouse, Geology of the Boston Basin, Civil Eng. Practice, 2011/2012 |
Barosh & Woodhouse, Geology of the Boston Basin, Civil Eng. Practice, 2011/2012
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Patrick J. Barosh & David Woodhouse, Geology of the Boston Basin, Civil Engineering Practice (2011/2012).
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No deposits between Carboniferous and Pleistocene in age were found until July, 1905, when a boring made at the Ames Building, on Washington at the head of State street, started at an elevation of 33 feet above mean tide, and was sunk to the unusual depth of 228 feet. A previous test here had reported bed-rock at a depth of 77 feet, directly underearth the drift. Not being satisfied with the
original report, the engineers decided to make a new test, with the result that in the 228-foot boring the following strata were penetrated. According to the system of the Transit Commission, samples were collected from the boring at intervals of every few feet, and are preserved at the office of the Commission, where they were seen by the writer through the courtesy of Mr. Howard A. Carson, chief engineer. Down to 77 feet from the surface the materials are the ordinary sand, gravel, clay and till of the region, shown by their character to he entirely of Pleistocene age. They are mostly rather wet and yield considerable water. The material below 77 feet is dry, and in a previous boring had been called rock and not entered by the drill. All the samples of this bed were seen by the writer and found to consist mostly of a very fine-grained gray to white clay, which became plastic when wet. It varied from very soft and putty-like to nearly as hard as the underlying slate. The material when examined by Dr. W. T. Schaller of the United States Geological Survey was found to consist of Si 0, = 59·18 per cent and (Al,O.,Fe,O.,P,O., Ti02)= 27·11 per cent, thus being a very pure clay. Two masses, one consisting of sandstone, the other of finegrained conglomerate, were found in the clay, and each measured about 1½ feet in thickness. These may be interstratified beds of rock, or, as tl10ir relations and character seem to indicate, they may be bowlders. Difference from Pleistocene clays.-This clay is important for the reason that it is unlike the general type of clay found at Boston. All the Pleistocene clays of the vicinity are of blue-gray to brown or buff colors; this clay is light gray to nearly white. The Pleistocene clays contain numerous bowlders and pebbles composed of all kinds of rock found in New England, but in this clay only two bowlders have been discovered, and these consist of rock only found in the vicinity of Boston, and which forms the bed-rock of the region. The Pleistocene clays are interstratified with glacial deposits; this clay rests on bed-rock and is separated from the overlying Pleistocene clay by a bed of till. This clay is much dryer than the overlying Pleistocene clay. the surface. Some of these fragments are as much as half an inch in diameter. Mr. B. F. Smith, a prominent well driller of Boston, reports a number of wells in the city, in which peculiar soft white deposits were found directly underneath the till. The material is said . to cave badly and sometimes contains much water. To Professor Crosby, who has made extensive investigations regarding the borings of Boston, belongs the credit of being the first to suggest the pre-Pleistocene age of this clay. Professor Crosby writes as follows:* "We may profitably note the fact that some of the borings repol'ting bed-rock in tbe section of Boston south and east of Beacon Hill have clearly not reached any of the hard and thoroughly solid rocks (slate, conglomerate, trap, etc.) such as make up the whole of the bed-rock surface wherever it is exposed in ledges and shallow excavations; but instead the drill has passed from the glacial drift to imperfectly consolidated sands, clays, marls, etc., in part of colors unknown to the drift, and probably rep1·esenting Tertiary strata underlying the drift and filling deep depressions and valleys in the harder formations or true bed-rocks of the region. The artesian well of N. Ward & Co., on Spectacle Island, 560 feet deep, passed through at least 360 feet of unconsolidated material, only part of which could be regarded as glacial drift; and the deep well at the corner of High and Purchase streets in Boston, reported as reaching the bottom of the drift at about 100 feet, is in soft materials comparable with the Tertiary deposits of Martha's Vineyard and Long Island, to a depth of at least 500 feet." Oonclusion8.-Samp1es of .the white clay from the Ames Building boring were compared at the office of the United States Geological Survey with samples of clay collected by Mr. Yeatch from a number of borings on Long Island, New York, and found to ,igree very closely with them in appearance. Mr. Veatch has correlated the Long Island deposits with the Raritan formation of New Jersey. t If this correlation is correct, it is possible that the Boston deposits may be of similar age. This is rendered more probable by the similarity of tlie material in the Boston borings to some of the clays on l\Iartha1s Vineyard, and by the fact that the beds on that island referred to by Professor Crosby as "Tertiary" are said by paleontologists to be in part of Cretaceous age.
original report, the engineers decided to make a new test, with the result that in the 228-foot boring the following strata were penetrated. According to the system of the Transit Commission, samples were collected from the boring at intervals of every few feet, and are preserved at the office of the Commission, where they were seen by the writer through the courtesy of Mr. Howard A. Carson, chief engineer. Down to 77 feet from the surface the materials are the ordinary sand, gravel, clay and till of the region, shown by their character to he entirely of Pleistocene age. They are mostly rather wet and yield considerable water. The material below 77 feet is dry, and in a previous boring had been called rock and not entered by the drill. All the samples of this bed were seen by the writer and found to consist mostly of a very fine-grained gray to white clay, which became plastic when wet. It varied from very soft and putty-like to nearly as hard as the underlying slate. The material when examined by Dr. W. T. Schaller of the United States Geological Survey was found to consist of Si 0, = 59·18 per cent and (Al,O.,Fe,O.,P,O., Ti02)= 27·11 per cent, thus being a very pure clay. Two masses, one consisting of sandstone, the other of finegrained conglomerate, were found in the clay, and each measured about 1½ feet in thickness. These may be interstratified beds of rock, or, as tl10ir relations and character seem to indicate, they may be bowlders. Difference from Pleistocene clays.-This clay is important for the reason that it is unlike the general type of clay found at Boston. All the Pleistocene clays of the vicinity are of blue-gray to brown or buff colors; this clay is light gray to nearly white. The Pleistocene clays contain numerous bowlders and pebbles composed of all kinds of rock found in New England, but in this clay only two bowlders have been discovered, and these consist of rock only found in the vicinity of Boston, and which forms the bed-rock of the region. The Pleistocene clays are interstratified with glacial deposits; this clay rests on bed-rock and is separated from the overlying Pleistocene clay by a bed of till. This clay is much dryer than the overlying Pleistocene clay. the surface. Some of these fragments are as much as half an inch in diameter. Mr. B. F. Smith, a prominent well driller of Boston, reports a number of wells in the city, in which peculiar soft white deposits were found directly underneath the till. The material is said . to cave badly and sometimes contains much water. To Professor Crosby, who has made extensive investigations regarding the borings of Boston, belongs the credit of being the first to suggest the pre-Pleistocene age of this clay. Professor Crosby writes as follows:* "We may profitably note the fact that some of the borings repol'ting bed-rock in tbe section of Boston south and east of Beacon Hill have clearly not reached any of the hard and thoroughly solid rocks (slate, conglomerate, trap, etc.) such as make up the whole of the bed-rock surface wherever it is exposed in ledges and shallow excavations; but instead the drill has passed from the glacial drift to imperfectly consolidated sands, clays, marls, etc., in part of colors unknown to the drift, and probably rep1·esenting Tertiary strata underlying the drift and filling deep depressions and valleys in the harder formations or true bed-rocks of the region. The artesian well of N. Ward & Co., on Spectacle Island, 560 feet deep, passed through at least 360 feet of unconsolidated material, only part of which could be regarded as glacial drift; and the deep well at the corner of High and Purchase streets in Boston, reported as reaching the bottom of the drift at about 100 feet, is in soft materials comparable with the Tertiary deposits of Martha's Vineyard and Long Island, to a depth of at least 500 feet." Oonclusion8.-Samp1es of .the white clay from the Ames Building boring were compared at the office of the United States Geological Survey with samples of clay collected by Mr. Yeatch from a number of borings on Long Island, New York, and found to ,igree very closely with them in appearance. Mr. Veatch has correlated the Long Island deposits with the Raritan formation of New Jersey. t If this correlation is correct, it is possible that the Boston deposits may be of similar age. This is rendered more probable by the similarity of tlie material in the Boston borings to some of the clays on l\Iartha1s Vineyard, and by the fact that the beds on that island referred to by Professor Crosby as "Tertiary" are said by paleontologists to be in part of Cretaceous age.
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Contract #W912DS-12-D-0002, DB01
Marine Geophysical and Geological Investigation,
Boston Harbor, Boston Massachusetts 2015
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Contract #W912DS-12-D-0002, DB01
Marine Geophysical and Geological Investigation,
Boston Harbor, Boston Massachusetts 2015
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Contract #W912DS-12-D-0002, DB01
Marine Geophysical and Geological Investigation,
Boston Harbor, Boston Massachusetts 2015
The depth to the upper surface of bedrock is very important in determining the cost and nature of building foundations and utilities placement. If the bedrock is shallow, buildings may be supported on piles, allowing the use of relatively cheap, locally abundant clay for the purpose of land fill. (It should be mentioned that this combination of piles and clay may result in "negative loading"- a problem which arises when the clay consolidates over a long period of time. See p. 2 5 .) If, however, the bedrock is buried under thick layers of highly compressible sediments, such as clay and mud, the cost of placing piles becomes prohibitively expensive, and one is forced to consider floating foundations placed in a non-compressible fill such as clean sand or gravel. In addition, since a thick layer of mud or clay is likely to compress under the added burden of fill material, settlement problems are likely to be encountered in areas where fill is to be placed upon thick deposits of clay or mud overlying deep bedrock.
Unfortunately, very little information is available concerning depth to bedrock within the harbor area. Upson and Spencer (1964) suggest the existence of several Pleistocene river valleys, or buried bedrock valleys, under the Dorchester Bay area. (see Figure 3.) This would place the bedrock surface 250 feet or more below sea level in parts of Dorchester Bay, and Phipps (1964) cites a depth of 360 feet to bedrock under Spectacle Island. On the other hand, outcrops of the Squantum formation exist above sea level in the Squantum Point area (Squaw Rocks and Chapel Rocks). Mr. Clifford Kaye of the U.S. Geological Survey, noted authority on the geology of Boston, explained that he personally places little confidence in the bedrock valley theory. He added that the bedrock is indeed quite deep in parts of the harbor, but the bedrock contours are too irregular and too uncertain to accurately predict the existence or the location of such bedrock valleys.
The most prominent sedimentary feature of the Boston Basin is the existence of a vast deposit of soft clay known as the Boston Blue clay. This blue clay is a still water deposit whose thickness ranges from 2 feet to over 200 feet. It is often separated from bedrock by thin layers of sand, gravel, or glacial till, and in some areas there are thin layers of sand, gravel, or silt within the clay. The upper layer of the blue clay has been oxidized to form a generally firmer yellowish clay, referred to by Boston engineers as "the stiff crust." This oxidation is believed to have taken place at a time when much of the harbor bottom was above sea level. There has since been a general subsidence of the entire Boston Basin area, with a subsequent rise in sea level. (Marmer, 1944). Above the yellow clay lies a thin (5 feet to 10 feet in most places) layer of organic silt covered by mud. This mud is of the black, carbonaceous variety. The high organic content of this mud is generally attributed to the many sewer outfalls in and near Boston Harbor. (Mencher, Copeland, and Payson, 1968) This mud is extremely soft and compressible and is generally incapable of supporting structures or landfill. (See p. 21 .) This mud often contains trapped gases, making it a reflector of seismic waves. This porperty makes some methods of seismic study impossible, notably the electronic pinger method developed by Dr. Harold Edgerton of M.I.T. (Payson 1963). The normal thickness of these mud deposits is less than 15 feet. However, in the Dorchester Bay area these mud deposits average about 20 feet in thickness (Barlow, 1966), and the thickest mud beds (40 feet) in the inner harbor are found in the Neponset River Channel, just west of Thompson Island.
From data obtained in the Boston Common area, Kaye (1961) concludes that four layers of glacial drift were deposited, separated by three layers of marine clay. He takes the bottom layer (Drift I) to be till of the Kansan or Nebraskan era, Drift II to be till, clay and gravel outwash of the Illinoian era, Drift III to be drumlin till of the Early Wisconsin era, and Drift IV to be outwash of the Late Wisconsin era. Using this information the following stratigraphic history has been inferred (Phipps, 1964). The initial advance of the ice sheet deposits the ground moraine. The ice sheet retreats and local outwash sand, gravel and clay are deposited and subsequently oxidized, followed by widespread deposition of clay. The ice sheet readvances, depositing till, and retreats, allowing another extensive layer of clay to be deposited. The final outwash layer is deposited, possibly by another advance of the ice sheet. The land is then elevated and eroded, bringing us to the present. The upper surface of the Boston Blue clay is highly irregular, showing a relief of over 100 feet within the harbor.
Hughes, D. J., Engineering Geology Of The Dorchester Bay Area Pertaining To Urban Development Of Thompson Island, MIT (1968).
Unfortunately, very little information is available concerning depth to bedrock within the harbor area. Upson and Spencer (1964) suggest the existence of several Pleistocene river valleys, or buried bedrock valleys, under the Dorchester Bay area. (see Figure 3.) This would place the bedrock surface 250 feet or more below sea level in parts of Dorchester Bay, and Phipps (1964) cites a depth of 360 feet to bedrock under Spectacle Island. On the other hand, outcrops of the Squantum formation exist above sea level in the Squantum Point area (Squaw Rocks and Chapel Rocks). Mr. Clifford Kaye of the U.S. Geological Survey, noted authority on the geology of Boston, explained that he personally places little confidence in the bedrock valley theory. He added that the bedrock is indeed quite deep in parts of the harbor, but the bedrock contours are too irregular and too uncertain to accurately predict the existence or the location of such bedrock valleys.
The most prominent sedimentary feature of the Boston Basin is the existence of a vast deposit of soft clay known as the Boston Blue clay. This blue clay is a still water deposit whose thickness ranges from 2 feet to over 200 feet. It is often separated from bedrock by thin layers of sand, gravel, or glacial till, and in some areas there are thin layers of sand, gravel, or silt within the clay. The upper layer of the blue clay has been oxidized to form a generally firmer yellowish clay, referred to by Boston engineers as "the stiff crust." This oxidation is believed to have taken place at a time when much of the harbor bottom was above sea level. There has since been a general subsidence of the entire Boston Basin area, with a subsequent rise in sea level. (Marmer, 1944). Above the yellow clay lies a thin (5 feet to 10 feet in most places) layer of organic silt covered by mud. This mud is of the black, carbonaceous variety. The high organic content of this mud is generally attributed to the many sewer outfalls in and near Boston Harbor. (Mencher, Copeland, and Payson, 1968) This mud is extremely soft and compressible and is generally incapable of supporting structures or landfill. (See p. 21 .) This mud often contains trapped gases, making it a reflector of seismic waves. This porperty makes some methods of seismic study impossible, notably the electronic pinger method developed by Dr. Harold Edgerton of M.I.T. (Payson 1963). The normal thickness of these mud deposits is less than 15 feet. However, in the Dorchester Bay area these mud deposits average about 20 feet in thickness (Barlow, 1966), and the thickest mud beds (40 feet) in the inner harbor are found in the Neponset River Channel, just west of Thompson Island.
From data obtained in the Boston Common area, Kaye (1961) concludes that four layers of glacial drift were deposited, separated by three layers of marine clay. He takes the bottom layer (Drift I) to be till of the Kansan or Nebraskan era, Drift II to be till, clay and gravel outwash of the Illinoian era, Drift III to be drumlin till of the Early Wisconsin era, and Drift IV to be outwash of the Late Wisconsin era. Using this information the following stratigraphic history has been inferred (Phipps, 1964). The initial advance of the ice sheet deposits the ground moraine. The ice sheet retreats and local outwash sand, gravel and clay are deposited and subsequently oxidized, followed by widespread deposition of clay. The ice sheet readvances, depositing till, and retreats, allowing another extensive layer of clay to be deposited. The final outwash layer is deposited, possibly by another advance of the ice sheet. The land is then elevated and eroded, bringing us to the present. The upper surface of the Boston Blue clay is highly irregular, showing a relief of over 100 feet within the harbor.
Hughes, D. J., Engineering Geology Of The Dorchester Bay Area Pertaining To Urban Development Of Thompson Island, MIT (1968).
Hughes, D. J., Engineering Geology Of The Dorchester Bay Area Pertaining To Urban Development Of Thompson Island, MIT (1968).
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Hughes, D. J., Engineering Geology Of The Dorchester Bay Area Pertaining To Urban Development Of Thompson Island, MIT (1968).
Hughes, D. J., Engineering Geology Of The Dorchester Bay Area Pertaining To Urban Development Of Thompson Island, MIT (1968).
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Hughes, D. J., Engineering Geology Of The Dorchester Bay Area Pertaining To Urban Development Of Thompson Island, MIT (1968).
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Hughes, D. J., Engineering Geology Of The Dorchester Bay Area Pertaining To Urban Development Of Thompson Island, MIT (1968).
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Marine Clay / Boston Blue Clay
"The outline of the Shawmut peninsular begins to emerge via the formation of a ridge of this marine clay through processes of both deposition and erosion. The Shawmut Neck was a narrow strip of land that formed the terminus of the Shawmut Peninsula. It was bordered on the west by the Roxbury Tidal Flats and on the east by South Boston Bay and was barely passable at low tide, making Boston essentially an island. The Neck ran along Orange Street (what is now Washington Street)."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
Marine Clay. "Fine sediment from an increasingly more distant ice front, coupled with a rising sea level that inundated much of the low-lying areas, produced a thick blanket of marine clay that smoothed out most of the relief across the underlying units. The infilling of topographic lows and channels around islands formed by the higher drumlins is seen in many cross-sections constructed from borehole data and offshore seismic profiles. The marine clay is found in broad channels under much of Massachusetts Bay, Boston Harbor and the rivers and surrounding lowlands that extend inland from the harbor. These areas include the old Mill Pond, Charles and Mystic rivers, the Back Bay and other former estuarine marshlands that are now filled. The marine clay deposit is commonly referred to as the "Boston Blue Clay" and is found throughout the low areas of Boston. The clay extends eastward beyond the harbor. It was mistakenly called the Leda Clay by Sears (1905) who observed it on the north side of the basin and northward, where shells confirmed its marine origin. The clay laps up onto the till of Beacon Hill and the drumlins of Charlestown. The outline of the Shawmut peninsular begins to emerge via the formation of a ridge of this marine clay through processes of both deposition and erosion.
The Shawmut Neck was a narrow strip of land that formed the terminus of the Shawmut Peninsula. It was bordered on the west by the Roxbury Tidal Flats and on the east by South Boston Bay and was barely passable at low tide, making Boston essentially an island. The Neck ran along Orange Street (what is now Washington Street). Yellow clay, possibly gladomarine, on the order of 3 meters (10 feet) thick was encountered during the drilling for the Wang Center on Tremont Street, which would represent the extreme western edge of the neck. Drilling on Washington Street between West Oak Street and Kneeland Street for a housing complex also found clay but only the thinner oxidized yellow crust (the marine day) commonly found around the Boston area. The day is typically light greenish-gray to medium-gray, rather than blue, and usually weathers yellowish in its upper portion. Blue color is seen in the clay under the Boston Company Building and Millennium Place, where deformation of the day was also observed. Throughout most of its thickness the clay is soft and plastic, but less so in the weathered zone and where it contains partings of fine sand, silt or sand lenses. Analyses show that the predominant day mineral is illite.
The clay is only slightly sensitive, with a natural water content of about 30 percent, but it also can be found in a sensitive state in certain areas, such as at Alewife Station. Where redeposited, the clay exhibits anisotropy. The contained lenses and layers of silt and sand increase with depth and in some places the clay grades downward into well stratified sand that becomes coarser with depth and finally grades into gravel (see Figure 3-98). Scattered through the clay are a few pebbles, cobbles and boulders (iceberg drop stones), which may reach several tons in weight. A very large erratic boulder is incorporated into its base in the Fort Point Channel at the MBTA Silver Line crossing (Leifer, 2006). The clay and interbedded silt and sand grade up the valley of the Charles River into littoral sand and, thence, into outwash consisting of sand and gravel deposited by rivers. Because the topographic trough provided by the Boston Basin was a major drainage way for glacial melt water from the west, Boston became the apex of a large, submarine clay delta of outwash origin. Besides the low tidal areas around the Shawmut Peninsula, the clay can also be found in the Squantum area of Quincy to the south; in Charlestown, Cambridge and Somerville to the north and west; the Fenway; Roxbury, South Boston and Dorchester to the south; and East Boston to the east.
The top 1 to 5 meters (3 to 16 feet) of the clay stratum became generally oxidized during a period when it was exposed; much of this area is currently below sea level in Boston Harbor. Borings indicate that where overlain by the Lexington outwash it is oxidized to a depth of I meter (3 feet) and to a maximum depth of 3 meters (10 feet) where exposed on the surface around the Back Bay and Cambridge. A stiff, yellow crust of subaerial origin causing oxidation was formed by the desiccation and resulting over-consolidation that has taken place. Significant over-consolidation is generally limited to the upper 5 meters (16.5 feet), although less over-consolidation can be found to depths of 10 to 12 meters (33 to 39 feet). At depths lower than 18 meters (60 feet), the clay becomes softer, gray and essentially normally consolidated. Where the oxidized clay was overlain by substantial organic matter (such as peat), a chemical reaction involving iron reduction- i.e., ferric iron, Fe+3, is changed to ferrous iron, Fe+2 by bacterial action (Kusel et al., 2008) - has created an upper zone of softened blue clay up to 2 meters (6.5 feet) thick. This zone, representing the top of the clay stratum, may also be silty or sandy and somewhat water bearing. The upper 2 to 5 meters (6.5 to 16 feet) of the clay is structurally disturbed and may be broken, folded and badly contorted in places in the Back Bay and parts of Boston Harbor. The upper clay also is seen in excavations to have prismatic structure or cubical jointing and fissuring, which appears to be evidence of having been frozen, probably at the time of the overlying upper outwash.
The clay may overlie any of the earlier surficial deposits or bedrock because of preceding erosion, and it is normally overlain by the Lexington outwash, Holocene organic deposits or fill. The unit is usually well-bedded with thin horizontal layers. However, both offshore and onshore profiles show it locally conforms to, or is draped over, the surface of the basement on which it was deposited, which imparts a folded appearance to the clay. There are deep funnel-shaped "downfolds" on the order of 100 to 200 meters (330 to 660 feet) across The topographic trough provided by the Boston Basin was a major drainageway for glacial melt water from the west and Boston became the apex of a large, submarine clay 'delta of outwash origin. The marine clay, because it often contains more silt than clay, has lower plastic and liquid limits of its claysized material than would mineral clay. The marine clay represents a high percentage of fine-grained rock flour component of outwash that was carried farther and deposited in coastal marine waters. Miller (2010) considers a significant amount is reworked clay eroded during the Lexington Substage from the older day-rich glaciomarine deposit based on evidence that indicates that the deposit was exposed at places during the time of Lexington day deposition. The occasional glacial gravel to boulder clasts in the clay are apparently iceberg dropstones, perhaps rafted when the ice front was close to Boston. The day is thickest in the lower valley of the Charles River and Boston Harbor, where it wrapped around Beacon Hill, and thins under Massachusetts Bay to the north, east and south. In the Back Bay, as well as along marginal waterfront areas, the clay is typically 15 to 38 meters (50 to 125 feet) thick.
Up the Charles River at the Mount Auburn Cemetery the clay is 25 meters (81 feet) thick near the present river and eroded to zero at
short distances to the north beneath the upper outwash. Even greater thicknesses - up to 60 meters (200 feet) - are found west of Massachusetts Avenue and in Cambridge, where parts of Harvard University and MIT rest on it. Known clay thicknesses, as great as 75 meters (246 feet), occur in the Charles River area. The top of the clay was deeply eroded by the ancestral Charles River and its tributaries during a drop in sea level associated with the Lexington Substage subsequent to 12,600 years ago and the contoured surface (Judson, 1949) shows a well developed stream pattern. The main channel is closely aligned with the present Charles River, a second channel enters from the north between Charlestown and East Boston, and a third exists beneath Fort Point Channel. The surface of the clay is now generally below sea level around the Shawmut Peninsula and is estimated to descend eastward to nearly elevation -60 meters (-200 feet) MSL by Judson (1949). Judson (1949) also reported it rising to about elevation 9 meters (30 feet) MSL on the north and east sides of Beacon Hill. Kaye (1961) only found it reaching to thickness of about 4.6 to 7.6 meters (15 to 2~ feet) over the upper till on the south side, which would be a Boston area local limit since the upper limit of the marine clay varies across New England because the post-glacial rebound has raised the land progressively higher to the north. At the northwest edge of the Boston area, clay has been found as high as elevation 22 meters (72 feet) MSL (Woodworth, 1897; Chute, 1959; Colgan & Rosen, 2001), but these deposits are apparently disturbed or from local ponding. I.B. Crosby (1934) found the highest known marine glacial clay in the environs of Boston (except for small deposits that obviously. formed in glacial lakes) at elevation 7.6 or 10.7 meters (25 or 35 feet) MSL."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
The Shawmut Neck was a narrow strip of land that formed the terminus of the Shawmut Peninsula. It was bordered on the west by the Roxbury Tidal Flats and on the east by South Boston Bay and was barely passable at low tide, making Boston essentially an island. The Neck ran along Orange Street (what is now Washington Street). Yellow clay, possibly gladomarine, on the order of 3 meters (10 feet) thick was encountered during the drilling for the Wang Center on Tremont Street, which would represent the extreme western edge of the neck. Drilling on Washington Street between West Oak Street and Kneeland Street for a housing complex also found clay but only the thinner oxidized yellow crust (the marine day) commonly found around the Boston area. The day is typically light greenish-gray to medium-gray, rather than blue, and usually weathers yellowish in its upper portion. Blue color is seen in the clay under the Boston Company Building and Millennium Place, where deformation of the day was also observed. Throughout most of its thickness the clay is soft and plastic, but less so in the weathered zone and where it contains partings of fine sand, silt or sand lenses. Analyses show that the predominant day mineral is illite.
The clay is only slightly sensitive, with a natural water content of about 30 percent, but it also can be found in a sensitive state in certain areas, such as at Alewife Station. Where redeposited, the clay exhibits anisotropy. The contained lenses and layers of silt and sand increase with depth and in some places the clay grades downward into well stratified sand that becomes coarser with depth and finally grades into gravel (see Figure 3-98). Scattered through the clay are a few pebbles, cobbles and boulders (iceberg drop stones), which may reach several tons in weight. A very large erratic boulder is incorporated into its base in the Fort Point Channel at the MBTA Silver Line crossing (Leifer, 2006). The clay and interbedded silt and sand grade up the valley of the Charles River into littoral sand and, thence, into outwash consisting of sand and gravel deposited by rivers. Because the topographic trough provided by the Boston Basin was a major drainage way for glacial melt water from the west, Boston became the apex of a large, submarine clay delta of outwash origin. Besides the low tidal areas around the Shawmut Peninsula, the clay can also be found in the Squantum area of Quincy to the south; in Charlestown, Cambridge and Somerville to the north and west; the Fenway; Roxbury, South Boston and Dorchester to the south; and East Boston to the east.
The top 1 to 5 meters (3 to 16 feet) of the clay stratum became generally oxidized during a period when it was exposed; much of this area is currently below sea level in Boston Harbor. Borings indicate that where overlain by the Lexington outwash it is oxidized to a depth of I meter (3 feet) and to a maximum depth of 3 meters (10 feet) where exposed on the surface around the Back Bay and Cambridge. A stiff, yellow crust of subaerial origin causing oxidation was formed by the desiccation and resulting over-consolidation that has taken place. Significant over-consolidation is generally limited to the upper 5 meters (16.5 feet), although less over-consolidation can be found to depths of 10 to 12 meters (33 to 39 feet). At depths lower than 18 meters (60 feet), the clay becomes softer, gray and essentially normally consolidated. Where the oxidized clay was overlain by substantial organic matter (such as peat), a chemical reaction involving iron reduction- i.e., ferric iron, Fe+3, is changed to ferrous iron, Fe+2 by bacterial action (Kusel et al., 2008) - has created an upper zone of softened blue clay up to 2 meters (6.5 feet) thick. This zone, representing the top of the clay stratum, may also be silty or sandy and somewhat water bearing. The upper 2 to 5 meters (6.5 to 16 feet) of the clay is structurally disturbed and may be broken, folded and badly contorted in places in the Back Bay and parts of Boston Harbor. The upper clay also is seen in excavations to have prismatic structure or cubical jointing and fissuring, which appears to be evidence of having been frozen, probably at the time of the overlying upper outwash.
The clay may overlie any of the earlier surficial deposits or bedrock because of preceding erosion, and it is normally overlain by the Lexington outwash, Holocene organic deposits or fill. The unit is usually well-bedded with thin horizontal layers. However, both offshore and onshore profiles show it locally conforms to, or is draped over, the surface of the basement on which it was deposited, which imparts a folded appearance to the clay. There are deep funnel-shaped "downfolds" on the order of 100 to 200 meters (330 to 660 feet) across The topographic trough provided by the Boston Basin was a major drainageway for glacial melt water from the west and Boston became the apex of a large, submarine clay 'delta of outwash origin. The marine clay, because it often contains more silt than clay, has lower plastic and liquid limits of its claysized material than would mineral clay. The marine clay represents a high percentage of fine-grained rock flour component of outwash that was carried farther and deposited in coastal marine waters. Miller (2010) considers a significant amount is reworked clay eroded during the Lexington Substage from the older day-rich glaciomarine deposit based on evidence that indicates that the deposit was exposed at places during the time of Lexington day deposition. The occasional glacial gravel to boulder clasts in the clay are apparently iceberg dropstones, perhaps rafted when the ice front was close to Boston. The day is thickest in the lower valley of the Charles River and Boston Harbor, where it wrapped around Beacon Hill, and thins under Massachusetts Bay to the north, east and south. In the Back Bay, as well as along marginal waterfront areas, the clay is typically 15 to 38 meters (50 to 125 feet) thick.
Up the Charles River at the Mount Auburn Cemetery the clay is 25 meters (81 feet) thick near the present river and eroded to zero at
short distances to the north beneath the upper outwash. Even greater thicknesses - up to 60 meters (200 feet) - are found west of Massachusetts Avenue and in Cambridge, where parts of Harvard University and MIT rest on it. Known clay thicknesses, as great as 75 meters (246 feet), occur in the Charles River area. The top of the clay was deeply eroded by the ancestral Charles River and its tributaries during a drop in sea level associated with the Lexington Substage subsequent to 12,600 years ago and the contoured surface (Judson, 1949) shows a well developed stream pattern. The main channel is closely aligned with the present Charles River, a second channel enters from the north between Charlestown and East Boston, and a third exists beneath Fort Point Channel. The surface of the clay is now generally below sea level around the Shawmut Peninsula and is estimated to descend eastward to nearly elevation -60 meters (-200 feet) MSL by Judson (1949). Judson (1949) also reported it rising to about elevation 9 meters (30 feet) MSL on the north and east sides of Beacon Hill. Kaye (1961) only found it reaching to thickness of about 4.6 to 7.6 meters (15 to 2~ feet) over the upper till on the south side, which would be a Boston area local limit since the upper limit of the marine clay varies across New England because the post-glacial rebound has raised the land progressively higher to the north. At the northwest edge of the Boston area, clay has been found as high as elevation 22 meters (72 feet) MSL (Woodworth, 1897; Chute, 1959; Colgan & Rosen, 2001), but these deposits are apparently disturbed or from local ponding. I.B. Crosby (1934) found the highest known marine glacial clay in the environs of Boston (except for small deposits that obviously. formed in glacial lakes) at elevation 7.6 or 10.7 meters (25 or 35 feet) MSL."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
| Geology of the Boston Basin cep-vol-26-27-05.pdf |
Boston Main Relief Drainage Sewer was built 1950, 3.8 km long, lined diameter of 3.1m, and about 70-80m deep. Built through clay.
Main Drainage Tunnel sewer was built 1954-1959, 11.5 km long, lined diameter of 3.1-3.5m, and about 89-92.5m deep. Built through clay.
Conveyance pipelines and tunnels have been the backbone of sewage collection and treatment in Boston since its initial conception in the late 1840s. Early in its history, Boston managed its sewage by using small pipes to discharge sewage at the nearest shore.
Barosh & Woodhouse, Boston Area Water Supply & Wastewater Tunnels, Civil Engineering Practice (2011/2012)
Main Drainage Tunnel sewer was built 1954-1959, 11.5 km long, lined diameter of 3.1-3.5m, and about 89-92.5m deep. Built through clay.
Conveyance pipelines and tunnels have been the backbone of sewage collection and treatment in Boston since its initial conception in the late 1840s. Early in its history, Boston managed its sewage by using small pipes to discharge sewage at the nearest shore.
Barosh & Woodhouse, Boston Area Water Supply & Wastewater Tunnels, Civil Engineering Practice (2011/2012)
The Cambridge Argillite is a shale, locally and weakly metamorphosed, and occasionally encountered with reworked tuffaceous material. It is generally hard and competent due to its poorly developed bedding planes and general lack of fissility. However, localized zones exist where alteration of the bedrock has produced zones of varying widths of kaolin, a "soil-like" material composed essentially of the clay mineral kaolinite. The abrupt and unpredictable change from the sound Argillite to the kaolinitic weak Argillite occurs in very short distances. To further complicate the situation, preglacial surficial weathering has created a variable
thickness of overlying weathered rock that appears to have characteristics similar to those of the kaolinized zones. Kaye's (1967) study of thin-sections from the altered zone revealed that the commonly present rock minerals, including quartz, have been replaced by sericite and kaolinite at varying levels. This observation led to the possible explanation that the younger igneous intrusions have hydrothermally altered the adjacent weaker rocks and created the zones of highly decomposed kaolinite-rich, clay-like "soil" adjacent and running parallel to the intrusions. The erratic occurrence of the weathered and altered zones, in conjunction with the steeply dipping bedding planes typical of the Argillite, causes difficulty in characterizing the bedrock for engineering purposes. Without test borings at each caisson location, it is difficult to predict the quality of the rock within which a foundation unit will bear (Figure 2 ) .
Bedrock characterization and design considerations for rock socketed caissons in the Greater Boston area Caractérisation du substratum rocheux et conception de caissons foncés dans la roche dans la région de Boston L. M. Brown, C. Soydemir, S.T. Parkhill, M. J. Lally & A. D. Smith - Haley & Aldrich, Inc., Cambridge, Mass., USA
thickness of overlying weathered rock that appears to have characteristics similar to those of the kaolinized zones. Kaye's (1967) study of thin-sections from the altered zone revealed that the commonly present rock minerals, including quartz, have been replaced by sericite and kaolinite at varying levels. This observation led to the possible explanation that the younger igneous intrusions have hydrothermally altered the adjacent weaker rocks and created the zones of highly decomposed kaolinite-rich, clay-like "soil" adjacent and running parallel to the intrusions. The erratic occurrence of the weathered and altered zones, in conjunction with the steeply dipping bedding planes typical of the Argillite, causes difficulty in characterizing the bedrock for engineering purposes. Without test borings at each caisson location, it is difficult to predict the quality of the rock within which a foundation unit will bear (Figure 2 ) .
Bedrock characterization and design considerations for rock socketed caissons in the Greater Boston area Caractérisation du substratum rocheux et conception de caissons foncés dans la roche dans la région de Boston L. M. Brown, C. Soydemir, S.T. Parkhill, M. J. Lally & A. D. Smith - Haley & Aldrich, Inc., Cambridge, Mass., USA
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MA DEP directly compared Boston Blue clay to SF and ‘San Francisco Bay Mud.” Also noted deposits exist in Chicago as well. Found in swamps and coastal regions. “Its all over Boston” says James Elliott of the DPW env div. Dep usually 20ft below surface and some up to 200ft thick. Researchers say the clay can make highways sag, buildings settle, and tunnels collapse.”
The Boston Globe Sun, Sep 29, 1974 ·Page 42
MA DEP and Boston want to give out blue clay for everyone to cap all the landfills and put most of it on Spectacle Island. US EPA says no don’t do that.
The Boston Globe, Fri, Dec 15, 1989 ·Page 80
In 1990s learned that Boston Blue Clay is more sensitive to heavy loads then previously thought. MIT.
The Boston Globe, Wed, Feb 17, 1993
The Clays of the Boston Basin, Robert Marshall Browns, Art. XLIII, American Journal of Science, Vol. s4-14, Issue 84, 1902, https://doi.org/10.2475/ajs.s4-14.84.445
This deposit is widely found in eastern MA, the greater Boston region, and southern New Hampshire, is locally known as Boston Blue Clay (BBC), and has a thickness that can vary from a few meters up to 60 meters. BBC has undergone extensive desiccation due to freezing, ground water table fluctuations, possible erosion, and anthropogenic activities. As a result, the deposit often has a stiff overconsolidated crust below which is soft, low overconsolidation BBC. Significant variations in thickness of the crust and the overall deposit are due to the complex depositional environment and subsequent geologic history in the region.
DeGroot, et al, Geology and engineering properties of sensitive Boston Blue Clay at Newbury, Massachusetts, AIMS Geosciences, 5(3): 412–447 (2019).
Organic sediment (silt, Peat, fossils)
It is important to note that assessment/investigation activities conducted by KEY for a real estate transaction discovered soils to be mostly fill overlaying peat at the Property. Therefore, KEY believes the Property was once a wetland that was filled. A short review of other sites in the area of the Property also indicates the same. The fill at the Property consists of mostly urban gray (i.e., urban soil typically found in the Boston areas) with coal ash (i.e., artifact of habitation and is due to the widespread practice of emptying
fireplaces, stoves, boilers, garage, etc. in urban areas over the past several hundred years) and some solid waste such as glass, wood, metal, tires, etc (i.e., also related to artifact of habitation over the past several hundred years). See photographs in Appendix A that show some of the soils at the Property. However, the source of the urban fill is unknown.
Class B-1 Response Action Outcome Statement, Commercial/Industrial Property, 25 Chesterton Street, Roxbury. MA 02119 (Feb. 2005).
fireplaces, stoves, boilers, garage, etc. in urban areas over the past several hundred years) and some solid waste such as glass, wood, metal, tires, etc (i.e., also related to artifact of habitation over the past several hundred years). See photographs in Appendix A that show some of the soils at the Property. However, the source of the urban fill is unknown.
Class B-1 Response Action Outcome Statement, Commercial/Industrial Property, 25 Chesterton Street, Roxbury. MA 02119 (Feb. 2005).
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"Organic Sediment. The organic silt and clay, and peat deposits that were laid down throughout much of the lower lying areas surrounding the Shawmut Peninsula following glaciation vary greatly in overall thickness and content, but are generally from 1.5 to 7.5 meters (5 to 25 feet) thick. In those filled-in areas of the Back Ba:» the layer has been compressed considerably due to the weight of the fill. Marsh gas that results from the decomposing organic matter is sometimes encountered in excavations."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geotechnical Factors in Boston, Civil Engineering Practice, Vol. 26-27 (2011/2012).
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geotechnical Factors in Boston, Civil Engineering Practice, Vol. 26-27 (2011/2012).
Organic Silt & Peat. "An organic deposit consisting of both fresh water and salt water peat, and organic silt and clay, is found in the present and former tidal flats, estuaries and coastal lowlands that flourished along the margins of what was then the Back Bay and the Mystic River, that were at that time more restricted than today. These marsh and tidal deposits gradually extended onshore and thickened as the sea level rose following the Lexington Substage... The deposit ranges from a feather edge up to a total thickness of 12 meters (40 feet) in the channels. The organic silt usually overlies marine clay, but silt also is found above outwash sand and, in some cases, till. A recent saltwater or brackish estuary peat, usually less than 1.5 meters (5 feet) thick, is found locally on top of the organic silt, as well as the extensive fill placed around the city. The thickness and nature of the organic deposit is determined from the many test borings made for the numerous structures built on, or over, the wetlands and from the many foundation caissons that were augured through it and belled at the top of the underlying marine clay... Dark-gray to black organic estuarine and marine silt with low plasticity and organic clay and fine sand were deposited on top of a basal fresh water peat layer and usually beneath a capping peat... The organic sediment, which smells of hydrogen sulfide (H2S); also contains methane, and is highly fossiliferous with plant fibers and traces of wood as well as remnants of oyster banks predominantly composed of whole or parts of shells belonging to the species Venus mercenarius. Some oyster shells reach 25 centimeters (10 inches) in length and 1 kilogram (2.25 pounds) in weight (Boston Transit Commission, 1913). The stratum rests on the marine clay that generally has a thin, 0 to 1 meter (0 to 3 foot) cover of rusty upper outwash sand that supports tree stumps locally...Dark gray to black generally fibrous peat, which ranges in thick-ness from less than 0.3 to 1.5 meters (1 to 5 feet), is made up of decaying plants and wood formed over the clay and outwash and some times interfingers with dark silt and silty sand of the channels in a complex manner.... The continued flooding of the estuary resulted in the peat being rapidly buried by bay mud that was rich in marine life."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
"Marine mollusks, starfish, foraminifera, sponge spicules, echinoid spines and some diatoms occur in the clay at West Lynn at the north edge of the Boston Basin (Sears, 1905; Nichols, 1946; Kaye, 1961). Mollusks, barnacles, foraminifera and ostracodes also are found nearby in clay in Lynn, Revere and Winthrop, and have yielded a mean radiocarbon age of 14,000 years ago (Colgan & Rosen, 2001). Some sparse foraminifera occur in the Back Bay (Stetson & Parker, 1942), along with a few barnacles such as Balanus hameri (Ascanius). The latter yields radiocarbon dates that range from 13,230 (±320) to 14,420 (±300) years ago and average about 14,000 years ~go (Kaye & Barghoom, 1964; Kaye, 1976a). However, locally the clay at Lynn and in other pits bordering Boston, as well as in the Back Bay, is devoid of diatoms and foraminifera (Conger, 1949; Phleger, 1949). Beaver-cut twigs and peat embedded in the upper part of the clay in the Boston Common yield two dates that average 12,200 years ago (Kaye, 1972 & 1976a). The clay also is older than the Lexington outwash sand of circa 12,000 ago that overlies it. The general lack of fossils, especially the microfossils, probably reflects the low salinity, excessive turbidity of the water causing diminishing light, and rapid deposition at the mouth of a major outwash river (Phleger, 1949; Conger, 1949). The clay thus appears to represent a Woodfordian, early Late Wisconsin, marine inundation, which is indicated to have occurred under cold conditions by the mollusks. The most abundant shell found by Sears (1905) was Yoldia arctica (Portlandia Arctica) which now lives in the Arctic at depths of 1 to 60 meters (3 to 197 feet). The spruce pollen that is very abundant in the upper 3 meters (10 feet) of the clay at West Lynn also supports a cold or periglacial depositional environment during its deposition (E.B. Leopold, in Kaye, 1961). The correlation by Sears (1905) with the Leda Clay is a misnomer since the Leda is the quick (sensitive) clay found in certain areas of Canada. The Presumpscot Clay in Maine, however, has a much greater clay particle content than the Boston clay (i.e., less silt), and has been leached of its depositional high salt and is unrelated. The Boston marine clay, however, does correlate northward of the basin with some older clay that has been grouped with the generally younger Presumpscot Formation, also known as the Maine "Blue Clay" that formed after the later Lexington Substage."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
The origin of glacial deposits was a mystery to early geologists and one geologist even considered them debris from a comet impact (Note: a version of the comet theory has been resurrected by West et al. [2006] and several colleagues who believe a comet or asteroid exploded over central Canada 12,900 years· ago to trigger the last, Younger-Dryas, glacial event, which is known in Boston as the Lexington Substage; others place the impact in Chesapeake Bay or Georgia.) However, great strides in describing the glacial deposits in eastern Massachusetts (and unraveling the events that produced them) were made quickly after the mid-nineteenth century. This unraveling occurred after Louis Agassiz arrived from Europe in 1846 with his knowledge of Swiss glacial deposits and theories on continental glaciation.
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012). (Donnelly, 1883).
NASA Earth Observatory has unveiled a new image of Boston Harbor's drumlin islands, highlighting these rare geological formations—one of only three worldwide—that were carved out by glaciers more than 20,000 years ago. NASA... explains that during the Wisconsin Glaciation—which began between 100,000–75,000 years ago and ended about 11,000 years ago—a massive ice sheet, more than one mile thick in places, entirely covered the land that is now occupied by the small islands. As the icy coat melted away, it left behind piles of sediment and glacial debris in hundreds of elongated, streamlined hills known as drumlins, which were later partially submerged by rising sea levels and turned into tiny islands. Today, several of these drumlin islands make up part of the Boston Harbor Islands National and State Park, which works to preserve many distinctive geological, historical, and natural resources in the area. According to NASA, these little glacial islands are the only partially submerged drumlin field in North America and one of only three in the world. A similar example can be spotted in Clew Bay, Ireland.
Newsweek, NASA Image Reveals Boston Feature That’s ‘One of Only Three in the World’, April 23 2025, https://www.newsweek.com/nasa-image-boston-harbor-drumlin-islands-2063016
NASA, The Drumlin Islands of Boston Harbor, 2025, https://science.nasa.gov/earth/earth-observatory/the-drumlin-islands-of-boston-harbor-154204/
"In fact, drumlin researcher Rebecca Lee says that no one knows exactly how drumlins form. Competing theories include formation by water or ice flow, or some combination of the two. A more complete understanding of how drumlins form could help researchers determine past ice conditions, such as how fast the ice was moving. It could also help them to interpret glacial deposits and where they came from."
NASA, Drumlin Field in Northern Canada, 2015, https://science.nasa.gov/earth/earth-observatory/drumlin-field-in-northern-canada-85506/
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012).
The origin of glacial deposits was a mystery to early geologists and one geologist even considered them debris from a comet impact (Note: a version of the comet theory has been resurrected by West et al. [2006] and several colleagues who believe a comet or asteroid exploded over central Canada 12,900 years· ago to trigger the last, Younger-Dryas, glacial event, which is known in Boston as the Lexington Substage; others place the impact in Chesapeake Bay or Georgia.) However, great strides in describing the glacial deposits in eastern Massachusetts (and unraveling the events that produced them) were made quickly after the mid-nineteenth century. This unraveling occurred after Louis Agassiz arrived from Europe in 1846 with his knowledge of Swiss glacial deposits and theories on continental glaciation.
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Geology of the Boston Basin, Civil Engineering Practice, Vol. 26-27 (2011/2012). (Donnelly, 1883).
NASA Earth Observatory has unveiled a new image of Boston Harbor's drumlin islands, highlighting these rare geological formations—one of only three worldwide—that were carved out by glaciers more than 20,000 years ago. NASA... explains that during the Wisconsin Glaciation—which began between 100,000–75,000 years ago and ended about 11,000 years ago—a massive ice sheet, more than one mile thick in places, entirely covered the land that is now occupied by the small islands. As the icy coat melted away, it left behind piles of sediment and glacial debris in hundreds of elongated, streamlined hills known as drumlins, which were later partially submerged by rising sea levels and turned into tiny islands. Today, several of these drumlin islands make up part of the Boston Harbor Islands National and State Park, which works to preserve many distinctive geological, historical, and natural resources in the area. According to NASA, these little glacial islands are the only partially submerged drumlin field in North America and one of only three in the world. A similar example can be spotted in Clew Bay, Ireland.
Newsweek, NASA Image Reveals Boston Feature That’s ‘One of Only Three in the World’, April 23 2025, https://www.newsweek.com/nasa-image-boston-harbor-drumlin-islands-2063016
NASA, The Drumlin Islands of Boston Harbor, 2025, https://science.nasa.gov/earth/earth-observatory/the-drumlin-islands-of-boston-harbor-154204/
"In fact, drumlin researcher Rebecca Lee says that no one knows exactly how drumlins form. Competing theories include formation by water or ice flow, or some combination of the two. A more complete understanding of how drumlins form could help researchers determine past ice conditions, such as how fast the ice was moving. It could also help them to interpret glacial deposits and where they came from."
NASA, Drumlin Field in Northern Canada, 2015, https://science.nasa.gov/earth/earth-observatory/drumlin-field-in-northern-canada-85506/
Boston_Basin_Map_Billings
"At the present time, the mud line varies from El. to El. -20 at the proposed location of the culvert while the depth of organic silt and fill is between 5 and 30 feet. Underlying the organic silt is a deep stratum overlay varying from stiff yellow clay toward the top surface to soft blue clay at greater depth. Thickness of the clay stratum varies between 40 and 100 ft. Below the clay are thin layers of pervious sands and glacial till, (hardpan) over a blue slaty shale.
The 1950 report by the Port of Boston Authority proposed that fill for the channel be obtained by hydraulic means from the Old Harbor area. Since the size of the area to be filled has been greatly reduced, fill to be placed to Dorchester Avenue only, and since considerable filling has occurred since 1950, it is believed that continued filling by conventional methods is indicated. It is estimated that 700,000 cu. yds. of fill are required to bring the Channel to approximately Elevation 10. Few limitations need to be placed on the type of earth fill used in the Channel once the culvert is constructed.
Foundations for all but the lightest temporary structures which may be constructed in the future will need to be founded on piles where earth fill overlies organic silt. It is considered important to exclude rock, boulders, granite blocks, brick walls, concrete and other hard materials from fill in order to minimize future pile driving and excavation problems. It will be desirable to place a minimum of 2 to 3 ft. of granular soil over the filled area, especially where organic soils are dumped, to provide a sanitary, stable, free draining surface. A careful survey of soil conditions throughout the area prior filling combined with restricted areas for various types of fill may be a promising way of providing a few desirable building sites. For example, if compacted granular fill were placed in areas where little or no organic silt and miscellaneous fill occurs, permanent one and two story structures could probably be built without piling.
Over a period of years the fill placed in the Channel is expected to subside as a result of compression from these sources: 1. Compression of fill material itself. 2. Compression of undisturbed soft organic silt. 3. Compression of deep stratum of "Boston blue clay".
Magnitudes of settlements estimated below are based largely on studies for the Dover Street embankment and from recent settlement observations on Expressway fills near the Fort Point Channel. Compression of earth fill will vary considerably, dependent largely on the type of fill. Where appreciable quantities of organic silt have already been placed, compression could exceed 1 ft. However, where granular fill is used, settlement will be small and will occur primarily during filling. Soft organic silt which has accumulated over a long period of time largely from natural causes will compress from 5 to 10 percent of its thickness under the weight of 20 ft. of fill, (to El. + 10.) Somewhat more than half this compression will occur during filling and within a year thereafter. Settlement of the area from compression in the underlying clay stratum will be small by comparison. Furthermore, the compress will be relatively uniform with gradual transitions as the thickness of clay and depth of fill vary. Maximum compression is not expected to exceed 8 inches over a 20 year period of which half will occur in 3 to 4 years. For purposes of estimating the quantity of fill required to bring the Channel to El. 10.0, an average compression of 1 ft. has been assumed for the area to be filled. Total settlement of the Fort Point Channel area following filling will be relatively non-uniform varying from 6 inches to over 2 ft."
Fort Point Channel and South Bay, Report of the Special Commission Relative to Filling and Improving South Bay and Part of Fort Point Channel in the City of Boston, Senate — No. 498 (1959).
The 1950 report by the Port of Boston Authority proposed that fill for the channel be obtained by hydraulic means from the Old Harbor area. Since the size of the area to be filled has been greatly reduced, fill to be placed to Dorchester Avenue only, and since considerable filling has occurred since 1950, it is believed that continued filling by conventional methods is indicated. It is estimated that 700,000 cu. yds. of fill are required to bring the Channel to approximately Elevation 10. Few limitations need to be placed on the type of earth fill used in the Channel once the culvert is constructed.
Foundations for all but the lightest temporary structures which may be constructed in the future will need to be founded on piles where earth fill overlies organic silt. It is considered important to exclude rock, boulders, granite blocks, brick walls, concrete and other hard materials from fill in order to minimize future pile driving and excavation problems. It will be desirable to place a minimum of 2 to 3 ft. of granular soil over the filled area, especially where organic soils are dumped, to provide a sanitary, stable, free draining surface. A careful survey of soil conditions throughout the area prior filling combined with restricted areas for various types of fill may be a promising way of providing a few desirable building sites. For example, if compacted granular fill were placed in areas where little or no organic silt and miscellaneous fill occurs, permanent one and two story structures could probably be built without piling.
Over a period of years the fill placed in the Channel is expected to subside as a result of compression from these sources: 1. Compression of fill material itself. 2. Compression of undisturbed soft organic silt. 3. Compression of deep stratum of "Boston blue clay".
Magnitudes of settlements estimated below are based largely on studies for the Dover Street embankment and from recent settlement observations on Expressway fills near the Fort Point Channel. Compression of earth fill will vary considerably, dependent largely on the type of fill. Where appreciable quantities of organic silt have already been placed, compression could exceed 1 ft. However, where granular fill is used, settlement will be small and will occur primarily during filling. Soft organic silt which has accumulated over a long period of time largely from natural causes will compress from 5 to 10 percent of its thickness under the weight of 20 ft. of fill, (to El. + 10.) Somewhat more than half this compression will occur during filling and within a year thereafter. Settlement of the area from compression in the underlying clay stratum will be small by comparison. Furthermore, the compress will be relatively uniform with gradual transitions as the thickness of clay and depth of fill vary. Maximum compression is not expected to exceed 8 inches over a 20 year period of which half will occur in 3 to 4 years. For purposes of estimating the quantity of fill required to bring the Channel to El. 10.0, an average compression of 1 ft. has been assumed for the area to be filled. Total settlement of the Fort Point Channel area following filling will be relatively non-uniform varying from 6 inches to over 2 ft."
Fort Point Channel and South Bay, Report of the Special Commission Relative to Filling and Improving South Bay and Part of Fort Point Channel in the City of Boston, Senate — No. 498 (1959).
The factors next in importance to glaciation and the kind of surface rock in forming deposits of peat are wave and stream action and coastal subsidence. Many peat deposits of salt-marsh and freshwater origin are seen in drowned valleys, where the coast has subsided and landlocked lagoons or deltas have been formed, and in flat, imperfectly drained areas farther inland. In some places saltmarsh peat overlies peat of fresh-water origin, indicating coastal subsidence.
One of the chief substances formed by plants during their growth is cellulose (C72H120060 ), which consists of carbon, hydrogen, and oxygen. These constituents are absorbed by the leaves from the atmosphere and by the roots from the soil. Cellulose, because of its complex composition, is an unstable compound and when attacked by fungi and bacteria decomposes rapidly. If at the end of the
growing season the plant debris falls upon drained soil it is vigorously attacked by these microorganisms, and the carbon and hydrogen of the cellulose unite with the atmospheric oxygen and with each other, forming carbon dioxide, water, and marsh gas. In other words, if oxidation is unhampered, the organic matter will disappear in a relatively short time. If, however, the plant matter falls into water or upon soil saturated with moisture, it undergoes a change different from the decay suffered by exposed vegetation. The atmospheric oxygen is largely excluded, and as the activity of fungi and bacteria is controlled by the supply of air, upon which they depend for their existence, decay is slow, the plant debris becomes buried, and a large proportion of the fixed carbon is retained. The salient features in the production of peat (C62H72024 ) from cellulose (C72H120O60 ) are the elimination of hydrogen and oxygen as water (H20) and of carbon and oxygen as carbon dioxide (C02 ) and the generation of methane (CH4 ). This is the process of carbonization.
If the surface conditions are unchanged, carbonization is largely arrested with the formation of peat, and the accumulation of organic matter may exist indefinitely as peat, unless the land is drained and decomposition begins again or unless the peat is deeply buried beneath superposed deposits, generally muds, sands, limestone, and other sedimentary beds, and subjected to pressure, accompanied by heat. Lignite, bituminous coal, anthracite, and graphite are succeeding stages in the process of carbonization of the buried vegetable debris. Most coals were once peats; most coal fields were formerly swamps, and the formation of peat in the bogs and swamps of this country to-day is an example of the first stage in the process of coal formation.
Salt-marsh peat. Salt-marsh peat, though formed in practically the same manner as fresh-water peat, differs from it somewhat in character. Few seed plants tolerate salt water, and the number of plant varieties found in salt marshes is therefore rather small. The most common types are salt-marsh grasses, rushes, and sedges. The entire vegetation of some of the New England salt marshes consists of one dominant and two or three subordinate species. In some of the coastal marshes of New England salt-marsh peat is underlain by peat of fresh-water origin, indicating the subsidence of that part of the Atlantic coast.
Native peat consists of partly decayed vegetable matter, inorganic minerals, and water in varying proportions, the usual ratio being 10 per cent of solid matter to 90 per cent of water. In specific gravity it ranges from 0.1 to 1.06 and in weight from 7 to 65 pounds per cubic foot. Aside from its high water content, peat is extremely variable, and scarcely any two deposits contain material that is exactly similar in physical properties. This diversity is due to many causes, the most notable of which are the variety of plants from which the peat was formed, and differences in climate, in the ages of the deposits, in water level, and in the quantity of sediment deposited during the accumulation of the peat.
Peat ranges in color from light yellow through various shades of brown to jet black, the color representing in a measure the degree of decomposition. Peat that is new or that has been well protected from the air is usually light yellow or brown; well-decomposed humified peat is jet black. Green peat, produced by the decomposition of algae and related aquatic plants, is found at the bottom of some filled-basin deposits.
The affinity of peat for moisture is proverbial. In fact, as previously explained, peat can not form unless the plant debris is saturated or covered with water. The peat in most deposits contains about 90 per cent of moisture, which is held both mechanically and chemically in the plant cells and intercellular spaces. In other words, a short ton of typical raw peat consists of about 200 pounds of solid matter to 1,800 pounds of water.
The affinity of peat for moisture is proverbial. In fact, as previously explained, peat can not form unless the plant debris is saturated or covered with water. A detailed study of the chemical properties of more than 500 samples of peat taken from deposits in different parts of the peat regions of this country leads to the following conclusions: Peat consists of carbon, hydrogen, oxygen, and relatively small quantities of nitrogen.
The ash in native peat, which renders it more or less impure, constitutes from 3 to 30 per cent of its dry weight and is traceable either to the plant cells or to the mineral matter carried in suspension or solution by the water in which the peat formed. The inorganic impurities of peat consist of silica, alumina, iron oxide, magnesia, lime, soda, potash, sulphuric acid, chlorine, and phosphoric acid. If the ash content exceeds 8 per cent, it is due to the mineral matter in the water that covered the peat during formation, and it usually consists of silica in the form of sand or silt or of alumina and silica in the form of clay. Mineral constituents other than silica and alumina in excess of 8 per cent are not common in peat and where found may be traced to the local ground and surface waters. The ash content of the best peats in the United States ranges from about 6 to 12 per cent, though many of the largest deposits in the Great Lakes area contain 15 per cent.
Massachusetts possesses a larger quantity of valuable peat than any other New England State except Maine. Aside from the salt marshes, which were formed by coastal subsidence and wave action, the peat deposits of Massachusetts originated in glacial depressions, Salt marshes near Revere Beach, adjoining the mouth of Saugus and Pines rivers (see fig. 23), are several square miles in area and contain muck about 10 feet in average depth. The surface of these deposits, which were formed from salt-marsh grasses, lies at about high-tide level. The dominant vegetation consists of saltmarsh grasses. The muck is very fibrous and contains a large proportion of clay and silt.
THE OCCURRENCE AND USES OP PEAT IN THE UNITED STATES BY E. K. SOPER AND C. C. OSBON, UNITED STATES GEOLOGICAL SURVEY Bulletin 728 (1922).
Diabse Dikes; Dedham-Medford Zone Plutonic Rocks (Granite)
"Dikes of the Boston district. Crosby describes the Triassic diabase dikes of the lower Neponset Valley as follows: * The diabase dikes * * * of the Boston Basin generally, are referable to two distinct series distinct in age, trend, and lithologic character. We may properly emphasize the chronologic distinction as of greatest geologic significance, by designating these two series provisionally the Carboniferous and the Triassic. Evidently the diabase dikes are not related in origin or composition to any of the other igneous rocks of the district, and in size, regularity, and continuity the two systems are essentially similar and normal. * * * The * * * dikes of this series adhere very closely to a north-south trend and vertical attitude, a hade of even a few degrees being very unusual. Their relation to the general geological structure of the region is distinctly transverse, and evidently they date from a period of gravity faulting without folding, such as the Triassic is known to have been. Transverse columnar jointing is commonly well developed. The greenstone alteration is wanting, and the rock yields readily to kaolinization, the tendency to pass by spheroidal weathering to a rusty brown earth being a marked feature of this! diabase."
GEOLOGY OF MASSACHUSETTS AND RHODE ISLAND (1917), https://pubs.usgs.gov/bul/0597/report.pdf
GEOLOGY OF MASSACHUSETTS AND RHODE ISLAND (1917), https://pubs.usgs.gov/bul/0597/report.pdf
"Boston terrane dikes have much higher average TiO2 and K2O contents and lower average SiO2 and MgO contents (for quartz and olive normative dikes, respectively).
Socci & Smith, Mafic dikes of the Avalon Boston terrane, Geology of the composite Avalon Terrane of southern New England (1990).
Socci & Smith, Mafic dikes of the Avalon Boston terrane, Geology of the composite Avalon Terrane of southern New England (1990).
DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Radiometric Ages on File In the Radiometric Age Data Bank (RADB) of Rocks from Massachusetts by Robert E. Zartman and Richard F. Marvin Open-File Report 87-170
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The Boston Basin is a topographic as well as a structural depression The Cambridge Argillite is a shale, locally and weakly metamorphosed, and occasionally encountered with reworked tuffaceous material.It is generally hard and competent due to its poorly developed bedding planes and general lack offissility. However, localized zones exist where alteration of the bedrock hasproduced zones of varying widths of kaolin, a "soil-like" material composed essentially of the clay mineral kaolinite. The abrupt and unpredictable change from the sound Argillite to the kaolinitic weak Argillite occurs in very short distances.The first reported foundation project including rock socketed caissons where "altered" Cambridge Argillite wasencountered was the Boston CompanyBuilding. Johnson (1973) described that "in certain zones the rock is highly altered and weakened to the point that the material can be crumbled between the fingers."
Brown, et al. Bedrock characterization and design considerations for rock socketed caissons in the Greater Boston area (Caractérisation du substratum rocheux et conception de caissons foncés dans la roche dans la région de Boston), (1997).
The Cambridge argillite, divided into 13 units, is almost exclusively argillite; the portion exposed in the tunnel is 3922 feet thick. It is noteworthy that this section of Cambridge argillite is 422 feet thicker than the maximum thickness estimated by LaForge ( 193 2) and others. The total thickness of that portion of the Boston Bay group exposed in the Main Drainage Tunnel is 56 78 feet.The size of the particles composing shales renders their microscopic study difficult. Thin sections show aligned felty or fibrous masses of clay-size material composed of chlorite, sericite and kaolinite in various proportions. Scattered fragments of angular quartz of silt size are common in some of the shale laminae. Small amounts of hematite color the purple shales, whereas the green shales contain relatively abundant chlorite. Quantitative X-ray analyses of five representative shale samples show that they all contain abundant quartz, and appreciable sericite and kaolinite. Chlorite appears in three samples, albite is present in three samples, and one sample contains zoisite or epidote in small amounts. As previously stated the lamination or bedding of the argillite is remarkably regularUnder the petrographic microscope scattered angular to subangular particles of quartz of silt size appear in a felty or fibrous matrix of intimately intergrown clay-size minerals. Sericite, chlorite and some kaolinite make up most of the fine-grained material. It is also probable that much finely divided quartz is present, but it is difficult to separate this from clay minerals of low birefringence such as kaolinite. Opaque matter consists of hematite, hydrous iron oxides, organic matter and scattered pyrite euhedra. Flakes of chlorite are particularly prominent in the green argillites. Organic matter is chiefly concentrated in the darker laminae. Samples of the darker laminae were examined by Professor Elso Barghoorn of Harvard University for possible plant spores or other microfossils. He reported abundant triturated organic matter in which no organic structures were recognizeable (personal communication).It is notworthy that the shales and argillites contain relatively abundant ferric iron in their chemical analyses. In what mineral this occurs has not been satisfactorily determined. Breccia is well developed along a few of the faults, but in general it is rare.
Rahm, D. A., 1962, Geology of the main drainage tunnel,. Boston, Massachusetts: Boston Soc. Civil Engineers, v. 49, p. 319-368.
[The Boylston St. Fishweir]. The lowest part of the Lower Unit overlies the Boston Blue Clay. While the top 1 to 3 m of the clay is typically marked by a yellowish color (CROSBY, 1903; KAYE, 1982; JOHNSON, 1942; HALEY and ALDRICH, 1985), only about 20 cm of this alteration was observed on this site. The upper 12 cm of the Blue Clay is a massive silty clay (7080% clay) with some fine sand, gravel and pebbles present. The Blue Clay sampled at -5.10 m (MLW) from Core 8 is dominated by fresh-brackish water diatoms (RICE, 1988). Rice interprets the environment at this depth to have been deposited above high tide level based on the low concentration of diatoms as well as the species found. The uppermost clay is penetrated by abundant Spartina alterniflora rhizomes from overlying peat. Some burrow or root-like features extend down into the clay to a maximum of 15 cm and are infilled with the grey silt from above. The pebbles are angular, with long axes (1 to 3.5 cm) parallel to bedding. The upper boundary of the Blue Clay is transitional, grading grey-blue then brown to black as it becomes more organic-rich over about 10 cm. As this contact represents a depositional hiatus, the transitional zone is likely due to reworking. Pollen and vegetation analysis at the base of the Lower Unit indicates sedge seeds and roots from herbaceous plants which suggests the initiation of a sedge peat on the site from fresh water encroachment into the area. Additional evidence from pollen suggests the presence of a swampy lowland forest in the area at the time (NEWBY and WEBB, 1988). The upper, gradational contact of the Blue Clay has occasional disarticulated shells of Crassostrea virginica, pebbles and cobbles, or, most commonly, a distinct salt marsh layer directly above the clay. This Lower Peat is a black, clay-rich (80%) deposit which reaches a maximum thickness of 30 cm. Quartz sand layers (approximately 0.5 cm thick) occur within the peat. No shells were found within the peat. Radiocarbon dating of sediment from this depth (-5.10 to -5.04 m MLW) puts the earliest transgression at 5,630 + 90 yr BP (Table 1). A layer of sand up to 6 cm thick occurs intermittently overlying the peat. The sand is poorly sorted and contains a few pebbles. Black/brown organic debris is mixed with the sand. Overlying this sand is a grey, clayey silt that forms most of the Lower Unit. The lower section of the clayey silt is characterized by intermittent traces of plant remains and shell fragments, and small gastropods, including Mulinea lateralis, Nassarius obsoleta, and Aequipecten irradians, all common to brackish estuarine conditions. Dark, sub-vertical, burrow-like features are common at this depth. One example, which extends into the Blue Clay, is 35 cm long with a 5 cm diameter. Isolated small patches of gravel and sand occur. Core 8 sediment shows an increase in abundance of more saline planktonic diatoms in the lower silt from -5.05 to -4.93 m MLW. In the interval of -4.93 to -4.76 m MLW, a decrease in salinity in the depositional environment is indicated by the presence of more brackish-water diatoms (RICE, 1988). Pollen data from this depth suggest that a freshwater marsh grew nearby, and that brackish conditions may have existed at this location (NEWBY and WEBB, 1988).
Holocene Evolution of Boston Inner Harbor, Rosent, et al., Journal of Coastal Research 9/2 363-377, Fort Lauderdale, Florida (Spring 1993).
Four or the initial five core borings encountered the argillite and occasional tuff, whereas the fifth boring penetrated an igneous rock, believed to be diabase, occurring as either a dike or sill. In certain zones the rock is highly altered and weakened to the point that the material can be crumbled between the fingers. This weakening of the normally sound rock formation is not unique at this location but has also been reported at other locations within the City. 2 Often fresh, unaltered strata are found interlayered with similar rocks which have undergone varying degrees of alteration. The entire system is generally steeply inclined, with individual layers ranging in thickness from a few inches to a few feet. The diabase encountered in Boring No. 1 was badly fractured and jointed, and thus has also been classified as a weak rock in-situ It was possible to examine directly the in situ argillite in zones where little or no core recovery had been achieved during the previous exploratory drilling. The rock was found to occur in a steeply inclined (60° to 80°) layered system of medium hard argillite, with occasional bands of medium to soft, clay-like altered zones of 1 /2 to 2 inches in thickness. It is concluded that perhaps the softer layers may have eroded during drilling, thus the cores tend to separate along the softer layers, resulting in little or no recovery. 2. The sounder portions of the argillite were highly jointed. The steep bedding planes tended to become highly lubricated with seepage, and often unstable. The instability was an advantage to the contractor during most of the excavation, since the rock tended to break out in flat or cubical shapes of up to 12+ in. maximum size. Shoring and rock bolting was required in many areas. 3. As noted on Figure 5, the bell in Caisson No. 2 was nearly bisected by a contact zone between the argillite and volcanic tuff. The tuff is hard, well indurated, relatively sound, but is highly jointed, although bedding planes are absent. The contact between the tuff and argillite was very irregular along one side of the bell and smooth along the opposite side. Both of the materials were quite stable during the excavation, except for a collapse zone noted in the argillite as it tended to break off along the weaker, lubricated bedding planes. 4. The bell at Caisson No. 1 also encountered both the argillite and the tuff. Bedding in the argillite dipped an average of 55° north, with an east-west strike, and was highly jointed. Very thin, kaolinized beds (1 /4 in.±) were noted within the relatively sound rock mass. 5. The bells of Caissons 3 and 4 were excavated totally within the argillite, which contained, typically, many high angle joints and an almost slatey cleavage. A few minor clay-filled joints were noted Bedrock Conditions 1. The rock core samples recovered during the design state indicated the erratic nature of the Cambridge Argillite formation. The widely varying sample recovery rates and visual examination revealed: - alternating zones of very soft, weathered argillite and sounder, but highly fractured, argillite and/or volcanic tuff. - the stratification is steeply inclined, at 60 to 80 degrees with the horizontal. - occasional dikes and sills of diabase intrusions 2. The zones of weathered or altered materials followed the orientation of the inclined strata, rather than occurring as a surface condition of limited depth. Thus, soft rock cores were sometimes encountered below zones of relatively sound rock
Johnson, E.G. (1973), "Unique Foundation Features, The Boston Company Building, Boston, Massachusetts," Journal of the Boston Soc. of Civil Eng., October, 170-189; originally presented in February 1969. (Principal, Haley & Aldrich, Inc., Cambridge, Mass.).
The argillite varies from decomposed to massive. Most of the argillite in the dredging prism is decomposed and highly weathered
Contract #W912DS-12-D-0002, DB01, Marine Geophysical and Geological Investigation, Boston Harbor, Boston Massachusetts; Department Of The Army, US Army Corps of Engineers (June 15, 2015)
Argillites within the Cambridge Formation of the Boston Bay Group have been known for quite some time to contain ring-like structures, generally interpreted to be body fossils of the Vendian organism Aspidella (Billings, 1872; Clark, 1923; Bailey & Bland, 2001; McMenamin, 2004). Cross-sections made through these ring structures have thin, dark, seemingly opaque laminations that usually run parallel to bedding, forming broad planar surfaces. Some of these dark layers demarcate the fossil surfaces, but others do not. In either case, the material of the thin dark layers has been interpreted to be a preserved pyritized biomat (Baily & Bland, 2001) which entombed the Aspidella, or was perhaps the organic remains of the fossil itself. The interpretation of the dark layers as pyritized biomats is not supported by data obtained in the present study, which began in the summer of 2007 when fresh float and in situ samples of rock bearing Aspidella were collected from the southern side of Boston Harbor. Energy dispersive spectrometry (EDS) with a Scanning Electron Microscope (SEM) reveals that the dark laminae contain no sulfur, unlike what would be expected from a pyritized layer. Further analysis of these rocks revealed the presence of small framboids, possibly of organic origin. Small pyrite framboids were found in most of the eleven thin sections from Hewitt’s Cove (Fig. 4); thus far, framboids have not been found in slides from the other two rocks. The pyrite is bright yellow in reflected light, and EDS analysis confirms the abundance of both sulfur and iron. The framboids range from 20 to 30 micrometers in diameter, with individual pyrite grains no more than a micron or two in width. About a third of the individual pyrite crystals appear to be subhedral cubes, others are almost spherical, or have irregular grain boundaries. These 1-2 μm pyrite grains are found both singly and in groups, comprising spherical framboids, or seemingly split open and scattered. In one slide, BB-HC-A6a, many of the framboidal crystals have dark centers, as shown in Figure 5. EDS analysis indicates that these framboids are rich in iron, but poor or entirely lacking in sulfur. Energy dispersive spectra also demonstrate that the more distinct the dark center of a framboid crystal, the richer in iron and poorer in sulfur it is. Many of these individual grains have a triangular or rhombohedral shape. The chemical composition indicated by EDS, combined with the shape of the grains, suggest that these framboids are made of hematite. These hematite framboids were located in a group of framboids, and were found toward the left and right ends of the cluster; the framboids in the center of the cluster, however, were rich in both iron and sulfur. The dark laminae in most slides, including those with Aspidella, were scrutinized and no framboids have been found within the Aspidella structures, or in any of the dark laminae. However, the framboids were most frequently found within silty regions a few millimeters above and/or below dark layers. The textures and mineralogy indicate that rocks of the Cambridge Argillite were buried to a depth between approximately 3.5 and 5 km, and were heated to temperatures between approximately 175°C and 250°C. The presence of foliation textures in the slate indicates it has undergone greater deformation than the argillite, as would be expected. Fossils could certainly be preserved in either of these environments, and microfossils have indeed been found in the Cambridge Argillite by Lenk et al. (1982) from tunnel excavation samples under the city of Boston.
Anderson, E. P., A PETROGRAPHIC AND SEM-EDS ANALYSIS OF ASPIDELLABEARING SILTSTONES AND SLATES OF THE CAMBRIDGE ARGILLITE, BOSTON BAY GROUP, MASSACHUSETT, 21st Annual Keck Symposium (2008)
Kaye, C.A. (1967), "Kaolinization of Bedrock of the Boston, Massachusetts Area," Geological Survey Professional Paper 575.
Brown, et al. Bedrock characterization and design considerations for rock socketed caissons in the Greater Boston area (Caractérisation du substratum rocheux et conception de caissons foncés dans la roche dans la région de Boston), (1997).
The Cambridge argillite, divided into 13 units, is almost exclusively argillite; the portion exposed in the tunnel is 3922 feet thick. It is noteworthy that this section of Cambridge argillite is 422 feet thicker than the maximum thickness estimated by LaForge ( 193 2) and others. The total thickness of that portion of the Boston Bay group exposed in the Main Drainage Tunnel is 56 78 feet.The size of the particles composing shales renders their microscopic study difficult. Thin sections show aligned felty or fibrous masses of clay-size material composed of chlorite, sericite and kaolinite in various proportions. Scattered fragments of angular quartz of silt size are common in some of the shale laminae. Small amounts of hematite color the purple shales, whereas the green shales contain relatively abundant chlorite. Quantitative X-ray analyses of five representative shale samples show that they all contain abundant quartz, and appreciable sericite and kaolinite. Chlorite appears in three samples, albite is present in three samples, and one sample contains zoisite or epidote in small amounts. As previously stated the lamination or bedding of the argillite is remarkably regularUnder the petrographic microscope scattered angular to subangular particles of quartz of silt size appear in a felty or fibrous matrix of intimately intergrown clay-size minerals. Sericite, chlorite and some kaolinite make up most of the fine-grained material. It is also probable that much finely divided quartz is present, but it is difficult to separate this from clay minerals of low birefringence such as kaolinite. Opaque matter consists of hematite, hydrous iron oxides, organic matter and scattered pyrite euhedra. Flakes of chlorite are particularly prominent in the green argillites. Organic matter is chiefly concentrated in the darker laminae. Samples of the darker laminae were examined by Professor Elso Barghoorn of Harvard University for possible plant spores or other microfossils. He reported abundant triturated organic matter in which no organic structures were recognizeable (personal communication).It is notworthy that the shales and argillites contain relatively abundant ferric iron in their chemical analyses. In what mineral this occurs has not been satisfactorily determined. Breccia is well developed along a few of the faults, but in general it is rare.
Rahm, D. A., 1962, Geology of the main drainage tunnel,. Boston, Massachusetts: Boston Soc. Civil Engineers, v. 49, p. 319-368.
[The Boylston St. Fishweir]. The lowest part of the Lower Unit overlies the Boston Blue Clay. While the top 1 to 3 m of the clay is typically marked by a yellowish color (CROSBY, 1903; KAYE, 1982; JOHNSON, 1942; HALEY and ALDRICH, 1985), only about 20 cm of this alteration was observed on this site. The upper 12 cm of the Blue Clay is a massive silty clay (7080% clay) with some fine sand, gravel and pebbles present. The Blue Clay sampled at -5.10 m (MLW) from Core 8 is dominated by fresh-brackish water diatoms (RICE, 1988). Rice interprets the environment at this depth to have been deposited above high tide level based on the low concentration of diatoms as well as the species found. The uppermost clay is penetrated by abundant Spartina alterniflora rhizomes from overlying peat. Some burrow or root-like features extend down into the clay to a maximum of 15 cm and are infilled with the grey silt from above. The pebbles are angular, with long axes (1 to 3.5 cm) parallel to bedding. The upper boundary of the Blue Clay is transitional, grading grey-blue then brown to black as it becomes more organic-rich over about 10 cm. As this contact represents a depositional hiatus, the transitional zone is likely due to reworking. Pollen and vegetation analysis at the base of the Lower Unit indicates sedge seeds and roots from herbaceous plants which suggests the initiation of a sedge peat on the site from fresh water encroachment into the area. Additional evidence from pollen suggests the presence of a swampy lowland forest in the area at the time (NEWBY and WEBB, 1988). The upper, gradational contact of the Blue Clay has occasional disarticulated shells of Crassostrea virginica, pebbles and cobbles, or, most commonly, a distinct salt marsh layer directly above the clay. This Lower Peat is a black, clay-rich (80%) deposit which reaches a maximum thickness of 30 cm. Quartz sand layers (approximately 0.5 cm thick) occur within the peat. No shells were found within the peat. Radiocarbon dating of sediment from this depth (-5.10 to -5.04 m MLW) puts the earliest transgression at 5,630 + 90 yr BP (Table 1). A layer of sand up to 6 cm thick occurs intermittently overlying the peat. The sand is poorly sorted and contains a few pebbles. Black/brown organic debris is mixed with the sand. Overlying this sand is a grey, clayey silt that forms most of the Lower Unit. The lower section of the clayey silt is characterized by intermittent traces of plant remains and shell fragments, and small gastropods, including Mulinea lateralis, Nassarius obsoleta, and Aequipecten irradians, all common to brackish estuarine conditions. Dark, sub-vertical, burrow-like features are common at this depth. One example, which extends into the Blue Clay, is 35 cm long with a 5 cm diameter. Isolated small patches of gravel and sand occur. Core 8 sediment shows an increase in abundance of more saline planktonic diatoms in the lower silt from -5.05 to -4.93 m MLW. In the interval of -4.93 to -4.76 m MLW, a decrease in salinity in the depositional environment is indicated by the presence of more brackish-water diatoms (RICE, 1988). Pollen data from this depth suggest that a freshwater marsh grew nearby, and that brackish conditions may have existed at this location (NEWBY and WEBB, 1988).
Holocene Evolution of Boston Inner Harbor, Rosent, et al., Journal of Coastal Research 9/2 363-377, Fort Lauderdale, Florida (Spring 1993).
Four or the initial five core borings encountered the argillite and occasional tuff, whereas the fifth boring penetrated an igneous rock, believed to be diabase, occurring as either a dike or sill. In certain zones the rock is highly altered and weakened to the point that the material can be crumbled between the fingers. This weakening of the normally sound rock formation is not unique at this location but has also been reported at other locations within the City. 2 Often fresh, unaltered strata are found interlayered with similar rocks which have undergone varying degrees of alteration. The entire system is generally steeply inclined, with individual layers ranging in thickness from a few inches to a few feet. The diabase encountered in Boring No. 1 was badly fractured and jointed, and thus has also been classified as a weak rock in-situ It was possible to examine directly the in situ argillite in zones where little or no core recovery had been achieved during the previous exploratory drilling. The rock was found to occur in a steeply inclined (60° to 80°) layered system of medium hard argillite, with occasional bands of medium to soft, clay-like altered zones of 1 /2 to 2 inches in thickness. It is concluded that perhaps the softer layers may have eroded during drilling, thus the cores tend to separate along the softer layers, resulting in little or no recovery. 2. The sounder portions of the argillite were highly jointed. The steep bedding planes tended to become highly lubricated with seepage, and often unstable. The instability was an advantage to the contractor during most of the excavation, since the rock tended to break out in flat or cubical shapes of up to 12+ in. maximum size. Shoring and rock bolting was required in many areas. 3. As noted on Figure 5, the bell in Caisson No. 2 was nearly bisected by a contact zone between the argillite and volcanic tuff. The tuff is hard, well indurated, relatively sound, but is highly jointed, although bedding planes are absent. The contact between the tuff and argillite was very irregular along one side of the bell and smooth along the opposite side. Both of the materials were quite stable during the excavation, except for a collapse zone noted in the argillite as it tended to break off along the weaker, lubricated bedding planes. 4. The bell at Caisson No. 1 also encountered both the argillite and the tuff. Bedding in the argillite dipped an average of 55° north, with an east-west strike, and was highly jointed. Very thin, kaolinized beds (1 /4 in.±) were noted within the relatively sound rock mass. 5. The bells of Caissons 3 and 4 were excavated totally within the argillite, which contained, typically, many high angle joints and an almost slatey cleavage. A few minor clay-filled joints were noted Bedrock Conditions 1. The rock core samples recovered during the design state indicated the erratic nature of the Cambridge Argillite formation. The widely varying sample recovery rates and visual examination revealed: - alternating zones of very soft, weathered argillite and sounder, but highly fractured, argillite and/or volcanic tuff. - the stratification is steeply inclined, at 60 to 80 degrees with the horizontal. - occasional dikes and sills of diabase intrusions 2. The zones of weathered or altered materials followed the orientation of the inclined strata, rather than occurring as a surface condition of limited depth. Thus, soft rock cores were sometimes encountered below zones of relatively sound rock
Johnson, E.G. (1973), "Unique Foundation Features, The Boston Company Building, Boston, Massachusetts," Journal of the Boston Soc. of Civil Eng., October, 170-189; originally presented in February 1969. (Principal, Haley & Aldrich, Inc., Cambridge, Mass.).
The argillite varies from decomposed to massive. Most of the argillite in the dredging prism is decomposed and highly weathered
Contract #W912DS-12-D-0002, DB01, Marine Geophysical and Geological Investigation, Boston Harbor, Boston Massachusetts; Department Of The Army, US Army Corps of Engineers (June 15, 2015)
Argillites within the Cambridge Formation of the Boston Bay Group have been known for quite some time to contain ring-like structures, generally interpreted to be body fossils of the Vendian organism Aspidella (Billings, 1872; Clark, 1923; Bailey & Bland, 2001; McMenamin, 2004). Cross-sections made through these ring structures have thin, dark, seemingly opaque laminations that usually run parallel to bedding, forming broad planar surfaces. Some of these dark layers demarcate the fossil surfaces, but others do not. In either case, the material of the thin dark layers has been interpreted to be a preserved pyritized biomat (Baily & Bland, 2001) which entombed the Aspidella, or was perhaps the organic remains of the fossil itself. The interpretation of the dark layers as pyritized biomats is not supported by data obtained in the present study, which began in the summer of 2007 when fresh float and in situ samples of rock bearing Aspidella were collected from the southern side of Boston Harbor. Energy dispersive spectrometry (EDS) with a Scanning Electron Microscope (SEM) reveals that the dark laminae contain no sulfur, unlike what would be expected from a pyritized layer. Further analysis of these rocks revealed the presence of small framboids, possibly of organic origin. Small pyrite framboids were found in most of the eleven thin sections from Hewitt’s Cove (Fig. 4); thus far, framboids have not been found in slides from the other two rocks. The pyrite is bright yellow in reflected light, and EDS analysis confirms the abundance of both sulfur and iron. The framboids range from 20 to 30 micrometers in diameter, with individual pyrite grains no more than a micron or two in width. About a third of the individual pyrite crystals appear to be subhedral cubes, others are almost spherical, or have irregular grain boundaries. These 1-2 μm pyrite grains are found both singly and in groups, comprising spherical framboids, or seemingly split open and scattered. In one slide, BB-HC-A6a, many of the framboidal crystals have dark centers, as shown in Figure 5. EDS analysis indicates that these framboids are rich in iron, but poor or entirely lacking in sulfur. Energy dispersive spectra also demonstrate that the more distinct the dark center of a framboid crystal, the richer in iron and poorer in sulfur it is. Many of these individual grains have a triangular or rhombohedral shape. The chemical composition indicated by EDS, combined with the shape of the grains, suggest that these framboids are made of hematite. These hematite framboids were located in a group of framboids, and were found toward the left and right ends of the cluster; the framboids in the center of the cluster, however, were rich in both iron and sulfur. The dark laminae in most slides, including those with Aspidella, were scrutinized and no framboids have been found within the Aspidella structures, or in any of the dark laminae. However, the framboids were most frequently found within silty regions a few millimeters above and/or below dark layers. The textures and mineralogy indicate that rocks of the Cambridge Argillite were buried to a depth between approximately 3.5 and 5 km, and were heated to temperatures between approximately 175°C and 250°C. The presence of foliation textures in the slate indicates it has undergone greater deformation than the argillite, as would be expected. Fossils could certainly be preserved in either of these environments, and microfossils have indeed been found in the Cambridge Argillite by Lenk et al. (1982) from tunnel excavation samples under the city of Boston.
Anderson, E. P., A PETROGRAPHIC AND SEM-EDS ANALYSIS OF ASPIDELLABEARING SILTSTONES AND SLATES OF THE CAMBRIDGE ARGILLITE, BOSTON BAY GROUP, MASSACHUSETT, 21st Annual Keck Symposium (2008)
Kaye, C.A. (1967), "Kaolinization of Bedrock of the Boston, Massachusetts Area," Geological Survey Professional Paper 575.
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Another sequence that was reported as being very similar to the Raritan by F.G. Clapp (1907) was from one of the borings in downtown Boston for the Ames Building, at the corner of Tremont and Court streets. This 41.5 meter (136 foot) section of light gray to white day, occurred between the Pleistocene and the argillite at elevations of -13.5 to -55 meters (-44 to -180 feet) MSL. But Woodhouse observed kaolinized argillite at similar depths in test borings drilled in the immediate area for the nearby Boston Company Building on Court Street, the New -England Merchants Building on Tremont Street and the 60 State Street building. It is, therefore, more likely that altered argillite underlies the Ames Building
Rock Altered by Intrusions. Altered Cambridge Argillite was first considered Cretaceous clay in early encotmters, but this practice soon changed. Clapp (1907) described 41.5 meters (136 feet) of light gray to almost white clay found between till and the Cambridge Argillite at a depth of 13.5 meters (44 feet) beneath the Ames Building in downtown Boston as very similar to samples of the Raritan Formation from Long Island. It varied from very soft and putty-like material to hard as a rock, and chemical analysis indicated that it was pure clay. Clapp also cited other deep borings in central Boston and on Spectacle Island that encountered similar material. Crosby (1903) had earlier suggested such soft white deposits were pre-Pleistocene and called attention to the fact that under the new Cambridge site for MIT, the normal argillite was "rotted to a whitish and more or less plastic clay" (Worcester, 1914). Since then, this type of alteration has been found at many places in the Boston Basin (see Figure 3-48). Woodhouse has encountered it in samples of borings he has observed in central Boston. The alteration occurs in all types of sedimentary rock, including conglomerate. The altered conglomerate is well exposed in several places, but for the most part these softened rocks lay deeply buried (see Figure 3-49). In places, secondary alteration has changed the hard argillite rock into a soft, bleached ·white, silty aggregate that can be dug with a hand shovel. These changes are due to the formation of sericite and kaolin at the expense of all primary minerals, including quartz (Kaye, 1979). The argillites, particularly the maroon and green tuffaceous argillites, seem to be most widely affected, especially under parts of downtown Boston, the Back Bay and the lower Charles River, where the cause-and-effect relationship of low topography and deep bedrock with altered rock is notable. The altered argillite varies from light-gray to dark-green in color. Soft rocks were the most deeply eroded ones during Pleistocene and earlier, and are thought to underlie most of the larger lowlands in the basin. The alteration is present in zones reaching in excess of 91 meters (300 feet) below the surface. Kaye (1967a) noted that the alteration appeared limited to certain beds, and thought the common association of igneous rocks also might suggest a genetic relation, but later Kaye (1984b) found a closer relation with shear zones, faults and dikes and a lesser one to stratigraphic horizons. However, the suggestion that it follmvs tuffaceous horizons in the argillite that were rapidly altered after their eruption (Hager & Stewart, 1995) does not match their relation to the surface of the argillite and lacks an origin. The cause of this soft-rock alteration is conjectural and could be either the result of hydrothermal activity or deep lateritic weathering during Tertiary time (Kaye, 1961 & 1967a). Kaye favored weathering similar to that forming bauxite, and being. the result of early deep weathering as altered clasts occur in the Tertiary deposits of Martha's Vmeyard. If it were due to hydrothermal alteration, this could have occurred when the basic dikes in the basin were altered by the Late Ordovician intrusions and volcanic activity.
The altered rock tends to be restricted to certain beds as noted by Rahm (1962) and Billings and Tierney (1964) in the tunnels under Boston constructed by the Metropolitan District Commission. In the tunnels studied the City Tunnel Extension and the Main Drainage Tunnel - soft rock was limited to certain beds or groups of beds. However, because tunnel observations are limited by the height of the tunnel, about 4 meters (13 feet) for the Boston tunnels, the alteration might cut across planes of stratifi~ cation out of view (Kaye, 1967a). In the western 1,310 meters (4,300 feet) of the Main Drainage Tunnel beneath Roxbury, altered argillite, called "shale" in the tunnel reports, and sandstone are interbedded with massive ·conglomerate and arkose, some of which appear from the description to be altered. Three diabase dikes and sills cut the soft rock (Rahm, 1962). Billings and Tierney (1964) also found "shale" in two places in the City Tunnel Extension. A section 12 meters (40 feet) thick of soft kaolinized argillite, interbedded with thin quartzite, . purple argillite, sandstone and conglomerate, occurs in the tunnel south of the Charles River in Allston at a depth of about 69 meters (225 feet) below the top of the bedrock surface. The deepest recorded occurrence of alteration beneath the surface of bedrock is reported to be 91 meters (300 'feet) from the City Tunnel Extension under Cambridge (Billings & Tierney, 1964). The altered rock obviously extends below the tunnel level to an even greater depth. No borings yield unequivocal evidence of having reached the base, or maximum depth, of a particular kaolinized zone:
Some indirect evidence suggests that most alteration dies out at relatively moderate depths. The distribution of altered rock is much more restricted in the Main Drainage Tunnel and the City Tunnel Extension than it is under the Shawmut Peninsula and adjoining Cambridge. The average elevation of the rock surface in the altered zones is about -30 meters (---100 feet) MSL, whereas the elevation of the tunnels ranges from -88 to -116 meters (-290 to -380 feet) MSL. However, because the tunnels do not pass under the highly altered zone of Boston and Cambridge, it cannot be demonstrated that the sparseness of alteration in the tunnels bears on the depth of bedrock alteration. In addition, altered argillite is abundant at an elevation of -85 meters (-280 feet) MSL in the North Metropolitan Relief Tunnel. This discussion is based largely on the results of geological mapping of four bedrock tunnels in the greater Boston area, constructed, between 1948 and 1960 under the supervision of the Construction Division of the Metropolitan District Commission. These four bedrock tunnels total slightly more than 32 kilometers (20 miles) in length.
Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE 2011/2012, PATRICK J. BAROSH & DAVID WOODHOUSE
Rock Altered by Intrusions. Altered Cambridge Argillite was first considered Cretaceous clay in early encotmters, but this practice soon changed. Clapp (1907) described 41.5 meters (136 feet) of light gray to almost white clay found between till and the Cambridge Argillite at a depth of 13.5 meters (44 feet) beneath the Ames Building in downtown Boston as very similar to samples of the Raritan Formation from Long Island. It varied from very soft and putty-like material to hard as a rock, and chemical analysis indicated that it was pure clay. Clapp also cited other deep borings in central Boston and on Spectacle Island that encountered similar material. Crosby (1903) had earlier suggested such soft white deposits were pre-Pleistocene and called attention to the fact that under the new Cambridge site for MIT, the normal argillite was "rotted to a whitish and more or less plastic clay" (Worcester, 1914). Since then, this type of alteration has been found at many places in the Boston Basin (see Figure 3-48). Woodhouse has encountered it in samples of borings he has observed in central Boston. The alteration occurs in all types of sedimentary rock, including conglomerate. The altered conglomerate is well exposed in several places, but for the most part these softened rocks lay deeply buried (see Figure 3-49). In places, secondary alteration has changed the hard argillite rock into a soft, bleached ·white, silty aggregate that can be dug with a hand shovel. These changes are due to the formation of sericite and kaolin at the expense of all primary minerals, including quartz (Kaye, 1979). The argillites, particularly the maroon and green tuffaceous argillites, seem to be most widely affected, especially under parts of downtown Boston, the Back Bay and the lower Charles River, where the cause-and-effect relationship of low topography and deep bedrock with altered rock is notable. The altered argillite varies from light-gray to dark-green in color. Soft rocks were the most deeply eroded ones during Pleistocene and earlier, and are thought to underlie most of the larger lowlands in the basin. The alteration is present in zones reaching in excess of 91 meters (300 feet) below the surface. Kaye (1967a) noted that the alteration appeared limited to certain beds, and thought the common association of igneous rocks also might suggest a genetic relation, but later Kaye (1984b) found a closer relation with shear zones, faults and dikes and a lesser one to stratigraphic horizons. However, the suggestion that it follmvs tuffaceous horizons in the argillite that were rapidly altered after their eruption (Hager & Stewart, 1995) does not match their relation to the surface of the argillite and lacks an origin. The cause of this soft-rock alteration is conjectural and could be either the result of hydrothermal activity or deep lateritic weathering during Tertiary time (Kaye, 1961 & 1967a). Kaye favored weathering similar to that forming bauxite, and being. the result of early deep weathering as altered clasts occur in the Tertiary deposits of Martha's Vmeyard. If it were due to hydrothermal alteration, this could have occurred when the basic dikes in the basin were altered by the Late Ordovician intrusions and volcanic activity.
The altered rock tends to be restricted to certain beds as noted by Rahm (1962) and Billings and Tierney (1964) in the tunnels under Boston constructed by the Metropolitan District Commission. In the tunnels studied the City Tunnel Extension and the Main Drainage Tunnel - soft rock was limited to certain beds or groups of beds. However, because tunnel observations are limited by the height of the tunnel, about 4 meters (13 feet) for the Boston tunnels, the alteration might cut across planes of stratifi~ cation out of view (Kaye, 1967a). In the western 1,310 meters (4,300 feet) of the Main Drainage Tunnel beneath Roxbury, altered argillite, called "shale" in the tunnel reports, and sandstone are interbedded with massive ·conglomerate and arkose, some of which appear from the description to be altered. Three diabase dikes and sills cut the soft rock (Rahm, 1962). Billings and Tierney (1964) also found "shale" in two places in the City Tunnel Extension. A section 12 meters (40 feet) thick of soft kaolinized argillite, interbedded with thin quartzite, . purple argillite, sandstone and conglomerate, occurs in the tunnel south of the Charles River in Allston at a depth of about 69 meters (225 feet) below the top of the bedrock surface. The deepest recorded occurrence of alteration beneath the surface of bedrock is reported to be 91 meters (300 'feet) from the City Tunnel Extension under Cambridge (Billings & Tierney, 1964). The altered rock obviously extends below the tunnel level to an even greater depth. No borings yield unequivocal evidence of having reached the base, or maximum depth, of a particular kaolinized zone:
Some indirect evidence suggests that most alteration dies out at relatively moderate depths. The distribution of altered rock is much more restricted in the Main Drainage Tunnel and the City Tunnel Extension than it is under the Shawmut Peninsula and adjoining Cambridge. The average elevation of the rock surface in the altered zones is about -30 meters (---100 feet) MSL, whereas the elevation of the tunnels ranges from -88 to -116 meters (-290 to -380 feet) MSL. However, because the tunnels do not pass under the highly altered zone of Boston and Cambridge, it cannot be demonstrated that the sparseness of alteration in the tunnels bears on the depth of bedrock alteration. In addition, altered argillite is abundant at an elevation of -85 meters (-280 feet) MSL in the North Metropolitan Relief Tunnel. This discussion is based largely on the results of geological mapping of four bedrock tunnels in the greater Boston area, constructed, between 1948 and 1960 under the supervision of the Construction Division of the Metropolitan District Commission. These four bedrock tunnels total slightly more than 32 kilometers (20 miles) in length.
Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE 2011/2012, PATRICK J. BAROSH & DAVID WOODHOUSE
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Rahm, D. A., 1962, Geology of the main drainage tunnel,. Boston, Massachusetts: Boston Soc. Civil Engineers, v. 49, p. 319-368.
Rahm, D. A., 1962, Geology of the main drainage tunnel,. Boston, Massachusetts: Boston Soc. Civil Engineers, v. 49, p. 319-368.
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Rahm, D. A., 1962, Geology of the main drainage tunnel,. Boston, Massachusetts: Boston Soc. Civil Engineers, v. 49, p. 319-368.
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"The ancient North American and African plates - referred to as Laurentia and Gondwana, respectively collided in a zone just west of Boston about 650 to 620 million years ago. The geologic character of this margin and the structure to the east of this belt are not seen elsewhere on the Atlantic coast of the United States. When the much later rifting of about 225 million years ago began to form the present North Atlantic Basin, the split left a piece of northwest Africa clinging to North America. This fragment became the foundation upon which Boston was built. The city and its harbor lie in their own much smaller rift basin formed during a pause in the collision at the very end of the Proterozoic (pre-Cambrian) about 600 million years ago. An array of volcanic debris, gravel, sand and mud gradually filled and overflowed the rift. As the collision ended in the late Ordovician about 440 million years ago, volcanic upheavals formed masses of granite on both sides of the rift to give added character. These and subsequent events produced numerous kinds of structures and rock representing almost all later geologic periods. The region remains active, with earthquake activity and other indications of crustal movement, and is not at all passive."
There were several major geological influences that affected the founding of Boston (Kaye, 1976a). Being a seafaring people, the early settlers looked for a safe harbor. Boston's island studded harbor is formed by a deep indentation in the coastline of Massachusetts. This indentation exists primarily because the underlying rock, mostly argillite, is softer and more easily eroded than the hard highland conglomerate and granitic rock, which surround the Boston Basin (the name of this large topographic and structural depression). Glacial ice further eroded the broad valley and, with subsequent melting of the ice, the sea level rose and flooded the depression, thereby forming the bay and "a safe and pleasant harbor." "This harbor is made by a great company of island, whose high cliffs shoulder out the boisterous seas, yet may easily deceive any unskilfull [sic] pilot; presenting many fair openings and broad sounds, which afford too shallow waters for any ships" (Wood, 1634)."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Settlement, Topography & Geologic Studies of Boston, Civil Engineering Practice, Vol. 26-27 (2011/2012).
There were several major geological influences that affected the founding of Boston (Kaye, 1976a). Being a seafaring people, the early settlers looked for a safe harbor. Boston's island studded harbor is formed by a deep indentation in the coastline of Massachusetts. This indentation exists primarily because the underlying rock, mostly argillite, is softer and more easily eroded than the hard highland conglomerate and granitic rock, which surround the Boston Basin (the name of this large topographic and structural depression). Glacial ice further eroded the broad valley and, with subsequent melting of the ice, the sea level rose and flooded the depression, thereby forming the bay and "a safe and pleasant harbor." "This harbor is made by a great company of island, whose high cliffs shoulder out the boisterous seas, yet may easily deceive any unskilfull [sic] pilot; presenting many fair openings and broad sounds, which afford too shallow waters for any ships" (Wood, 1634)."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Settlement, Topography & Geologic Studies of Boston, Civil Engineering Practice, Vol. 26-27 (2011/2012).
"Three detrital zircon samples from Ediacaran sandstone of the Roxbury Conglomerate and a quartzite clast in the Squantum diamictite in the Boston Basin (Fig. 1C) show abundant ca. 610–580 Ma and ca. 2.1–1.0 Ga zircon and few ca. 2.8–2.5, ca. 3.0, and ca. 3.2 Ga grains based on isotope dilution–thermal ionization mass spectrometry (ID-TIMS) methods (Thompson and Bowring, 2000; Thompson et al., 2014)."
Kuiper, Yvette D., Daniel P. Murray, Sonia Ellison, and James L. Crowley. "U-Pb detrital zircon analysis of sedimentary rocks of the southeastern New England Avalon terrane in the U.S. Appalachians: Evidence for a separate crustal block." Special Paper of the Geological Society of America 554, (2022). doi: 10.1130/ 2021.2554(05).
The Squantum Member (Roxbury Conglomerate Formation) crops out in the Boston Basin, northeast USA, and was deposited on the Avalonian palaeoplate in the late Neoproterozoic. It is reasonably well-constrained in age by radiometric dates to between 595.8 ± 1.2 Ma (from the underlying Lynn-Mattapan volcanic complex; Thompson et al. 2007) and Ma (from the overlying Cambridge Argillite; Thompson and Bowring 2000). The Boston Basin in the Ediacaran was a back-arc basin setting within the Avalon Terrane, the modern extent of which is strongly demarcated by faults (Carto and Eyles 2012). The Ediacaran stratigraphy in the Boston Basin is organized into the Boston Bay Group, which comprises two formations: the lower Roxbury Conglomerate and upper Cambridge Argillite. The Roxbury Conglomerate is divided into three members. The lower Brookline Member is predominantly composed of conglomerates, the middle Dorchester Member is primarily fine-grained (argillaceous) turbidites with minor conglomerates, and the upper Squantum Member is mostly diamictite facies interbedded with sandstones and siltstones (Carto and Eyles 2012). The Cambridge Argillite comprises approximately 5.5 km of tuff-rich finely laminated to thinly bedded fine-grained (argillaceous) turbidites (Carto and Eyles 2012). Three primary and conformably associated facies are recognized across the Roxbury Conglomerate (Carto and Eyles 2012): (1) conglomerate and sandstone, (2) diamictite, and (3) mudrock.
The diamictite facies (2), typically associated with the upper Squantum Member, comprises massive 1097 or chaotic matrix-supported polymict diamictites with pebble- to boulder-sized, moderately to well 1098 sorted subrounded to angular clasts of predominantly local lithologies (felsic and mafic volcanics, 1099 granodiorite, quartzite, siltstone, and sandstone), similar to the clasts of the conglomerate facies 1100 (Carto and Eyles 2012). The diamictites vary in thickness from approximately 8 m in outcrop to 215 1101 m in the subsurface (Tierney et al. 1968; Carto and Eyles 2012). The diamictite facies is typified by 1102 sharp, erosive basal contacts and transitional upper boundaries as the diamictites grade into 1103 sandstones or conglomerates (Carto and Eyles 2012). The chaotic diamictites can show crude 1104 stratification with large rafts of conglomerate, sandstone, and mudrock poorly mixed through the diamictite (Carto and Eyles 2012). Facetted clasts have not been found, and striated clasts identified by earlier workers (Sayles 1914) were not found by subsequent investigators (Carto and Eyles 2012). Carto and Eyles (2012) note that the distinction between the Roxbury Conglomerate diamictite and 1108 conglomerate facies is the proportion of matrix (10 vol.% to 30 vol.% in the conglomerates and 80 vol.% in the diamictites) with other characteristics, particularly clast composition and form, being remarkably similar – a similarity noted by previous workers who were also dubious of a glaciogenic origin (Dott 1961; Socci and Smith 1990). The diamictite facies is interpreted as the result of the 1112 earlier stages of downslope mixing of conglomeratic and muddy material deriving from primary fan or slope-deposited conglomerates, an interpretation that is supported by the conformable, interbedded, relationship between the diamictite facies with coarse- and fine-grained turbidites (Carto and Eyles 2012).
The argillite facies (3) refers to “rhythmically laminated (0.1 to 1 mm thick) muddy-siltstones that grade subtly into mudstone” (Carto and Eyles 2012, p. 8). The argillite facies is typical of the Cambridge Argillite but is found interbedded with both the conglomerate and diamictite facies throughout the Boston Bay Group on scales of centimetres to hundreds of metres thickness (Carto 1120 and Eyles 2012). Sedimentary structures include parallel to wavy laminations, cross-lamination, and both large- and small-scale slump folds which occur throughout the unit in argillite interbedded with conglomerate and diamictite facies as well as with discrete tuff horizons (Carto and Eyles 2012). The argillite facies has been consistently interpreted as a deep marine (below storm wave base) low-density turbidite (Dott 1961; Thompson and Bowring 2000; Carto and Eyles 2012)
A fourth facies, or perhaps a sub-facies of the argillite facies, is also considered: the pebbly argillite (Carto and Eyles 2012). Stratigraphically and geographically limited to approximately 0.5 m in total directly underlying diamictites at Squantum Head is a laminated argillite with matrix-supported pebbles (approximately 75 vol.% matrix) that is interbedded with non-pebbly laminated and graded argillites (Carto and Eyles 2012). The clasts are all small, typically gravel-size or smaller, rounded to subrounded, and composed of the same local lithologies as the conglomerate and diamictite facies (Carto and Eyles 2012). The pebbly layers form couplets with overlying thin laminae of massive pebble-free argillite (Carto and Eyles 2012). The ‘diamictite-argillite couplets’ have been interpreted as ice-rafted debris or dropstones – and this remains possible – but an interpretation as debrite-turbidite couplets where finer material is sheared off the top of a dilute debris flow as a turbidity current and settles out onto the deposited debrite is more parsimonious with the surrounding facies (Carto and Eyles 2012). Important to this non-glaciogenic origin is the apparent absence of any larger (cobble- or boulder-sized) clasts from the pebbly argillite (Carto and Eyles 2012). The pebbly argillite is interpreted as a slightly more distal equivalent of the diamictite facies, representing part of a debris flow that ran away from or further than the main flow resulting in a more dilute flow (Carto 1140 and Eyles 2012). The most parsimonious interpretation for the depositional context of the Squantum Member diamictite facies may be as part of a continuum of deposits resulting from downslope remobilization of fan or slope sediments including sandstones, mudstones, and conglomerates. The conglomerates may have an originally glaciogenic origin, but that is not certain and could equally, or perhaps more plausibly given the absence of strongly facetted or striated clasts, have a fluvial origin.
Wong Hearing, Thomas & Tindal, Ben & Vandyk, Thomas & Na, Lin & Pohl, Alexandre & Liu, Alexander & Harvey, Thomas & Williams, Mark. (2025). Ediacaran coupling of climate and biosphere dynamics. 10.31223/X5S42P.
The Squantum 'Tillite' (c. 593-570 Ma) consists of thick (up to 215 m) massive and crudely-stratified diamictites conformably interbedded with subaqueously-deposited conglomerates and sandstones within a thick (~7 km) Boston Basin fill which is dominated by argillite turbidites. The Squantum Tillite was first interpreted as being glacigenic in origin in 1914 because of the presence of diamictites; argillites were interpreted as glaciolacustrine 'varves' with rare ice-rafted debris, and conglomerates as glaciofluvial outwash. More recently these have been shown to be the product of deep marine mass flow processes with no glacial influence, yet because of its age equivalence with the deep marine, glacially-influenced Gaskiers Formation, the Squantum Tillite is still seen by some as supporting evidence for a widespread 'Snowball Earth' event at c. 580 Ma. New sedimentological work confirms that conglomerate and sandstone facies are deep marine sediment gravity flows genetically related to massive (homogeneous) and crudely-stratified (heterogeneous) diamictites produced subaqueously by downslope mixing of gravel and cobbles with muddy facies. Rare horizons of 'ice rafted debris' in thin-bedded and laminated turbidite facies interbedded with thick debrites show a weak but positive correlation of lamina thickness with grain size, suggesting these facies are non-glacial co-genetic 'debrite-turbidite' couplets. A significant volcanic influence on sedimentation is identified from reworked lapilli tuff beds and reworked ash in turbidites. The depositional setting of the Squantum 'Tillite' appears to be that of a submarine slope/fan setting in an open marine volcanic arc basin receiving large volumes of poorly-sorted sediment on the mid-latitude active margin of Gondwana. No direct glacial influence is apparent.
Carto, S.L., and Eyles, N., 2012, Sedimentology of the Neoproterozoic (c. 580 Ma) Squantum ‘Tillite’, Boston Basin, USA: Mass flow deposition in a deep-water arc basin lacking direct glacial influence: Sedimentary Geology, v. 269–270, p. 1–14, doi:10.1016/j.sedgeo.2012.03.011.
Kuiper, Yvette D., Daniel P. Murray, Sonia Ellison, and James L. Crowley. "U-Pb detrital zircon analysis of sedimentary rocks of the southeastern New England Avalon terrane in the U.S. Appalachians: Evidence for a separate crustal block." Special Paper of the Geological Society of America 554, (2022). doi: 10.1130/ 2021.2554(05).
The Squantum Member (Roxbury Conglomerate Formation) crops out in the Boston Basin, northeast USA, and was deposited on the Avalonian palaeoplate in the late Neoproterozoic. It is reasonably well-constrained in age by radiometric dates to between 595.8 ± 1.2 Ma (from the underlying Lynn-Mattapan volcanic complex; Thompson et al. 2007) and Ma (from the overlying Cambridge Argillite; Thompson and Bowring 2000). The Boston Basin in the Ediacaran was a back-arc basin setting within the Avalon Terrane, the modern extent of which is strongly demarcated by faults (Carto and Eyles 2012). The Ediacaran stratigraphy in the Boston Basin is organized into the Boston Bay Group, which comprises two formations: the lower Roxbury Conglomerate and upper Cambridge Argillite. The Roxbury Conglomerate is divided into three members. The lower Brookline Member is predominantly composed of conglomerates, the middle Dorchester Member is primarily fine-grained (argillaceous) turbidites with minor conglomerates, and the upper Squantum Member is mostly diamictite facies interbedded with sandstones and siltstones (Carto and Eyles 2012). The Cambridge Argillite comprises approximately 5.5 km of tuff-rich finely laminated to thinly bedded fine-grained (argillaceous) turbidites (Carto and Eyles 2012). Three primary and conformably associated facies are recognized across the Roxbury Conglomerate (Carto and Eyles 2012): (1) conglomerate and sandstone, (2) diamictite, and (3) mudrock.
The diamictite facies (2), typically associated with the upper Squantum Member, comprises massive 1097 or chaotic matrix-supported polymict diamictites with pebble- to boulder-sized, moderately to well 1098 sorted subrounded to angular clasts of predominantly local lithologies (felsic and mafic volcanics, 1099 granodiorite, quartzite, siltstone, and sandstone), similar to the clasts of the conglomerate facies 1100 (Carto and Eyles 2012). The diamictites vary in thickness from approximately 8 m in outcrop to 215 1101 m in the subsurface (Tierney et al. 1968; Carto and Eyles 2012). The diamictite facies is typified by 1102 sharp, erosive basal contacts and transitional upper boundaries as the diamictites grade into 1103 sandstones or conglomerates (Carto and Eyles 2012). The chaotic diamictites can show crude 1104 stratification with large rafts of conglomerate, sandstone, and mudrock poorly mixed through the diamictite (Carto and Eyles 2012). Facetted clasts have not been found, and striated clasts identified by earlier workers (Sayles 1914) were not found by subsequent investigators (Carto and Eyles 2012). Carto and Eyles (2012) note that the distinction between the Roxbury Conglomerate diamictite and 1108 conglomerate facies is the proportion of matrix (10 vol.% to 30 vol.% in the conglomerates and 80 vol.% in the diamictites) with other characteristics, particularly clast composition and form, being remarkably similar – a similarity noted by previous workers who were also dubious of a glaciogenic origin (Dott 1961; Socci and Smith 1990). The diamictite facies is interpreted as the result of the 1112 earlier stages of downslope mixing of conglomeratic and muddy material deriving from primary fan or slope-deposited conglomerates, an interpretation that is supported by the conformable, interbedded, relationship between the diamictite facies with coarse- and fine-grained turbidites (Carto and Eyles 2012).
The argillite facies (3) refers to “rhythmically laminated (0.1 to 1 mm thick) muddy-siltstones that grade subtly into mudstone” (Carto and Eyles 2012, p. 8). The argillite facies is typical of the Cambridge Argillite but is found interbedded with both the conglomerate and diamictite facies throughout the Boston Bay Group on scales of centimetres to hundreds of metres thickness (Carto 1120 and Eyles 2012). Sedimentary structures include parallel to wavy laminations, cross-lamination, and both large- and small-scale slump folds which occur throughout the unit in argillite interbedded with conglomerate and diamictite facies as well as with discrete tuff horizons (Carto and Eyles 2012). The argillite facies has been consistently interpreted as a deep marine (below storm wave base) low-density turbidite (Dott 1961; Thompson and Bowring 2000; Carto and Eyles 2012)
A fourth facies, or perhaps a sub-facies of the argillite facies, is also considered: the pebbly argillite (Carto and Eyles 2012). Stratigraphically and geographically limited to approximately 0.5 m in total directly underlying diamictites at Squantum Head is a laminated argillite with matrix-supported pebbles (approximately 75 vol.% matrix) that is interbedded with non-pebbly laminated and graded argillites (Carto and Eyles 2012). The clasts are all small, typically gravel-size or smaller, rounded to subrounded, and composed of the same local lithologies as the conglomerate and diamictite facies (Carto and Eyles 2012). The pebbly layers form couplets with overlying thin laminae of massive pebble-free argillite (Carto and Eyles 2012). The ‘diamictite-argillite couplets’ have been interpreted as ice-rafted debris or dropstones – and this remains possible – but an interpretation as debrite-turbidite couplets where finer material is sheared off the top of a dilute debris flow as a turbidity current and settles out onto the deposited debrite is more parsimonious with the surrounding facies (Carto and Eyles 2012). Important to this non-glaciogenic origin is the apparent absence of any larger (cobble- or boulder-sized) clasts from the pebbly argillite (Carto and Eyles 2012). The pebbly argillite is interpreted as a slightly more distal equivalent of the diamictite facies, representing part of a debris flow that ran away from or further than the main flow resulting in a more dilute flow (Carto 1140 and Eyles 2012). The most parsimonious interpretation for the depositional context of the Squantum Member diamictite facies may be as part of a continuum of deposits resulting from downslope remobilization of fan or slope sediments including sandstones, mudstones, and conglomerates. The conglomerates may have an originally glaciogenic origin, but that is not certain and could equally, or perhaps more plausibly given the absence of strongly facetted or striated clasts, have a fluvial origin.
Wong Hearing, Thomas & Tindal, Ben & Vandyk, Thomas & Na, Lin & Pohl, Alexandre & Liu, Alexander & Harvey, Thomas & Williams, Mark. (2025). Ediacaran coupling of climate and biosphere dynamics. 10.31223/X5S42P.
The Squantum 'Tillite' (c. 593-570 Ma) consists of thick (up to 215 m) massive and crudely-stratified diamictites conformably interbedded with subaqueously-deposited conglomerates and sandstones within a thick (~7 km) Boston Basin fill which is dominated by argillite turbidites. The Squantum Tillite was first interpreted as being glacigenic in origin in 1914 because of the presence of diamictites; argillites were interpreted as glaciolacustrine 'varves' with rare ice-rafted debris, and conglomerates as glaciofluvial outwash. More recently these have been shown to be the product of deep marine mass flow processes with no glacial influence, yet because of its age equivalence with the deep marine, glacially-influenced Gaskiers Formation, the Squantum Tillite is still seen by some as supporting evidence for a widespread 'Snowball Earth' event at c. 580 Ma. New sedimentological work confirms that conglomerate and sandstone facies are deep marine sediment gravity flows genetically related to massive (homogeneous) and crudely-stratified (heterogeneous) diamictites produced subaqueously by downslope mixing of gravel and cobbles with muddy facies. Rare horizons of 'ice rafted debris' in thin-bedded and laminated turbidite facies interbedded with thick debrites show a weak but positive correlation of lamina thickness with grain size, suggesting these facies are non-glacial co-genetic 'debrite-turbidite' couplets. A significant volcanic influence on sedimentation is identified from reworked lapilli tuff beds and reworked ash in turbidites. The depositional setting of the Squantum 'Tillite' appears to be that of a submarine slope/fan setting in an open marine volcanic arc basin receiving large volumes of poorly-sorted sediment on the mid-latitude active margin of Gondwana. No direct glacial influence is apparent.
Carto, S.L., and Eyles, N., 2012, Sedimentology of the Neoproterozoic (c. 580 Ma) Squantum ‘Tillite’, Boston Basin, USA: Mass flow deposition in a deep-water arc basin lacking direct glacial influence: Sedimentary Geology, v. 269–270, p. 1–14, doi:10.1016/j.sedgeo.2012.03.011.
The softest Boston rocks are argillite (related to slate and shale but harder) and volcanic ash. The argillite was originally deposited as clay in either a lake or marine embayment; the volcanic ash was blown out of the many volcanoes that were active in the area during the time the clay or mud was being deposited. Gravel was interlayered with the clay and cemented into a hard rock called "conglomerate," locally called "puddingstone." This rock crops out widely in Roxbury, Dorchester, and Brookline. Also interlayered with these sediments were volcanic flows, ashes, cinders, and the great variety of deposits formed by volcanoes. These deposits now hard rock are best seen in nearby Mattapan, Hyde Park, Milton, Lynn, and Saugus.
What was the ancient landscape like at the time the Boston rocks were being laid down? In all probability, the region was a broad lowland surrounded by hills or mountains of granite. A large body of water, either a large lake or perhaps an arm of the sea, occupied part of the lowland. Small volcanoes and at least one large cone that rose a mile or more in height dotted the plain and surrounding uplands. The ash from these volcanoes blanketed the plain and was carried by the rivers to the lake or sea where it mixed with silt and clay that was being carried by rivers from the upland. The rivers descending onto the plain from the surrounding highlands also carried gravel, which was deposited in flood plains and -which makes up much of the Roxbury "puddingstone."
Kaye, C. A., The Geology and Early History of the Boston Area of Massachusetts, A Bicentennial Approach, Geological, Survey Bulletin 1476 (1976).
What was the ancient landscape like at the time the Boston rocks were being laid down? In all probability, the region was a broad lowland surrounded by hills or mountains of granite. A large body of water, either a large lake or perhaps an arm of the sea, occupied part of the lowland. Small volcanoes and at least one large cone that rose a mile or more in height dotted the plain and surrounding uplands. The ash from these volcanoes blanketed the plain and was carried by the rivers to the lake or sea where it mixed with silt and clay that was being carried by rivers from the upland. The rivers descending onto the plain from the surrounding highlands also carried gravel, which was deposited in flood plains and -which makes up much of the Roxbury "puddingstone."
Kaye, C. A., The Geology and Early History of the Boston Area of Massachusetts, A Bicentennial Approach, Geological, Survey Bulletin 1476 (1976).
The Boston Basin originated in the Late Precambrian as a failed rift/successor basin, during the opening of the lapetus Ocean. Early sedimentation was characterized by a suite of bimodal volcanics and coarse debris flow deposits in the form of a fan. The geochemistry of the volcanics at the base of the succession suggests that the basin was connected to the open ocean very early in its history.
Subsequent rapid progradation of submarine slope and fan deposits occurred in the Late Precambrian/(?)Cambrian, including ice-derived diamictons and remobilized detritus, which show evidence of transport toward the north-northeast. During periods of more equable climate the slope became the site of sand and mud deposited by gravity and current. The last depositional event was characterized by the development of an overlap sequence in the (?)Late Precambrian/Cambrian. Global climate amelioration is attributed to a tectonically induced eustatic rise in sea level.
This stratigraphic history reflects the evolution of a passive margin during the opening of the lapetus, and eventual closing of the lapetus and comprehensive deformation of the Boston Basin from Ordovician to Carboniferous time. Sediment transport in the basin was largely longitudinal and toward the east-northeast.
Anthony D. Socci, Geoffrey W. Smith, Evolution of the Boston Basin: A Sedimentological Perspective, Sedimentary Basins and Basin-Forming Mechanisms — Memoir 12, 1987, Pages 87-99, Extensional Basins
The Boston basin is one of several late Paleozoic nonmarine sedimentary basins that developed in eastern New England subsequent to the Acadian revolution. Most of the sedimentary rocks in these basins are known to be Pennsylvanian in age; those in the Boston basin are presumably of this age. The principal map units—except for the Blue Hills and Nahant—are the Precambrian basement, the Mattapan and Lynn Volcanic Complexes (Mississippian?), and the Boston Bay Group (Pennsylvanian?). The Boston Bay Group consists of the Cambridge Argillite and the Roxbury Conglomerate. The Roxbury Conglomerate in turn is subdivided, from bottom to top, into the Brookline, Dorchester, and Squantum Members.
During the past 25 years, a series of bedrock tunnels, driven for water supply and drainage purposes, have added greatly to our knowledge. The tunnels, 3 to 3.5 m in diameter, are at a depth of 30 to 90 m below the surface. The total length of these tunnels is 39.57 km; the Dorchester Tunnel, under construction, is another 10.19 km long.
New observations and interpretations are as follows: (1) The maximum thickness of the Boston Bay Group is 5,700 m. (2) The Boston Bay Group thins to the south. (3) The Roxbury Conglomerate, with a maximum thickness of 1,310 m, is a southerly facies of the lower part of the Cambridge Argillite. (4) The Cambridge Argillite reaches a maximum thickness of 5,700 m in the northern part of the basin. (5) The sedimentary rocks were derived from a highland to the south.
The most important new results that bear on structure are that (1) the Northern border fault, where exposed in a tunnel, dips 55°N; (2) the Charles River syncline, exposed in two tunnels 10.5 km apart, plunges 19° in a direction N84°E; and (3) many minor folds and faults complicate the structure.
Although no tunnel crosses the Blue Hills, a new interpretation of the structure is presented. The volcanic complex of that area was erupted onto flat-lying Cambrian sedimentary rocks. The Quincy Granite and Blue Hill Granite Porphyry were injected into the horizontal Cambrian strata and volcanic complex. After a period of uplift and erosion, the Pennsylvanian strata of the Norfolk basin were deposited. All the rocks were then folded into a syncline, the vertical north limb of which is now the Blue Hills. The Blue Hills were then thrust northward over the Boston basin.
Marland P. Billings, Geology of the Boston Basin, January 01, 1976, https://doi.org/10.1130/MEM146-p5
Subsequent rapid progradation of submarine slope and fan deposits occurred in the Late Precambrian/(?)Cambrian, including ice-derived diamictons and remobilized detritus, which show evidence of transport toward the north-northeast. During periods of more equable climate the slope became the site of sand and mud deposited by gravity and current. The last depositional event was characterized by the development of an overlap sequence in the (?)Late Precambrian/Cambrian. Global climate amelioration is attributed to a tectonically induced eustatic rise in sea level.
This stratigraphic history reflects the evolution of a passive margin during the opening of the lapetus, and eventual closing of the lapetus and comprehensive deformation of the Boston Basin from Ordovician to Carboniferous time. Sediment transport in the basin was largely longitudinal and toward the east-northeast.
Anthony D. Socci, Geoffrey W. Smith, Evolution of the Boston Basin: A Sedimentological Perspective, Sedimentary Basins and Basin-Forming Mechanisms — Memoir 12, 1987, Pages 87-99, Extensional Basins
The Boston basin is one of several late Paleozoic nonmarine sedimentary basins that developed in eastern New England subsequent to the Acadian revolution. Most of the sedimentary rocks in these basins are known to be Pennsylvanian in age; those in the Boston basin are presumably of this age. The principal map units—except for the Blue Hills and Nahant—are the Precambrian basement, the Mattapan and Lynn Volcanic Complexes (Mississippian?), and the Boston Bay Group (Pennsylvanian?). The Boston Bay Group consists of the Cambridge Argillite and the Roxbury Conglomerate. The Roxbury Conglomerate in turn is subdivided, from bottom to top, into the Brookline, Dorchester, and Squantum Members.
During the past 25 years, a series of bedrock tunnels, driven for water supply and drainage purposes, have added greatly to our knowledge. The tunnels, 3 to 3.5 m in diameter, are at a depth of 30 to 90 m below the surface. The total length of these tunnels is 39.57 km; the Dorchester Tunnel, under construction, is another 10.19 km long.
New observations and interpretations are as follows: (1) The maximum thickness of the Boston Bay Group is 5,700 m. (2) The Boston Bay Group thins to the south. (3) The Roxbury Conglomerate, with a maximum thickness of 1,310 m, is a southerly facies of the lower part of the Cambridge Argillite. (4) The Cambridge Argillite reaches a maximum thickness of 5,700 m in the northern part of the basin. (5) The sedimentary rocks were derived from a highland to the south.
The most important new results that bear on structure are that (1) the Northern border fault, where exposed in a tunnel, dips 55°N; (2) the Charles River syncline, exposed in two tunnels 10.5 km apart, plunges 19° in a direction N84°E; and (3) many minor folds and faults complicate the structure.
Although no tunnel crosses the Blue Hills, a new interpretation of the structure is presented. The volcanic complex of that area was erupted onto flat-lying Cambrian sedimentary rocks. The Quincy Granite and Blue Hill Granite Porphyry were injected into the horizontal Cambrian strata and volcanic complex. After a period of uplift and erosion, the Pennsylvanian strata of the Norfolk basin were deposited. All the rocks were then folded into a syncline, the vertical north limb of which is now the Blue Hills. The Blue Hills were then thrust northward over the Boston basin.
Marland P. Billings, Geology of the Boston Basin, January 01, 1976, https://doi.org/10.1130/MEM146-p5
The bathymetry and sidescan-sonar data show natural features and sea floor modification from anthropogenic activities. Dredging and other anthropogenic activities are generally focused in the shipping channels. Evidence of dredging is visible within the imagery as straight-sided channels, unnatural-appearing roughness and/or linear features on the sea floor that are typically oriented parallel to a channel. Disposal of dredged material is clearly displayed within the multibeam echosounder data as rounded mounds; often occurring in a straight line, some have a central high and a surrounding moat thought to be created as the material was deposited on the sea floor. The mounds sometimes are identified in the sidescan-sonar by high backscatter intensity, but are not always resolved. Other anthropogenic features on the sea floor include wrecks of small boats and barges, pipelines, and piles of debris. Almost all of the Inner Harbor from Castle Island to Long Wharf was mapped by multibeam echosounder. In the Outer Harbor and the Harbor Approaches, the 2-m resolution multibeam echosounder data are displayed with the 30-m resolution single-beam echosounder data; interpretation of features and their spatial extent is limited by these mixed observations.
The sea-floor landscape varies from gently sloping sub-tidal flats to areas of rugged elevation exhibiting as much as 7 m of local relief (sheet 1, fig. 3.6). The acoustic backscatter intensity (sheet 3, fig. 3.7) illustrates the general distribution of surficial sediment. The approaches to Boston Harbor and the dredged navigation channels around the Harbor Islands are generally characterized by high backscatter, bedrock, boulder, cobbles, or dense shell beds. The Inner and Outer Harbor are primarily composed of fine-grained sediments, such as fine sand or mud, which displays as low backscatter within the sidescan-sonar imagery (fig. 4.3).
Sea-floor topography and surficial character in the study area vary at scales of several meters and less. For example, high relief bedrock and bouldery glacial deposits (till) are commonly exposed on the sea floor in close proximity to flat-lying deposits of finer sediment (sand, mud). Rocky areas sometimes contain isolated accumulations of shelly sediment that are ponded in small cracks or low-lying areas between rock outcrops.
High-Resolution Geologic Mapping of the Inner Continental Shelf: Boston Harbor and Approaches, Massachusetts
https://pubs.usgs.gov/of/2006/1008/html/discussion.html
Paleocontinental reconstructions (Fig. 1) show that the Iapetus Ocean was initiated by the separation of continental landmasses that had previously collided during the ca. 1.1–0.9 Ga Grenville orogeny during the amalgamation of Rodinia (e.g. Cawood et al., 2001; Rivers, 2009). Geologic, geochronologic and paleomagnetic data from rocks
formed along the Laurentian margin of Iapetus are consistent with a multi-stage rift history in which separation from Baltica occurred between 620 and 570 Ma, and separation from West Gondwana occurred at ca. 570 Ma (Cawood et al., 2001; Cawood and Pisarevsky, 2006). The development of the classic Humber Zone passive margin,
which extends ca. 2000 km along the Laurentian margin, occurred in the interval ca. 540–535 Ma with the separation of peri-Laurentian microcontinents (e.g. Meert et al., 1998; Cawood et al., 2001) known as the Dashwood terrane in the northern Appalachians (Waldron and van Staal, 2001) and the Precordillera in the southern Appalachians
(Thomas and Astini, 1996, 1999). The mechanisms responsible for the origin of Iapetus are unclear, largely because of uncertainties in the paleomagnetic record between
615 and 550 Ma, when high latitude and low latitude positions for Laurentia (and by inference West Gondwana) are permissible (Pisarevsky et al., 2000, 2001, 2008; Murphy et al., 2004; Dalziel, in press). According to Pisarevsky et al. (2008), the location of magmatic provinces and orientations of dyke swarms emplaced during this
interval support the existence of a mantle plume located between Laurentia, Baltica and Amazonia, which then evolved to become a rift– rift–rift triple junction (see also Cawood et al., 2001). In this context, the Dashwood microcontinent would reflect a late-stage branch of the rift system that propagated inboard of the Laurentian margin (Waldron and van Staal, 2001). According to Thomas and Astini (1999), the separation of the Precordillera from Laurentia was accomplished by asymmetric rifting
along a low-angle detachment with the western Precordillera positioned along the lower plate. These relationships indicate that both the Dashwood terrane and the Precordillera formed microcontinental blocks within the Iapetus Ocean during the Cambrian.Comparative evolution of the Iapetus and Rheic Oceans: A North America perspective, Gondwana Research 17 (2010) 482–499
Avalonian terrane makes up a large portion of eastern Massachusetts but is not the only microcontinent terrane found in the northeastern part of North America, because Avalonia was not the only ancient microcontinent that made a journey from the edge of Gondwana to Laurentia (more on that here). Ganderia, which was accreted to Laurentia before Avalonia, formed off the edge of the Amazonian craton rather than in the ocean like Avalonia. Ganderia and Avalonia were geographically in proximity to each other when they were rifted away from Gondwana by the opening of the Rheic Ocean. Ganderia’s accretion to Laurentia happened with the closing of the Iaepetus Ocean and resulted in a significant mountain building event, the Salinian orogen. When Avalonian contacted Ganderian terrane, it was thrust under it; the contact point between Avalonian terrane and Ganderian (Nashoba) terrane is clearly demarcated in Massachusetts by the Bloody Bluff fault (see Figure at left). Like much of New England, the bedrock of Avalonia is extensively igneous or metamorphosed igneous rock.
A bedrock geologic map developed by Thompson et al. 2014 (figure left below) provides more detail on the Avalonian rocks of the Boston basin. The red outline in the map shows the boundaries of Brookline. The rock formations within the Boston basin mark the transition of this part of Avalonia from subduction-island arc magmatism (Act 3) to transform faulting-rifting magmatism and to passive margin sedimentation (Act 4). The Boston basin may originally have formed from shear stress exerted on the Avalonian island arc during subduction, as the angle of subduction was relatively low, exerting significant transverse stress on the arc margin (Figure right below). The combination of this subduction and the shear stress could have initiated pull-apart basins in the arc.
The rocks of the Boston Basin are a repository of a specific period in Avalonia, during the time it made a transition from an active volcanic arc to a passive arc platform. The period of time these rocks cover is about 610 Ma to 540 Ma (Neoproterozoic--Early Cambrian). The figure below shows the time period for deposit of these different rocks. Ages have been established through a variety of isotopic techniques appropriate to rocks of this age. The summary below covers these rocks in chronological order of their formation.
The Avalonian Rocks of the Boston Basin and Their Ancient Depositional Environment, https://www.brooklinerocks.org/avalonian-rocks-of-brookline
A GRAVITY SURVEY OF THE BOSTON BASIN REGION By Merrill S. Ginsburg Submitted to the Department of Geology and Geophysics on October 10, 1959, in partial fulfillment of the requirements for the degree of Master of Science in Geology and Geophysics. ABSTRACT One hundred sixty eight gravity stations were occupied in the Boston Basin area, and Bouguer anomalies were ascertained for the purpose of determining or corroborating facts about the geology and structure of the basin and surrounding region. The relative accuracy of the anomaly determinations is 0.22 milligals. The Bouguer anomaly contour map bears out some of the known phenomena in the region outside of the Boston Basin, but fails to indicate others. Three predominant highs are associated with the occurrence of Salem gabbro-diorite - the rock of greatest density in the region. Hence, it is concluded that the situation and thickness of the Salem throughout the region is the primary cause for the pattern of the gravity contours. Over the main part of the Boston Basin, the gravity contour lines trend east-west. The gradient of over +2 milligals to the north is greater than, and nearly perpendicular to, the regional trend of the area. In the southwest corridor of the basin, a gravity ridge is seen to be in correlation with the stratified formations which are of greater density than the bordering igneous rocks of the area. Hence, the Boston Basin is manifested by the iso-anomaly map. Two profiles, taken in a general north-south direction across the main portion of the Boston Basin, are approximately "U "-shaped, with the low centered over the Quincy granite, which borders the basin on the south. It was found that the profiles could best be interpreted by considering the flanks of each profile "U" separately. The right flanks indicate that the density contrast between the Salem gabbro-diorite and, to the north, the Dedham granodiorite and Quincy granite extends to a maximum depth of over 4400 feet. The left flanks show manifestations of two of the three principal structural units of the basin: the central anticline and southern shingle-block zone. The contour map indicates a gentle plunging of the Boston Basin sediments to the east, corroborating the findings of geologic investigators. But the contours also indicate a sharp upswing of dense basement rocks in Boston Bay. This contradicts the belief of certain investigators. A northern boundary fault is implied by s-shaped offsets of the gravity contour lines. The fault may be continuous from Lynn to Natick, although the s-shaped offset pattern is not apparent between Arlington and Waltham. The northern boundary fault is also manifested slightly on one of the northsouth profiles. No conclusive evidence is found for the presence of a southern boundary fault. Interpretation is hampered by the low density contrast between the major rock formations of the region - only 0. 3 gm/cm3 separates the densities of the rocks of greatest and least density - and is also hampered by a thin layer of low density glacial deposits of undertermined thickness, the total effect of which is not definitely known. Thesis Supervisor: William F. Brace Title: Assistant Professor of Geology
https://dspace.mit.edu/bitstream/handle/1721.1/60436/32553214-MIT.pdf?sequence=2&isAllowed=y
The sea-floor landscape varies from gently sloping sub-tidal flats to areas of rugged elevation exhibiting as much as 7 m of local relief (sheet 1, fig. 3.6). The acoustic backscatter intensity (sheet 3, fig. 3.7) illustrates the general distribution of surficial sediment. The approaches to Boston Harbor and the dredged navigation channels around the Harbor Islands are generally characterized by high backscatter, bedrock, boulder, cobbles, or dense shell beds. The Inner and Outer Harbor are primarily composed of fine-grained sediments, such as fine sand or mud, which displays as low backscatter within the sidescan-sonar imagery (fig. 4.3).
Sea-floor topography and surficial character in the study area vary at scales of several meters and less. For example, high relief bedrock and bouldery glacial deposits (till) are commonly exposed on the sea floor in close proximity to flat-lying deposits of finer sediment (sand, mud). Rocky areas sometimes contain isolated accumulations of shelly sediment that are ponded in small cracks or low-lying areas between rock outcrops.
High-Resolution Geologic Mapping of the Inner Continental Shelf: Boston Harbor and Approaches, Massachusetts
https://pubs.usgs.gov/of/2006/1008/html/discussion.html
Paleocontinental reconstructions (Fig. 1) show that the Iapetus Ocean was initiated by the separation of continental landmasses that had previously collided during the ca. 1.1–0.9 Ga Grenville orogeny during the amalgamation of Rodinia (e.g. Cawood et al., 2001; Rivers, 2009). Geologic, geochronologic and paleomagnetic data from rocks
formed along the Laurentian margin of Iapetus are consistent with a multi-stage rift history in which separation from Baltica occurred between 620 and 570 Ma, and separation from West Gondwana occurred at ca. 570 Ma (Cawood et al., 2001; Cawood and Pisarevsky, 2006). The development of the classic Humber Zone passive margin,
which extends ca. 2000 km along the Laurentian margin, occurred in the interval ca. 540–535 Ma with the separation of peri-Laurentian microcontinents (e.g. Meert et al., 1998; Cawood et al., 2001) known as the Dashwood terrane in the northern Appalachians (Waldron and van Staal, 2001) and the Precordillera in the southern Appalachians
(Thomas and Astini, 1996, 1999). The mechanisms responsible for the origin of Iapetus are unclear, largely because of uncertainties in the paleomagnetic record between
615 and 550 Ma, when high latitude and low latitude positions for Laurentia (and by inference West Gondwana) are permissible (Pisarevsky et al., 2000, 2001, 2008; Murphy et al., 2004; Dalziel, in press). According to Pisarevsky et al. (2008), the location of magmatic provinces and orientations of dyke swarms emplaced during this
interval support the existence of a mantle plume located between Laurentia, Baltica and Amazonia, which then evolved to become a rift– rift–rift triple junction (see also Cawood et al., 2001). In this context, the Dashwood microcontinent would reflect a late-stage branch of the rift system that propagated inboard of the Laurentian margin (Waldron and van Staal, 2001). According to Thomas and Astini (1999), the separation of the Precordillera from Laurentia was accomplished by asymmetric rifting
along a low-angle detachment with the western Precordillera positioned along the lower plate. These relationships indicate that both the Dashwood terrane and the Precordillera formed microcontinental blocks within the Iapetus Ocean during the Cambrian.Comparative evolution of the Iapetus and Rheic Oceans: A North America perspective, Gondwana Research 17 (2010) 482–499
Avalonian terrane makes up a large portion of eastern Massachusetts but is not the only microcontinent terrane found in the northeastern part of North America, because Avalonia was not the only ancient microcontinent that made a journey from the edge of Gondwana to Laurentia (more on that here). Ganderia, which was accreted to Laurentia before Avalonia, formed off the edge of the Amazonian craton rather than in the ocean like Avalonia. Ganderia and Avalonia were geographically in proximity to each other when they were rifted away from Gondwana by the opening of the Rheic Ocean. Ganderia’s accretion to Laurentia happened with the closing of the Iaepetus Ocean and resulted in a significant mountain building event, the Salinian orogen. When Avalonian contacted Ganderian terrane, it was thrust under it; the contact point between Avalonian terrane and Ganderian (Nashoba) terrane is clearly demarcated in Massachusetts by the Bloody Bluff fault (see Figure at left). Like much of New England, the bedrock of Avalonia is extensively igneous or metamorphosed igneous rock.
A bedrock geologic map developed by Thompson et al. 2014 (figure left below) provides more detail on the Avalonian rocks of the Boston basin. The red outline in the map shows the boundaries of Brookline. The rock formations within the Boston basin mark the transition of this part of Avalonia from subduction-island arc magmatism (Act 3) to transform faulting-rifting magmatism and to passive margin sedimentation (Act 4). The Boston basin may originally have formed from shear stress exerted on the Avalonian island arc during subduction, as the angle of subduction was relatively low, exerting significant transverse stress on the arc margin (Figure right below). The combination of this subduction and the shear stress could have initiated pull-apart basins in the arc.
The rocks of the Boston Basin are a repository of a specific period in Avalonia, during the time it made a transition from an active volcanic arc to a passive arc platform. The period of time these rocks cover is about 610 Ma to 540 Ma (Neoproterozoic--Early Cambrian). The figure below shows the time period for deposit of these different rocks. Ages have been established through a variety of isotopic techniques appropriate to rocks of this age. The summary below covers these rocks in chronological order of their formation.
The Avalonian Rocks of the Boston Basin and Their Ancient Depositional Environment, https://www.brooklinerocks.org/avalonian-rocks-of-brookline
A GRAVITY SURVEY OF THE BOSTON BASIN REGION By Merrill S. Ginsburg Submitted to the Department of Geology and Geophysics on October 10, 1959, in partial fulfillment of the requirements for the degree of Master of Science in Geology and Geophysics. ABSTRACT One hundred sixty eight gravity stations were occupied in the Boston Basin area, and Bouguer anomalies were ascertained for the purpose of determining or corroborating facts about the geology and structure of the basin and surrounding region. The relative accuracy of the anomaly determinations is 0.22 milligals. The Bouguer anomaly contour map bears out some of the known phenomena in the region outside of the Boston Basin, but fails to indicate others. Three predominant highs are associated with the occurrence of Salem gabbro-diorite - the rock of greatest density in the region. Hence, it is concluded that the situation and thickness of the Salem throughout the region is the primary cause for the pattern of the gravity contours. Over the main part of the Boston Basin, the gravity contour lines trend east-west. The gradient of over +2 milligals to the north is greater than, and nearly perpendicular to, the regional trend of the area. In the southwest corridor of the basin, a gravity ridge is seen to be in correlation with the stratified formations which are of greater density than the bordering igneous rocks of the area. Hence, the Boston Basin is manifested by the iso-anomaly map. Two profiles, taken in a general north-south direction across the main portion of the Boston Basin, are approximately "U "-shaped, with the low centered over the Quincy granite, which borders the basin on the south. It was found that the profiles could best be interpreted by considering the flanks of each profile "U" separately. The right flanks indicate that the density contrast between the Salem gabbro-diorite and, to the north, the Dedham granodiorite and Quincy granite extends to a maximum depth of over 4400 feet. The left flanks show manifestations of two of the three principal structural units of the basin: the central anticline and southern shingle-block zone. The contour map indicates a gentle plunging of the Boston Basin sediments to the east, corroborating the findings of geologic investigators. But the contours also indicate a sharp upswing of dense basement rocks in Boston Bay. This contradicts the belief of certain investigators. A northern boundary fault is implied by s-shaped offsets of the gravity contour lines. The fault may be continuous from Lynn to Natick, although the s-shaped offset pattern is not apparent between Arlington and Waltham. The northern boundary fault is also manifested slightly on one of the northsouth profiles. No conclusive evidence is found for the presence of a southern boundary fault. Interpretation is hampered by the low density contrast between the major rock formations of the region - only 0. 3 gm/cm3 separates the densities of the rocks of greatest and least density - and is also hampered by a thin layer of low density glacial deposits of undertermined thickness, the total effect of which is not definitely known. Thesis Supervisor: William F. Brace Title: Assistant Professor of Geology
https://dspace.mit.edu/bitstream/handle/1721.1/60436/32553214-MIT.pdf?sequence=2&isAllowed=y
Kuiper, Yvette D., et al. "U-Pb, Lu-Hf, and trace element zircon data from plutonic rocks of the New England Avalon terrane, USA." Atlantic Geoscience, volume 61, 2025, p. 281–304. https://doi.org/10.4138/atlgeo.2025.011
Glacial deposits mantle much of the bedrock ana drumlins are prominent features. Recent alluvium and marine deposits also are extensive. The submarine topography suggests that glacial deposits form much of the submarine area along with bedrock outcrops. Glacial ground moraine deposits comprise the major part of the sediments in the area. Ground moraine is characteristically heterogeneous in composition and arrangement, and this condition is fully met in Boston Harbor. Deposits are found to be patchy, and adjacent deposits commonly vary greatly in texture and thickness. In summary, the sediments underlying Boston Harbor are probably mostly glacial in origin. There has been little later sedimentation.
Literature Survey of Oceanographic Information Concerning Boston Harbor, Office of Naval Research, Woods Hole, Massachusetts, Reference No. 51- 84, (October 1951).
Literature Survey of Oceanographic Information Concerning Boston Harbor, Office of Naval Research, Woods Hole, Massachusetts, Reference No. 51- 84, (October 1951).
A very large erratic boulder is incorporated into its base in the Fort Point Channel at the MBTA Silver Line crossing (Leifer, 2006) During dredging for the immersed tube placement, a 6 by 6 by 2.4 meter (20 by 20 by 8 foot) glacial erratic boulder was found at the clay-till contact and had to be broken up in place before dredging could be completed. Leifer, A.L., 2006, tunnel information for new Silver Line, written communication, March, 31, 2006, 1 p.
The integrated thickness of the Cambridge Formation in the northern Boston Basin is 5350 m assuming that CTE and MDT-NMRT sections overlap as shown in figure 8. This total is in unexpectedly good agreement with the 5700 m estimate of
Billings (1975 and 1976) given the revised interpretations outlined above in each of the tunnels.
Thompson, AVALONIAN ARC-TO-PLATFORM TRANSITION IN SOUTHEASTERN NEW ENGLAND
The tectonic significance of the Cambridge Formation which is poorly exposed in the Boston Basin, eastern Massachusetts, but transected by 50 km of tunnels beneath the mainland and Boston Harbor. The youngest detrital zircon in a sample from the northern Braintree Weymouth Tunnel establishes a maximum depositional age of 584.09 1.98 Ma, consistent with sources in sills of that age in underlying Roxbury Conglomerate. A 551.22 0.20 Ma ash bed from the Mystic Quarry in Somerville, Massachusetts lies near the top of an approximately 5350 m thick, dominantly argillaceous section measured in subsurface cross sections. These were constructed from attitudes reported in pre-1960 tunnels and from mapping logs obtained from tunnels completed decades later during the federally ordered clean-up of Boston Harbor. A 488.58 0.16 Ma aplite sill intruding argillite 800 m above the ash bed sets the minimum depositional age on the north side of the Basin. A tighter constraint comes from trilobite-bearing strata of the lower Cambrian Weymouth Formation located south of Boston that overlies the Cambridge Formation without obvious break in the Braintree Weymouth Tunnel. If Cambridge deposition was continuous after 584 Ma, the depositional interval would exceed 40 million years. An estimated 20 Ma depositional hiatus seems more likely because the base of the Cambridge Formation appears to define a regional unconformity above which argillite rests variously on magmatic arc-related units of both the 595 to 584 Ma Roxbury Conglomerate and the 597 to 593 Ma Lynn-Mattapan Volcanic Complex. Cambridge deposition set in once arc activity in more northerly “West” Avalonian terranes extending through Atlantic Canada to the Avalon Peninsula, Newfoundland had given way to wrench faulting and bimodal magmatism. This regime is manifested structurally in Boston-area tunnels by later-reactivated normal faults in which hanging wall blocks of Cambridge argillite were originally downthrown relative to older footwall units. Pyroclastic volcanic textures and thin basaltic flows with soft sediment contacts are present in argillite of the City Tunnel Extension, and whole rock major element and REE compositions reveal mixed terrigenous and volcanic components deposited under marine conditions throughout the Basin. Proposed sources for the latter are voluminous eruptions recorded in the 560 to 550 Ma Coldbrook Group in New Brunswick’s Caledonia terrane.
More accurate and precise crystallization ages now available for Boston-area granites and volcanic rocks (table 1) reinforce cross-terrane correlations with arc-related sequences in more northerly Avalonian terranes (fig. 1). However, a combined chemical abrasion-thermal ionization mass spectrometry [CA-TIMS] and laser ablation-inductively coupled mass spectrometry [LA-ICPMS] approach to dating the Roxbury Conglomerate establishes 595 to 584 Ma U-Pb age constraints that also fall within the interval of arc activity (M. Thompson and others, 2014). Here we present LA-ICPMS and CA-TIMS results indicating that aerially more extensive and much thicker marine deposits of the Cambridge Formation (fig. 2) are significantly younger than the previously reported maximum age of 570 Ma (Thompson and Bowring, 2000). The new dates suggest a considerable Roxbury-Cambridge depositional hiatus and raise many questions about the tectonic scenario preceding platform deposition in SE New England.
Thompson & Crowley; AVALONIAN ARC-TO-PLATFORM TRANSITION IN SOUTHEASTERN NEW ENGLAND: U-Pb GEOCHRONOLOGY AND STRATIGRAPHY OF EDIACARAN CAMBRIDGE “ARGILLITE,” BOSTON BASIN, MASSACHUSETTS, USA; American Journal of Science, Vol. 320, May, 2020, P. 405–449, DOI 10.2475/05.2020.01]
More accurate and precise crystallization ages now available for Boston-area granites and volcanic rocks (table 1) reinforce cross-terrane correlations with arc-related sequences in more northerly Avalonian terranes (fig. 1). However, a combined chemical abrasion-thermal ionization mass spectrometry [CA-TIMS] and laser ablation-inductively coupled mass spectrometry [LA-ICPMS] approach to dating the Roxbury Conglomerate establishes 595 to 584 Ma U-Pb age constraints that also fall within the interval of arc activity (M. Thompson and others, 2014). Here we present LA-ICPMS and CA-TIMS results indicating that aerially more extensive and much thicker marine deposits of the Cambridge Formation (fig. 2) are significantly younger than the previously reported maximum age of 570 Ma (Thompson and Bowring, 2000). The new dates suggest a considerable Roxbury-Cambridge depositional hiatus and raise many questions about the tectonic scenario preceding platform deposition in SE New England.
Thompson & Crowley; AVALONIAN ARC-TO-PLATFORM TRANSITION IN SOUTHEASTERN NEW ENGLAND: U-Pb GEOCHRONOLOGY AND STRATIGRAPHY OF EDIACARAN CAMBRIDGE “ARGILLITE,” BOSTON BASIN, MASSACHUSETTS, USA; American Journal of Science, Vol. 320, May, 2020, P. 405–449, DOI 10.2475/05.2020.01]
"The bottom in the Boston Harbor area is characterized by considerable relief. Isolated hills and depressions closely resemble the land topography. The bedrock surface in the vicinity of Boston appears to be extremely irregular. A few elongated deep areas may represent former stream valleys of Pleistocene or pre-Pleistocene age. Construction of a detailed bedrock contour map of the harbor area is not warranted because of the sparsity of data. Under Boston Harbor, data on the depth of bedrock are practically nonexistent except for determinations for dredging in the main ship channel in the inner harbor.
Known deep points in the bedrock generally lie below low areas in the surface topography. The more important deep areas include... deep areas at the west side of Fort Point Channel both north and south of South Station... Locations of the deep points in the bedrock suggest a valley from the Malden bridge southeast across Little Mystic Channel to the inner harbor where it joins a valley trending northerly from Fort Point Channel through the present inner harbor to the northwestern part of East Boston. Another valley appears to cross the Boston peninsula near the Public Garden in a southeasterly direction toward the Fort Point Channel."
Literature Survey of Oceanographic Information Concerning Boston Harbor, Office of Naval Research, Woods Hole, Massachusetts, Reference No. 51- 84, (October 1951).
Known deep points in the bedrock generally lie below low areas in the surface topography. The more important deep areas include... deep areas at the west side of Fort Point Channel both north and south of South Station... Locations of the deep points in the bedrock suggest a valley from the Malden bridge southeast across Little Mystic Channel to the inner harbor where it joins a valley trending northerly from Fort Point Channel through the present inner harbor to the northwestern part of East Boston. Another valley appears to cross the Boston peninsula near the Public Garden in a southeasterly direction toward the Fort Point Channel."
Literature Survey of Oceanographic Information Concerning Boston Harbor, Office of Naval Research, Woods Hole, Massachusetts, Reference No. 51- 84, (October 1951).
"The future site of Boston then lay at the east side of the lapetus Ocean in a volcano-studded region along the coast of the larger continent now called Gondwana, which is now northwest Africa. The region was undergoing considerable volcanic activity, with sand and more heterogeneous volcanic rock being deposited. Clean sand was laid down near shore between influxes of fine volcanic material and eventually overwhelmed by an irregular mixture of mafic lava flows, including some pillow basalt, basaltic and rhyolitic tuff and some calcareous mud. A deeper marine basin that lay offshore, perhaps around volcanic islands, was filling with a thick sequence of volcaniclastic sediments carried in from the southeast. These sediments were dirty sands with much andesitic debris, interlayered with tuff, tuffaceous silt and mud, aluminous mud, limy mud, thin calcareous material and mafic (dark, low-silica rock such as basalt and andesite) flows. Much of the sediment was carried by turbidity flows and laid down as graded beds that shaped the Nashoba and adjacent formations west of Boston. The Gondwana plate moved relatively westward to squeeze the Iapetus Ocean. The western edge of its plate boundary collided against the deep marine basin offshore of Laurentia and slid under it during the Late Proterozoic.
The initial terrestrial and near-shore rhyolite and andesite flow and ash were spewed from volcanoes that were triggered by the faulting and rose up the fault zone bordering the south side of the basin. The debris intermixed with, and then was succeeded by, boulder fans shed northward from the adjacent highlands. The boulder deposit and sand graded north, downslope into mud and silt, which quietly accumulated in marine waters that invaded from the east as the area began to subside. Earthquakes during deposition caused occasional basinward slumping and sliding of the sediments."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Regional Geologic Setting for the
Boston Area, Civil Engineering Practice, Vol. 26-27 (2011/2012).
The initial terrestrial and near-shore rhyolite and andesite flow and ash were spewed from volcanoes that were triggered by the faulting and rose up the fault zone bordering the south side of the basin. The debris intermixed with, and then was succeeded by, boulder fans shed northward from the adjacent highlands. The boulder deposit and sand graded north, downslope into mud and silt, which quietly accumulated in marine waters that invaded from the east as the area began to subside. Earthquakes during deposition caused occasional basinward slumping and sliding of the sediments."
Barosh & Woodhouse, A City Upon a Hill: Geology of the City of Boston & Surrounding Area, Regional Geologic Setting for the
Boston Area, Civil Engineering Practice, Vol. 26-27 (2011/2012).
Geology of the Boston Basin (2011/2012)
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"There is surprisingly little unconsolidated fault breccia or gouge lining the basin faults. However, some faults have silicified cataclastic material, which is difficult to characterize. At present, the average lateral-spacing throughout the area for the larger faults is. indicated to be about 150 meters (490 feet), measured in any direction, although the density of faults varies from place to place and many more small ones exist, as can be seen in all areas of good exposure (Crosby, 1893 & 1894; Bell, 1975a; Wolf, 1976; Ross & Bailey, CML 2001; Ross 2001; Metcalf & Eddy, 1990b) and in tunnels (Clarke, 1888; Kaye, 1980a; Barosh & Woodhouse, 1990; Davidson, 2003). Detailed mapping in the Wellesley Extension Interceptor Tunnel in Dedham, just off the southwest side of the basin, shows closely spaced faults and very complex joint systems (see Figure 3-53). Seven joint trends, chiefly with steep dips, are recognized near the Blue Hills (Chute, 1966) and most, if not all, are throughgoing ones that reflect the fault sets present. The fault pattern varies with the amount of data. That pattern in central Boston appears rather simple (see Figure 3-51), but just to the south between Dorchester and Milton the surface and tunnel exposures show a complexity more representative of the basin (see Figure 3-54). In that area, the general east-northeast structural trends are cut by northeast i.')nd north faults along with a few northwest-trend- .ing ones. The late north-trending faults are prominent across the southern -side of the basin and the late northwest-trending ones more common in the northeast, as is reflected in the bottom topography in Massachusetts Bay and outer islands and trend of the late dikes (see Figure 3-52). Any folds within the longitudinal fault blocks trend parallel to the fault blocks and may pluhge to the east (Crosby, 1880; LaForge, 1932; Kaye, 1980a). Various anticlines and synclines have been interpreted in the Boston Bas.in in the past, but the designations and placements have varied greatly between researchers depending on the data available. However, none of these proposed folds continue along strike into the older metamorphosed strata and granitic rock to the west (Nelson; 1975a & 1975b; Barosh, 1977b) nor are any seen to the north or south of the basin. The older rock to the west does show broad folds, but the fold axes trend northerly (Barosh, 1972 & 2005, Barash & Hermes, 1981) and a nose of a broad northerly plung.ing fold (see Figures 2-17 & 2-18) lies just west of the Boston Bas.in (Nelson, 1975b). These are syntectonic folds, which formed during the intrusion of the . batholithic granite in the Late Proterozoic prior to the formation of the Boston Basin (Barash, 1972 & 2005) and no younger regional folds have been found to cross them. What folds that may be present in the Boston Basin, therefore, cannot be related to any regional folding involving the basement, but are shallow features related to various motions on the longitudinal faults and are therefore drag folds. Many of the earlier interpreted folds are discovered to be tilted fault blocks by the various tunnel exposures. Laforge (1932) in effect labeled each area of older rock in the basin an anticline, although he also showed faults bounding their south sides (see Figure 1-30). Billings (1929, 1976a & 1976b) hypothesized that some of LaForge's faults could be additional folds and later enhanced the size of small folds when he summarized his student's tunnel mapping. He likely interpreted the slump, drag and small-scale folds as indicative of large-scale features (see Figure 1-33). The large-scale map of the City Tunnel Extension shows considerable faulting and apparent drag folds, but the generalized section by Billings and Tierney (1964) does not, in contrast to Kaye's (1980) faulted summary of the geology. Kaye (1984a) considered possible folds between the longitudinal faults, but his map data (1980a) show many of these are fault repetitions. Others, such as Cazier (1987), interpreted different periods of folding in the Norfolk Basin from cleavage where mapping only indicates drag folds. Just a single large fold, the Needham-Savin anticline, appears supported by the dips of the strata at present iri. the basin, but its axis lies to the south of · where LaForge placed it (see Figure 1-30),
Reinterpretations of LaForge's data as folds by Billings have not been supported by additional mapping. Billings (1982a) renamed the faults and folds shown by Laforge (1932) and · -reinterpreted a slice of Roxbury Conglomerate in LaForge's Rock Island Fault on Hough's Neck on the southeast side of the basin as the core of a Hough's Neck Anticline (see Figures 1-30 & 1-33). Borings and mapping of the Braintree-Weymouth Tunnel in Hingham, which crosses this area, shows that not only is the Rock Island Fault present (see Figure 3-55), but many smaller faults are as well (Davidson, 2003). Another example is the Northern or Charles River syncline, which was shown with an axis along the Charles River (see Figure 1-33). The contrasting lithology across the axial zone was interpreted as due to fades change, but the change is so abrupt it demonstrates instead the presence of a fault (see Figure 3-56). The reversal of dips at the river marks a fault zone and the south-dipping strata to the north of it are in a rotated block between this fault and the Northern Boundary Fault. However, some smaller folds are present along the northern edge of the basin. These folds appear to be from drag associated with the reverse faults of the Northern Boundary Fault Zone. Billings (1929) removed faults between three of LaForge' s anticlines to form a single large Central anticline (see Figures 1- 30, 1-32 & 1-33), but recent data show that these faults are present (Kaye, 1982b ). The Needham-Savin Anticline lies along the south side of this "Central Anticline." . The original rift basin was a half-graben, dropping the rock down to form a basin to the north. It had extended much farther to the north, but this portion was later cut off and its original width is unknown. The basin (see Figure 3-10) may have faced a narrow arm of the sea or a gulf to the north and had a similar tectonic and structural setting as the contemporary rift at Saint Johns, New Brunswick (Barosh, 1995), and both basins apparently formed as part of a broader basin-and-range topography (Kaye, 1984a). The position of the Mattapan Volcanic Complex centered in Needham apparently was controlled by the initial faults. The Blue Hills igneous complex, which was a volcanic center in the Late Ordovician, was controlled apparently by the border faults as well. The complex interfingering of the volcanic rock, conglomerate and argillite (along with the slumps, mass movement and graded beds) indicates a rapidly ris"' ing upland to the south consistent with longitudinal normal faults being very active during deposition (Datt, 1961 small faults show a minor left-lateral component as well. Such rotational movement elsewhere in the basin could account for tilted blocks and explain some of the misinterpreted folds. The primary longitudinal faults can --be difficiilftopkkoiifin places because of the later reactivation and offset.
BAROSH & WOODHOUSE, Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE (2011/2012)
Reinterpretations of LaForge's data as folds by Billings have not been supported by additional mapping. Billings (1982a) renamed the faults and folds shown by Laforge (1932) and · -reinterpreted a slice of Roxbury Conglomerate in LaForge's Rock Island Fault on Hough's Neck on the southeast side of the basin as the core of a Hough's Neck Anticline (see Figures 1-30 & 1-33). Borings and mapping of the Braintree-Weymouth Tunnel in Hingham, which crosses this area, shows that not only is the Rock Island Fault present (see Figure 3-55), but many smaller faults are as well (Davidson, 2003). Another example is the Northern or Charles River syncline, which was shown with an axis along the Charles River (see Figure 1-33). The contrasting lithology across the axial zone was interpreted as due to fades change, but the change is so abrupt it demonstrates instead the presence of a fault (see Figure 3-56). The reversal of dips at the river marks a fault zone and the south-dipping strata to the north of it are in a rotated block between this fault and the Northern Boundary Fault. However, some smaller folds are present along the northern edge of the basin. These folds appear to be from drag associated with the reverse faults of the Northern Boundary Fault Zone. Billings (1929) removed faults between three of LaForge' s anticlines to form a single large Central anticline (see Figures 1- 30, 1-32 & 1-33), but recent data show that these faults are present (Kaye, 1982b ). The Needham-Savin Anticline lies along the south side of this "Central Anticline." . The original rift basin was a half-graben, dropping the rock down to form a basin to the north. It had extended much farther to the north, but this portion was later cut off and its original width is unknown. The basin (see Figure 3-10) may have faced a narrow arm of the sea or a gulf to the north and had a similar tectonic and structural setting as the contemporary rift at Saint Johns, New Brunswick (Barosh, 1995), and both basins apparently formed as part of a broader basin-and-range topography (Kaye, 1984a). The position of the Mattapan Volcanic Complex centered in Needham apparently was controlled by the initial faults. The Blue Hills igneous complex, which was a volcanic center in the Late Ordovician, was controlled apparently by the border faults as well. The complex interfingering of the volcanic rock, conglomerate and argillite (along with the slumps, mass movement and graded beds) indicates a rapidly ris"' ing upland to the south consistent with longitudinal normal faults being very active during deposition (Datt, 1961 small faults show a minor left-lateral component as well. Such rotational movement elsewhere in the basin could account for tilted blocks and explain some of the misinterpreted folds. The primary longitudinal faults can --be difficiilftopkkoiifin places because of the later reactivation and offset.
BAROSH & WOODHOUSE, Geology of the Boston Basin, CIVIL ENGINEERING PRACTICE (2011/2012)
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Barosh PJ (2016) New England-Morocco Connection and the Northwestern Pan-African Collision. Int J Earth Sci
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Minute Man National Historical Park
Geologic Resources Inventory Report
Natural Resource Report NPS/NRSS/GRD/NRR—2017/1523
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Patrick J. Barosh & David Woodhouse, Geology of the Boston Basin, Civil Engineering Practice (2011/2012).
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Geology of the Boston Basin (2011/2012)
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Barosh & Woodhouse, Geology of the Boston Basin, Civil Eng. Practice, 2011/2012
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Barosh & Woodhouse, Geology of the Boston Basin, Civil Eng. Practice, 2011/2012
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THE MEMORIAL HISTORY OF BOSTON, INCLUDING SUFFOLK COUNTY, MASSACHUSETTS. 1630 1880. EDITED BY JUSTIN WINSOR, IN FOUR VOLUMES. VOL. I. THE EARLY AND COLONIAL PERIODS. BOSTON: JAMES R. OSGOOD AND COMPANY. 1881.THE MEMORIAL HISTORY OF BOSTON: PREHISTORIC PERIOD AND NATURAL PERIOD. CHAPTER I. OUTLINE OF THE GEOLOGY OF BOSTON AND ITS ENVIRONS. BY NATHANIEL SOUTHGATE SHALER, S. D., Professor of Paleontology in Harvard University.
Within the peninsula of Boston, the seat of the old town, these older rocks that were caught in the mountain folds do not come to the level of the sea. They are deeply covered by the waste of the glacial period.But in Roxbury, Dorchester, Somerville, Brookline, and many other adjacent towns, they are extensively exposed. They consist principally ofclay-slates and conglomerates, a mingled series, with a total thickness of from five to ten thousand feet. The slates are generally fine-grained and flag-like in texture, their structure showing that they were laid down in a sea at some distance from the shore. The conglomerates were evidently laid down in the sea at points near the shore ; and they are probably the pebble-waste resulting from a glacial period that occurred in the Cambrian age, or at a time when the recorded organic history of the earth was at its very beginning. These rocks represent a time when the waters of this shore were essentially destitute of organic life. In the whole section we have only about three hundred feet of beds among the lower layers that hold any remains of organic life ; and these remains are limited to a few species of trilobites, that lived in the deep sea. From the slates and conglomerates of the Cambridge and Roxbury series the first quarried stones of this Colony were taken.Although the rocks of this vicinity are extensively intersected by dykes and veins, those agents that in other regions aid the gathering together of the precious metals, no ore-bearing deposits have ever been found very near Boston. After the ice had lain for an unknown period over this region, climatal changes caused it to shrink away slowly and by stages, until it disappeared altogether. As it disappeared it left a very deep mass of waste, which was distributed in an irregular way over the surface, at some places much deeper than at others. At many points this depth exceeded one hundred feet. As the surface of the land lay over one hundred feet below the present level in the district of Massachusetts Bay when the sea began to leave the shore, the sea had free access to this incoherent mass of debris, and began rapidly to wash it away. We can still see a part of this work of destruction of the glacial beds in the marine erosion going on about the islands and headlands in the harbor and bay. The same sort of work went on about the glacial beds, at the height of one hundred feet or more above the present tide-line.During this period of re-elevation, the greater part of the drift-deposits of the region about Boston was worked over by the water. Where the gravel happened to lie upon a ridge of rock that formed, as it were, a pedestal for it, it generally remained as an island above the surface of the water. As the land seems to have risen pretty rapidly when the ice-burden was taken off, probably on account of this very relief from its load, the sea did not have time to sweep away the whole of these islands of glacial waste.Many of them survive in the form of low, symmetrical bow-shaped hills.Parker's Hill, Corey's Hill, Aspinwall, and the other hills on the south side of Charles River, Powderhorn and other hills in Chelsea and Winthrop, are conspicuously beautiful specimens of this structure. Of this nature were also the three hills that occupied the peninsula of Boston, known as Sentry or Beacon, Fort, and Copp's hills. Whenever an open cut is driventhrough these hills, we find in the centre a solid mass of pebbles and clay, all confusedly intermingled, without any distinct trace of bedding. This mass, termed by geologists till, or boulder-clay, is the waste of the glacier, lying just where it dropped when the ice in which it was bedded ceased to move, and melted on the ground where it lay. All around these hills, with their central core of till, there 'are sheets of sand, clay, and gravel, which have been washed from the original mass, and worked over by the tides and rivers. This reworked boulder-clay constitutes by far the larger part of the dry lowland surface about Boston : all the flat-lands above the level of the swamps which lay about the base of the three principal hills of old Boston lands on which the town first grew were composed of the beddedsands and gravels derived from the waste of the old boulder-clay. These terraces of sand and gravel from the reasserted boulder-clay make up by far the greater part of the low-lying arable lands of eastern Massachusetts ; and of this nature are about all the lands first used for town-sites and tillage by the colonists, notwithstanding the soil they afford is not as rich nor as enduring as the soils upon the unchanged boulder-clay. The reason these terrace deposits were the most sought for town-sites and cultivation is that they were the only tracts of land above the level of theswamps that were free from large boulders. Over all the unchanged drift these large boulders were originally so abundant that it was a very laborious work to clear the land for cultivation ; but on these terraces of stratified drift there were never boulders enough to render them difficult of cultivation.The result was that the first colonists sought this class of lands. One of the advantages of the neighborhood of Boston was the large area of these terrace deposits found there. There was an area of fifteen or twenty thousand acres within seven or eight miles of the town that could have beenquickly brought under the plough, and which was very extensively cultivated before the boulder-covered hills began to be tilled.After the terrace-making period had passed away, owing to the rising of the land above the sea, there came a second advance of the glaciers, which had clung to the higher hills, and had not passed entirely away from the land. This second advance did not cover the land with ice ; it only caused local glaciers to pour down the valleys. The Neponset, the Charles, and the Mystic valleys were filled by these river-like streams, which seem never to have attained as far seaward as the peninsula of Boston. This second advance of the ice seems to have been very temporary in its action, not having endured long enough to bring about any great changes. At about thetime of its retreat, the last considerable change of line along these shores seems to have taken place. This movement was a subsidence of the land twenty feet or more below the former high-tide mark. This is shown by the remains of buried roots of trees, standing as they grew in the harbor and coast-lands about Boston. These have been found at two points on the shore of Cambridge, a little north of the west end of West Boston Bridge, and in Lynn harbor. Since this last sinking, the shore-line in this district shows no clear indications of change.With the cessation of the disturbances of the glacial period and at the beginning of the present geological conditions, the last of the constructive changes of this coast began. Hitherto mechanical forces alone had done their work on the geography of the region ; henceforward, to the present day, organic life, driven away from the shore and land by the glacial period, again takes a share in the constructive work. This is still going on about us. The larger part of it is done by the littoral sea-weeds and the swamp grasses. Along the estuaries of the Saugus, Mystic, Charles, and Neponset rivers there are some thousands of acres of lands which have beenrecovered from the sea by these plants. The operation is in general as follows : The mud brought down by these streams, consisting in part of clay and in part of decomposed vegetable matter, derived from land and water plants, coats the sandy bottoms or under-water terraces. In this mud, even at considerable depths, eel-grass and some sea-weeds take root, and their stems make a dense jungle. In this grass more mud is gathered, and kept from the scouring action of the tide by being boundtogether by the roots and cemented by the organic matter. This mass slowly rises until it is bare at low-tide. Then our marsh-grasses creep in, and in their interlaced foliage the waste brought in by the tide is retained, and helps to raise the level of the swamp higher. The streams from the land bring out a certain amount of mud, which at high-tide is spread in a thin sheet over the surface of the low plain. Some devious channels are kept open by the strong scouring action of the tide, but the swamp rapidly gains a level but little lower than high-tide. Except when there is some chance deposit of mud or sand from the bluffs along its edges, these swamps are never lifted above high-tide mark, for the forces that build them work only below that level. Their effect upon the harbor of Boston has been disadvantageous. They have diminished the area of storage for the tide-water above the town, and thereby enfeebled the scouring power of the tidal currents. Except at the very highest tides, the Charles, Mystic, and Neponset rivers now pour their mud directly into the harbor, instead of unloading it upon the flats where these marshes have grown up. There are other forces at work to diminish the depth of water in the harbor. The score or more of islands that diversify its surface are all sources of waste, which the waves tend to scatter over the floor. For the first two hundred years after the settlement, the erosion of these islands was not prevented by sea-walls ; and in this time the channels were doubtless much shoaled by river-waste. Just after the glacial period these channels were very deep.Borings made in the investigations for the new sewerage system showed that the channel at the mouth of the Neponset had been over one hundred feet deeper than at present, the filling being the rearranged glacial drift brought there by just such processes as have recently shoaled the channels of the harbor.Of the ancient life of this district there is hardly a trace. The two great and conspicuous formations in the basin the flags and conglomerates of the Roxbury series and the drift deposits of the last geological age are both very barren in organic remains, for the reason that they are probably both the product of ice periods. The rocks older than the Roxbury series are too much changed to have preserved any trace of the organisms they may have once contained. In the rearranged drift there are some very interesting remains of buried forests that have not yet received from naturalists the attention they deserve. These buried trees lie at a considerable depth below low-tide mark, and are not exposed, except by thechance of the few excavations along the shore that penetrate to some depth below the water-line. When found, these trees seem all to be species of coniferous woods. The cone-bearing trees appear from this and other evidence to have been the first to remake the forests of this region, after the cessation of the last ice time. Even the larger animals that once inhabited this district the moose, caribou, etc. have left little trace oftheir occupation. It is rare, indeed, that a bone of their skeletons is found, except among the middens accumulated around the old camping-grounds of the aborigines.On the extreme borders of the Boston basin there are extensive fossilbearing strata. At Mansfield, on the south, which is just outside of thissynclinal, and within the limits of the Rhode Island trough of the same nature, there is a broad section of the coal-measures exposed in some mines now unworked. These beds are extremely rich in fossil plants.At Gloucester there is a small deposit of beds, containing shells of mollusks that lived in the early part of the present period, that lie just above the high-tide mark. But neither of these interesting deposits extends into the limits of the Boston basin.
Within the peninsula of Boston, the seat of the old town, these older rocks that were caught in the mountain folds do not come to the level of the sea. They are deeply covered by the waste of the glacial period.But in Roxbury, Dorchester, Somerville, Brookline, and many other adjacent towns, they are extensively exposed. They consist principally ofclay-slates and conglomerates, a mingled series, with a total thickness of from five to ten thousand feet. The slates are generally fine-grained and flag-like in texture, their structure showing that they were laid down in a sea at some distance from the shore. The conglomerates were evidently laid down in the sea at points near the shore ; and they are probably the pebble-waste resulting from a glacial period that occurred in the Cambrian age, or at a time when the recorded organic history of the earth was at its very beginning. These rocks represent a time when the waters of this shore were essentially destitute of organic life. In the whole section we have only about three hundred feet of beds among the lower layers that hold any remains of organic life ; and these remains are limited to a few species of trilobites, that lived in the deep sea. From the slates and conglomerates of the Cambridge and Roxbury series the first quarried stones of this Colony were taken.Although the rocks of this vicinity are extensively intersected by dykes and veins, those agents that in other regions aid the gathering together of the precious metals, no ore-bearing deposits have ever been found very near Boston. After the ice had lain for an unknown period over this region, climatal changes caused it to shrink away slowly and by stages, until it disappeared altogether. As it disappeared it left a very deep mass of waste, which was distributed in an irregular way over the surface, at some places much deeper than at others. At many points this depth exceeded one hundred feet. As the surface of the land lay over one hundred feet below the present level in the district of Massachusetts Bay when the sea began to leave the shore, the sea had free access to this incoherent mass of debris, and began rapidly to wash it away. We can still see a part of this work of destruction of the glacial beds in the marine erosion going on about the islands and headlands in the harbor and bay. The same sort of work went on about the glacial beds, at the height of one hundred feet or more above the present tide-line.During this period of re-elevation, the greater part of the drift-deposits of the region about Boston was worked over by the water. Where the gravel happened to lie upon a ridge of rock that formed, as it were, a pedestal for it, it generally remained as an island above the surface of the water. As the land seems to have risen pretty rapidly when the ice-burden was taken off, probably on account of this very relief from its load, the sea did not have time to sweep away the whole of these islands of glacial waste.Many of them survive in the form of low, symmetrical bow-shaped hills.Parker's Hill, Corey's Hill, Aspinwall, and the other hills on the south side of Charles River, Powderhorn and other hills in Chelsea and Winthrop, are conspicuously beautiful specimens of this structure. Of this nature were also the three hills that occupied the peninsula of Boston, known as Sentry or Beacon, Fort, and Copp's hills. Whenever an open cut is driventhrough these hills, we find in the centre a solid mass of pebbles and clay, all confusedly intermingled, without any distinct trace of bedding. This mass, termed by geologists till, or boulder-clay, is the waste of the glacier, lying just where it dropped when the ice in which it was bedded ceased to move, and melted on the ground where it lay. All around these hills, with their central core of till, there 'are sheets of sand, clay, and gravel, which have been washed from the original mass, and worked over by the tides and rivers. This reworked boulder-clay constitutes by far the larger part of the dry lowland surface about Boston : all the flat-lands above the level of the swamps which lay about the base of the three principal hills of old Boston lands on which the town first grew were composed of the beddedsands and gravels derived from the waste of the old boulder-clay. These terraces of sand and gravel from the reasserted boulder-clay make up by far the greater part of the low-lying arable lands of eastern Massachusetts ; and of this nature are about all the lands first used for town-sites and tillage by the colonists, notwithstanding the soil they afford is not as rich nor as enduring as the soils upon the unchanged boulder-clay. The reason these terrace deposits were the most sought for town-sites and cultivation is that they were the only tracts of land above the level of theswamps that were free from large boulders. Over all the unchanged drift these large boulders were originally so abundant that it was a very laborious work to clear the land for cultivation ; but on these terraces of stratified drift there were never boulders enough to render them difficult of cultivation.The result was that the first colonists sought this class of lands. One of the advantages of the neighborhood of Boston was the large area of these terrace deposits found there. There was an area of fifteen or twenty thousand acres within seven or eight miles of the town that could have beenquickly brought under the plough, and which was very extensively cultivated before the boulder-covered hills began to be tilled.After the terrace-making period had passed away, owing to the rising of the land above the sea, there came a second advance of the glaciers, which had clung to the higher hills, and had not passed entirely away from the land. This second advance did not cover the land with ice ; it only caused local glaciers to pour down the valleys. The Neponset, the Charles, and the Mystic valleys were filled by these river-like streams, which seem never to have attained as far seaward as the peninsula of Boston. This second advance of the ice seems to have been very temporary in its action, not having endured long enough to bring about any great changes. At about thetime of its retreat, the last considerable change of line along these shores seems to have taken place. This movement was a subsidence of the land twenty feet or more below the former high-tide mark. This is shown by the remains of buried roots of trees, standing as they grew in the harbor and coast-lands about Boston. These have been found at two points on the shore of Cambridge, a little north of the west end of West Boston Bridge, and in Lynn harbor. Since this last sinking, the shore-line in this district shows no clear indications of change.With the cessation of the disturbances of the glacial period and at the beginning of the present geological conditions, the last of the constructive changes of this coast began. Hitherto mechanical forces alone had done their work on the geography of the region ; henceforward, to the present day, organic life, driven away from the shore and land by the glacial period, again takes a share in the constructive work. This is still going on about us. The larger part of it is done by the littoral sea-weeds and the swamp grasses. Along the estuaries of the Saugus, Mystic, Charles, and Neponset rivers there are some thousands of acres of lands which have beenrecovered from the sea by these plants. The operation is in general as follows : The mud brought down by these streams, consisting in part of clay and in part of decomposed vegetable matter, derived from land and water plants, coats the sandy bottoms or under-water terraces. In this mud, even at considerable depths, eel-grass and some sea-weeds take root, and their stems make a dense jungle. In this grass more mud is gathered, and kept from the scouring action of the tide by being boundtogether by the roots and cemented by the organic matter. This mass slowly rises until it is bare at low-tide. Then our marsh-grasses creep in, and in their interlaced foliage the waste brought in by the tide is retained, and helps to raise the level of the swamp higher. The streams from the land bring out a certain amount of mud, which at high-tide is spread in a thin sheet over the surface of the low plain. Some devious channels are kept open by the strong scouring action of the tide, but the swamp rapidly gains a level but little lower than high-tide. Except when there is some chance deposit of mud or sand from the bluffs along its edges, these swamps are never lifted above high-tide mark, for the forces that build them work only below that level. Their effect upon the harbor of Boston has been disadvantageous. They have diminished the area of storage for the tide-water above the town, and thereby enfeebled the scouring power of the tidal currents. Except at the very highest tides, the Charles, Mystic, and Neponset rivers now pour their mud directly into the harbor, instead of unloading it upon the flats where these marshes have grown up. There are other forces at work to diminish the depth of water in the harbor. The score or more of islands that diversify its surface are all sources of waste, which the waves tend to scatter over the floor. For the first two hundred years after the settlement, the erosion of these islands was not prevented by sea-walls ; and in this time the channels were doubtless much shoaled by river-waste. Just after the glacial period these channels were very deep.Borings made in the investigations for the new sewerage system showed that the channel at the mouth of the Neponset had been over one hundred feet deeper than at present, the filling being the rearranged glacial drift brought there by just such processes as have recently shoaled the channels of the harbor.Of the ancient life of this district there is hardly a trace. The two great and conspicuous formations in the basin the flags and conglomerates of the Roxbury series and the drift deposits of the last geological age are both very barren in organic remains, for the reason that they are probably both the product of ice periods. The rocks older than the Roxbury series are too much changed to have preserved any trace of the organisms they may have once contained. In the rearranged drift there are some very interesting remains of buried forests that have not yet received from naturalists the attention they deserve. These buried trees lie at a considerable depth below low-tide mark, and are not exposed, except by thechance of the few excavations along the shore that penetrate to some depth below the water-line. When found, these trees seem all to be species of coniferous woods. The cone-bearing trees appear from this and other evidence to have been the first to remake the forests of this region, after the cessation of the last ice time. Even the larger animals that once inhabited this district the moose, caribou, etc. have left little trace oftheir occupation. It is rare, indeed, that a bone of their skeletons is found, except among the middens accumulated around the old camping-grounds of the aborigines.On the extreme borders of the Boston basin there are extensive fossilbearing strata. At Mansfield, on the south, which is just outside of thissynclinal, and within the limits of the Rhode Island trough of the same nature, there is a broad section of the coal-measures exposed in some mines now unworked. These beds are extremely rich in fossil plants.At Gloucester there is a small deposit of beds, containing shells of mollusks that lived in the early part of the present period, that lie just above the high-tide mark. But neither of these interesting deposits extends into the limits of the Boston basin.
Report on the GEOLOGY,MINERALOGY, BOTANY, AND ZOOLOGYOf Massachusetts.MADE AND PUBLISHED BY ORDER OP THE GOVERNMENT OF THAT STATE: IN POUR parts:PART I. ECONOMICAL GEOLOGY.PART n. TOPOGRAPHICAL GEOLOGY.PART III. SCIENTIFIC GEOLOGY. ^PART IV. CATALOGUES OF ANIMALS AND PLANTS.WITH A DESCRIPTIVE LIST OP THESPECIMENS OF ROCKS AND MINERALS COLLECTED FOR THE GOVERNMENT.ILLUSTRATED BY NUMEROUS WOOD CUTS AND AN ATLAS OP PLATES.SECOND EDITION, CORRECTED AND ENLARGED.BY EDWARD HITCHCOCK, A. M.Professor of Chemistry anil Natural History in Amherst College: Member of the Americaa Academy of Arts and Sciences: Of the Academy of Natural Sciences, Philadelphia: Of the American Geological Society, &c.AMHERST:PUBLISHED BY J. S. AND C. ADAMS.1835.According to the Messrs. Danas, trunks of trees, generally of some species of pine, occur in peat, several feet below the surface in the marshes of Charles river.Power of Ice in the removal of Boivlders in Ponds-I am not aware that this phenomenon has been noticed on the eastern continent ; and it has been but rarely observed on our own. Its effects in modifying the face of the globe must be very limited ; yet they deserve enumeration.It is well known that water, by an apparent exception to a general law, expands with great force when freezing, and even farbelow the freezing point. Over a large extent of surface this effect may be very considerable ; and when bowlder stones, lying in shallow ponds, become partially enveloped in the ice, they must feel the effect of this expansion, and be driven towards the shore : since the force must always act in that direction. As no counter force exists to bring back the rock to its original position, the ultimate effect must be to crowd it entirely out of the pond ; and perhaps to this cause we may impute the fact, that on the margin of some ponds we find a ridge of bowlders ; while the bottom, for a considerable extent, is free from them.The removal of rock masses in this manner was first noticed in Salisbury, Ct. ; and a statement published in Vol. 9th of the American Journal of Science. I have seen no similar instance in Massachusetts ; but Rev. Sylvester Holmes, of ^evr Bedford, informs me, that an undoubted example of these travelling bowlders exists in a pond in Carver, Plymouth county ; and that their track in the mud is quite obvious.But all the islands outwards from the GreatBrewster, are nierely naked masses of rock, and it would be natural to infer that the diluvium had been removed from these, even if we did not actually detect the process. But on the Great Brewster, the work is going on before our eyes. Its eastern side is a high bank of diluvium, obviously wasting away by the action of the waves that roll in upon It from the wide Atlantic ; while the extensive beach along iis southern side, is composed of the materials that have been swept away from its outer coast.It seems to me that no man, accustomed to reason correctly from geological facts to their causes, can hesitate, in view of the appearances which these islands exhibit, to infer that all those outside of the Great Brewster have been deprived of their diluvial coat by the action of the ocean. Nor when we consider thefrequency and violence of northeast winds and storms upon this coast, need we fear that the cause is inadequate to the effect ; although it is not less than two and a half miles from the GreatBrewster to the outermost of the Graves. It does not, indeed, follow, that all the intervening space between these outer islands was once solid land ; so that the ocean has actually worn away 2 1-2 miles ; and yet, this seems highly probable. Indeed, the mind is irresistably led to inquire whetlier the whole harbor has not been produced by the same cause ; and when we see so many islands scattered overfits bosom, which seem obviously the wrecks of one continuous diluvial formation, and perceive that the rocks, wherever they occur, are only a continuation of those occurring on the mainland, the most cautious reasoner can hardly avoid the conclusion that such was the origin of this harbor : or, at least, that this was a powerful auxiliary cause in its forniation. Nay, it is difficult to see why the same reasoning will not apply to the whole of Massachusetts Bay ; and when we see with what tremendous force the ocean must, for ages, have battered the hardsienitic rocks of Cape Ann, and what an immense accumulation of sand, gravel, and bowlders, has been made along the south shore of this Bay we feel almost prepared to adopt this theory.And yet, we are staggered in our belief when we reflect on the immense period of time requisite for such a work ; and doubt whether other geological facts do not indicate a later commencement to the present order of things on the globe.The proper place for learning the dynamical effect of northeast storms upon our coast, is on the north east side of Cape Ann.Rocks of many tons weight have been is this manner moved from their beds, and driven inward a considerable distance. One has only to visit this coast to be astonished at the marks everywhere exhibited of the powerful agency of a stormy ocean, and to be satisfied that nothing but the extreme hardness and unstratified structure of the rocks has enabled them to resist its violence.And when we learn that the rocks of Boston Harbor are softer and schistose, we see a sufficient reason why they have given way before the breakers ; while Cape Ann, and the shores of Cohassetand Scituate maintain their position.Since the publication of the first edition of this Report, T have received the following statement from Mr. Benjamin Haskell of Sandy Bay, on the northeast side of Cape Ann, illustrative of the power of the stormy waves of the ocean upon that coast.' The northeast extremity of this Cape, known by the name of Flat Point, differs from the general features of the coast, by extending into the sea with a gradual slope, instead of the bolder aspect of the adjacent shore. Upon this point the sea beats during a northeast storm with a violence conceivable only to those who have witnessed it. Here, at the distance of from 60 to 100 feet above high water mark, lies what a farmer would call awinrovv of bowlders, which there is every reason to believe have been thrown up within a few years.'' These bowlders are irregular in form, and angular, their corners being scarcely rounded by attrition. They exceed in size any thing of the kind in this vicinity. A number of them would weigh 10 or 15, and some even 20 tons. But there is one far more interesting than all the rest; both on account of its greater bulk, and comparative regularity of shape, which renders the former easy to be estimated, and thus affords the means of ascertaining the maximum force of the Ocean in its anger. This rock was originally attached to a ledge about 5 feet above the level of the sea.The broken surfaces correspond so exactly as leave no room to doubt from whence it was detached. From this spot to the spot where it now lies, the direction is south, a little westerly. The distance 106 feet: but between the two positions there is a hollowing of the ledge (not a recent one) over which it must havepassed, so that the ascent of the rock up this old-fashioned railway cannot have been less than 10 feet.'' The weight of this bowlder has been calculated with care, due allowance having been made for irregularities of surface, and found to be rising of 28 tons. What an illustration ofHydrodynamics ?'Several cliffs of clay and sand along the coast exhibit the combined effects of the ocean, rains, frost, &c., in wearing away the land. In Chilmark, on Martha's Vineyard, is one of these facing the southeast, and at least a mile in length. It is novi^ rare that the breakers rise high enough to impinge directly against the cliff: but they wash away whatever materials have been brought down by the rains. Gay Head, which is the western extremity of the same island, presents a cliff of variegated clays, sands, fcc. not less than 150 feet high ; and which standing exposed to the buffetings of winds and waves from the sea, and to the wastes ofstorms from above, exhibits perhaps the most instructive example along the shore, of the effects of these agents. In the second part of my Report, I described this cliff as a most picturesque object of scenery ; but there is not likewise a more interesting spot in the State, to the Geologist. And among other things he cannot but notice the numerous fantastic forms into which the lofty masses of clay have been worn, while the numerous bowlders andpebbles along the beach attest the violent action of the sea. The following sketch, hastily taken, will give some idea of the aspect of the northwestern part of this cliff, as seen by a person standing on the beach below, close to the water- To exhibit it in perfection, the various lively colors of the different kinds of clay should be put upon it. similar bank of clay occurs at the Light House in Truro, near the extremity of Cape Cod. It lies exposed to the unbroken fury of the wide Atlantic, and the marks of slow encroachment upon the land are quite manifest. Indeed, it is the prevailing opinion in that region, that this Cape is wearing away along the whole extent of its eastern extremity, and extending farther into Massachusetts Bay on the opposite side. I have no doubt that this isthe case. For the general current on that coast is towards the south.The same I presume is true of a considerable portion of the eastern shore of Nantucket. From data, on which Lt. Prescott places considerable confidence, he infers, that in one place, the loss of land within half a century, has amounted to 3 or 4 rods in width.This advance of the ocean, however, must not all be imputed to the action of currents. For when once a sand bank of considerable height has been raised on the coast, the sea breezes will drive it inwards farther than tiie land breezes will bring it back. This inland march is quite obvious on Chatham Beach, in the situation af a swamp, which, 50 years ago, was in the centre of thebeach ; but now lies near the eastern shore ; the body of the sands having moved farther west. A salt meadow formerly situated on the western side of the beach, adjoining the old north passage into Chatham harbor, has been covered up, and now begins to be disinterred on the eastern shore. A similar change of sides has taken place in a peat swamp on Nauset Beach ; which lies north of Chatham Beach, joining the mainland at Eastham.It is now generally admitted by geologists, that all stratified rocks must have been originally deposited in nearly horizontal layers, and subsequently elevated to their present inclined position by a force acting beneath. Such a disturbance must have produced many violent and extensive fractures in the strata and valleys of every shape. And since in the mountainous parts of Massachusetts, the strata are mostly primary and highly inclined, probably this is the manner in \Ahich most of our mountain valleys have been produced. If, as is now also generally admitted, llie strata were elevated from the bottom of the ocean, the retiring waters must iiave acted powerfully upon the irregular surface, and considerably modified the forms of the valleys. The agency of rains,snows, and rivers, since that period, must have given them still farther modifications. Nor ought we to leave out of the account any other deluges of a date subsequent to that of the elevation of the strata, that may have swept over the land.When the strata of rocks on the opposite sides of a valley coincide, the conclusion seems inevitable that they once formed acontinuous stratum, and that the valley has been subsequently ex cavated. The appearance in such cases indicates that it has been scooped out by running waters : and yet, this might be the appearance if water had only modified the sides and bottom of a fissure produced by other causes. And in some cases, at least, it seeras necessary to call in the aid of other causes.DILUVIUM.Under this term I include that coating of gravel, bowlders, sand, and loam, which is spread over almost every part of the surface, and which has been obviously mingled confusedly together by powerful currents of water, subsequent to the deposition of the regular strata. Hence geologists have referred it to the agency of a general deluge ; and since it occupies the highest place in the rock series, except alluvial and volcanic rocks, most of them have regarded that deluge as identical with the one described in the Christian Scriptures. But recently some respectable geologists maintain, that existing causes, operating as they now do, might in the course of ages, have produced all the phenomena of the rock formations. Hence they deny the existence of such a deposit as diluvium ; or, rather, they impute it to rivers, rains, frost, and other existing agencies, and include it under alluvium. Others, however, regard diluvium as the result of various agencies, operating at different periods ; among which are the floods produced by the elevation of the rock strata at various times. But they do not admit that we have in this diluvium any evidence of a deluge contemporaneous with that described by Moses.It ought to be remarked, however, that these geologists do not deny the occurence of such a deluge as is described in the Bible.Some of them, indeed, are clergyman, and they merely^say, that geology does not furnish any evidence of such a catastrophe, although it affords no evidence to the contrary, but rather a presumption in its favor, in the fact so abundantly proved by the records of geology, that numerous extensive, if not universal deluges, have occured since the creation.That a transient deluge, like that described in the Scriptures, could have produced, and brought into its present situation, all the diluvium which is now spread over the surface of this continent, will not, it seems to me, be admitted for a moment by any impartial observer. It has obviously been the result of different agencies, and of different epochs; the result of causes sometimes operating feebly and slowly, and at other times violently and powerfully.But the conclusion to which I have been irresistibly forced by an examination of this stratum in Massachusetts, is, that all the diluvium, ivhichhad been previously accumulated by various agencies, has Tjeen modified by a poiverful deluge, sioeeping from, the north and norlhivest, over every part of the State ; not excepting its highest mountains. And since that deluge, none but alluvial agencies have been operating to change the surface. I shall now proceed to give a history of this diluvium, with the reasons that prevent me from assigning its present modified state to any other cause than a recent deluge.Tocography of Diluvium.The most extensive diluvial deposite on the Map, is in Plymouth and Barnstable counties. Indeed, nearly the whole of those counties (with the exception of the north part of the former,) might have been thus colored with perfect justice. But as I had good reason to believe that a granite ridge occurs where it is marked, concealed by a few feet of diluvium only, I thought myself justified in extending that rock on the Map nearly to the extremityof Cape Cod. 1 saw, however, no example of rocks in placethroughout the whole extent of the Cape, except perhaps a single fissured rock, which has been powerfully acted upon by water ; and which, if it be in place, is only the wreck of a granite ledge.A view of this rock will be given farther on. In Plymouthcounty, except at its northern part, the granite rarely appears, and but seldom forms a cliff even fifty feet high. Every thing, indeed, is buried by diluvium ; and, as the streams are few and small there, it is extremely difficult to ascertain what is its geology, except to say that it is diluvial.The diluvium of Plymouth and Barnstable counties consistsalmost entirely of white sand, some pebbles, and a very large number of bowlders of primary rocks. These bowlders consist chiefly of granite, sienite, and gneiss, with occasional masses of graywacke conglomerate, compact feldspar, and porphyry. They all correspond with the rocks found in place along the coast, in the vicinity of Boston, and on Cape Ann ; and no one, it seems to me, can see the marks of degradation along that coast, who will not be convinced that a large portion of the pebbles and bowlders of Plymouth and Barnstable counties, must have come fromthence. Along the range of elevated, and for that part of the State, even mountainous land, which is colored as granite on the Map, the bowlders are so enormously large, and so thick, that I cannot believe they have been ever removed far from their native beds. They are sometimes from 10 to 20 and even 30 feet in diameter, and frequently occupy nearly the whole surface ; so that one can hardly persuade himself, when he examines them from a little distance, that they are not genuine ledges. Indeed, I have repeatedly been deceived by their appearance, until I had gone among them, and ascertained that they were detached bowlders.On the road from Sandwich to Falmouth is perhaps asstriking an exhibition of this phenomenon as in any place, unless it be in the western part of Martha's Vineyard, in Tisbury and Chihnark. The same appearance is striking, also, in Brewster, on the Cape ; and I doubt not that genuine ledges of granite may be found in those places ; although (with the exception of Brewster perhaps,) I did not make the discovery. I have been informed, however, that rocks in situ, do exist in Dennis. But I have been so often deceived in this matter in that region, that I dare not state any thing as fact concerning it, which I have not carefully examined with my own eyes. At any rate, I cannot believe that bowlders so large and numerous have been removed many miles; for powerful as has been the diluvial current in the eastern part of the State, I have seen no well ascertained instance where whole mountains have been torn up and transported, as they must have been in this case, if they came from the region of Scituate and Cohasset, 40 or 50 miles ; and that too, through a region of sand. And although much of the granite of thesebowlders resembles that of Cohasset and Scituate, yet I doubt whether it is identical with it. Some of it I know to be quite different.The sand, which is the predominant ingredient of the diluvium in the counties above named, was undoubtedly derived from a tertiary formation, which has been broken up by diluvial action.Remnants of this formation are occasionally seen on Cape Cod ; and in Truro, so lofty and distinct are the cliffs of clay, that they have been noted on the Map. Clay is found in other places on the Cape ; but not in large quantities, and generally at a low level.On Martha's Vineyard and Nantucket, this formation is much more abundant and obvious along the coast ; though covered for the most part in the interior with diluvium several feet thick.Very likely this formation once occupied no small part of Massachusetts Bay, and probably also Buzzard's Bay.In almost every part of the State the diluvium is piled up into elevations whose surfaces exhibit curves of every description ; while the correspondent cavities are of various shapes. These convexities and concavities resemble very much the sandy or gravelly bottom of existing streams, where the current has been very violent ; except that generally those in the diluvium are on a vastly larger scale. The following sketch may aid in imparting a correct idea of these diluvial irregularities. It was taken in the southeast part of Amherst, and exhibits several elevations from 10 to 20 feet high, composed entirely of gravel with no blocks large enough to be called bowlders.Standing upon the burying ground where rest the remains of our Pilgrim Fathers in Plymouth, we have around us on almost every side, and for a considerable distance, a fine example of these elevations and depressions. I mention this spot, not because it is more remarkable than many others for diluvial phenomena ; but merely because it is so frequently visited.In Truro, near the extremity of Cape Cod, the magnitude of these elevations and depressions is truly astonishing. One finds himself in a hilly and even mountainous country; the elevations being often from 200 to 300 feet high, and very numerous; and yet these are most obviously diluvial hills and valleys ; that is, they are as obviously the result of currents of water, as those inequalities of surface, of exactly the same shape, which we find in the dry bed of a river. The fact is, this Cape, below Orleans, consists almost entirely of coarse sand, which is more easily piled up and scooped out than gravel ; and this explains the striking features of the diluvium in the region of Truro, which is well worth a journey thither to examine. But one has only to look at a map of Massachusetts, to see that the idea of these effects having resulted from the action of any existing stream, is absurd ; since no current of water, deserving the name of a river, can exist on that part of the Cape ; whereas the Mississippi, or St. Lawrence, pouring through a mountain gorge upon a sandy plain, would be scarcely adequate to produce the effects here witnessed. And as to this being the result of the retiring or returning wave, when the strata were first elevated, I shall lake occasion to show, before concluding this section, that the opinion is improbable.The same idea, of a force vastly greater than any now in action in the State, having been exerted in the production of our diluvium, forces itself upon the attention in many other places besides Truro.All the eastern part of the State presents evidence of having been swept over by a prodigiously strong current of water. Nantucket, Dukes county, and the Elizabeth islands, are almost entirely covered with a vast quantity of bowlders, gravel, and sand, most of which must have come from the continent. On Nantucket, bowlders and gravel are rare ; only four or five large blocks occurring on the island ; although masses two or three feet in diameter are not unfrequently met with : and these, consisting of granite, gneiss, and quartz, were obviously transported from the continent. On the Vineyard, the bowlders are very numerous, and some of them very large ; and although some of them unquestionably proceeded from the mainland, yet in one or two places, as in Chilmark, I strongly suspect the existence of granite ledges a few feet below the surface, from the quantity and size of the bowlders : and yet one often sees very large blocks in the diluvial covering of the clay cliffs, as for instance at Gay Head ; where one or two of them that have rolled down to the base, are from 20 to 30 feet in diameter.The Elizaljeth islands are entirely covered by a thick coat of diluvium of a similar character ; and so is the whole coast, from Cape Cod to Rhode Island, except that south of Rochester there is much less of sand ; but tiie quantity of bowlders is prodigious ; so that one often travels many miles without seeing a rock in place ; although the surface is almost entirely covered by rounded masses of almost every size. This is seen on almost any road from New Bedford to Rhode Island,Erratic Blocks and Roclcing Stones,Passing northerly from Buzzard's Bay, the whole country east of a line drawn from Providence to Boston, except the summits of a few of the highest mountains, and some alluvial valleys, is covered with diluvii'.l blocks and gravel. But from Boston to the extremity of Cape Ann, embracing a considerable proportion of Essexcounty, the amount of bowlders is prodigious; and some ofthem are not less than 30 or 40 feet in diameter ; and yet so powerful was the diluvial current, that these must have been removed from their original position, and many of them now occupy the summits of the highest hills in that region : presenting often a most singular outline to the landscape. When one of these erratic blocks is so poised upon a rock in place, as to be easily moved it constitutes a rocking stone. Some of these, weighing from 10 to 100 tons, can be perceptibly moved by the strength of a single man, applied to a lever; though the combined efforts of a hundred cannot move them, but a few inches. Of two of these rocking stones 1 have taken a sketch, on account of their peculiar appearance.The valley of Worcester abounds in diluvium ; especially in the north part of the county. Proceeding towards the Merrimack, through Sterling, Lancaster, and Groton, we find large accumulations of diluvial gravel, exhibiting the irregular convexities and concavities already described. We find in this region, however, much fewer large bowlders than in most other parts of the State.These become more numerous as we follow the Merrimack to its mouth. Much of the diluvium, however, from Worcester toNewburyport, consists of shirigle ; by whicb, I mean partially rounded fragments of slate, and quartz rock ; resembling very much the pebbles occurring on the sea shore.The level part of the basin of the Connecticut, exhibits less striking marks of diluvial action than the smaller elevations on the margin of this tertiary plain. Some might even doubt whether the tertiary deposite of this valley is not postdiluvian. But I think that upon the whole, marks of diluvial action are too strong on its surface to be referred to the currents of an ancient lake. For the dilluvial coat is several feet thick in almost every place. We could not expect that a general deluge, of depth sufficient to rise above our highest mountains, would act as powerfully upon low and broad plains, as in the vicinity of mountain defiles and gorges, through which the water must have rushed with great power, even though its general movement was moderate. And this view accords with the present disposition of diluvium in Massachusetts. In Bernardston, Franklin county, for instance, which lies at the northernextremity of the Connecticut valley, we find a large amount of diluvium, which was evidently washed from the region of argillaceous slate lying north, through two or three narrow valleys,running north and south, down which the current must haverushed with great force. Accordingly we here find, on the road towards Northfield, a mile or two east of Bernard ston centre, an example of diluvial elevations and depressions scarcely equalled in the State ; exhibiting, as it were, the very gyrations of the mighty torrent. But when this stream spread out over the broad valley of the Connecticut, its violence and strength would greatly diminish ; and hence this diluvium was not driven very far into that valley. Yet at the east end of Mount Holyoke, where it approaches the primary hills in Belchertown, we find a very powerful diluvial agency to have been at work, in consequence of the rush of waters through the gorge between ihe mountains, and also through the valleys on both sides of Mount Toby, and among Pelham hills on the north. So that in the southeast part of Amherst, and indeed through its whole eastern part, as well as in Belchertown and Ludlow, the diluvial sand and gravel are piled up andscooped out in a striking manner. And in general, as we begin to rise from the tertiary plain of the Connecticut basin, we find a greater accumulation of this stratum than on the plain itself, or high up among the primitive mountains.How common may be consolidated diluvium in this country, I cannot say. But I believe no account of any other locality has been published. In Europe, geologists describe a similar rock, if Brongniart's Terrains Clysmiens is synonymous with diluvium ; for he says that 'the parts of the rocks of that class are sometimes united by a base or cement chemically produced ; that is by solution.'* At any rate, the consolidated shingle bed, described byMr. Mantell in his Geology of Sussex, as occurring at Brighton, in England, must be regarded as of the same character as that in Pownall above described.The first part of this evidence consists in tracing erratic bowlders to the parent rock from ivhich they were derived.When I began an examination of the State, I travelled east and west ; commencing with the line of towns bordering upon Connecticut, and returning through the line of towns next north.Thus essentially have I gone over the whole State. And 1 had not thus doubled my course many times, before [found, uniformly, that in order to trace bowlders to their original beds, I must travel north a greater or less distance. The discovery was frequently of great service to me ; and I do not recollect that the principle ever failed me. I have, indeed, sometimes found a straggling block east or west, and even north of ledges of the same kind of rock; but never anything more than lonely stragglers. It will be expected, however, that on such a point I should refer to particular instances.t have already remarked that granite and sienite constitute the great mass of the bowlders scattered over the southeast part of the State ; and that these correspond to the rocks of this character on the coast that bounds Boston harbor. But similar rocks also occur in place, occasionally, in the region where the bowlders are found ; and, therefore, we cannot be sure that they were transported from a distance ; although in many cases the exact correspondence between the specimens would leave little room to doubtthat such was the fact. But scattered among these primary bowlders, we frequently find others of porphyry, compact feldspar, and graywacke conglomerate ; rocks, which (except the conglomerate,) occur only within a kv^ miles of Boston, both north and south. I have found masses of porphyry as far down Cape Cod as Orleans ;and near the southern extremity of Martha's Vineyard, the pebbles of this rock are quite numerous. In Tisbury I have seen amass of peculiar blood-red, compact feldspar, which occurs in place in Hingham ; which would indicate the course of the diluvial current to be a few degrees east of south. The porphyry pebblesmerely indicate a southern direction to the current ; since the occurrence of porphyry at Half-way-rock, east of Marblebead, showsthat this rock might formerly have extended far into the ocean.Graywacke conglomerate occurs in the graywacke formation in patches, from Rhode Island to Newburyport; and the bowlders of it above spoken of, must, therefore, have been transported in a direction a little east of south, in order to reach the west part of the Vineyard, where I found themPerhaps the example more definite and decisive than any other on the subject under consideration, occurs in Rhode Island. In Cumberland a large hill exists of magnetic iron ore ; a considerable part of which contains distinct crystals of feldspar, so as to become beautifully porphyritic. A rock so peculiar cannot be confounded with any other. Now if we pass along the north, east,and west sides of this bed of ore, even very near it, no scattered fragments of it are seen among the bowlders. But on the south side, they occur all tho way to Providence, decreasing in size.Whether they may be found on the west side of Narraganset Bay, south of Providence, I cannot say : but I met with several pieces at the southern extremity of Rhode Island, in Newport, of only a few inches in diameter. These must have travelled nearly 35 inies from their bed, in a direction a few degrees east of south. * m several places in the southeast part of Worcester county, j met with bowlders of a variety of porphyritic granite, distinguished from every other kind in the State, by its remarkably large imbedded crystals of white feldspar. But it was not till I cameto Harvard, that I found this rock in place. On the north of the ledge, I never met with a single fragment. In Waltham, however, I did meet with one bowlder of this rock.Upon the whole, I have no hypothesis on this subject to propose, more free from difficulties, than that which imputes the removal of these quartz and graywacke bowlders, in a southeasterly direction, to the same debacle of waters, which, in other parts of the State, has swept the detritus southerly. What local cause should have deflected the current towards the east, in the western part of the State, and in the eastern part of New York, I can hardly conceive : though, I shall shortly endeavor to show, there was considerable irregularity in its direction in that region ; enough, perhaps, to lead to the suspicion, that the deep valleys and ravines, through which the waters must have rushed, might have considerably modified their course. But I think that the change of a few degrees in the direction of the current, is not so great an objection to this hypothesis, as the Sisyphean task, which must have been accomplished, if it be true, of urging upwards, over so long and steep inclined planes, bowlders so large and so numerous. Making every allowance for the reduction of the gravity of these bowlders when in water, I confess, 1 cannot conceive how such a work could have been effected by this agency. Yet neither can I conceive how those diluvial elevations and depressions, that have been described in various parts of the State, could have been produced by a deluge. For tliey are on so large a scale, as to transcend by far, the maximum effect, which 1 can conceive to be produced by a flood of waters. Still it is undeniable that these did result from such an agency. Hence I may underrate the power of that same agency in the removal of detritus.I acknowledge, however, that I should be inclined to refer the diluvial phenomena in the western part of the State, to a different and an earlier deluge than the last—perhaps to the retiring waves when the strata were first elevated—did not facts forbid it. I have mentioned some of these, and shall soon mention another still more conclusive.Diluvial Grooves and Scratches upon the Rocks in Place:The second argument that shows the direction of the last diluvial current in Massachusetts to have been- towards the south and southeast, is based upon the existence of grooves, furrows, and scratches, upon the surfaces of the rocks, that have never been moved from their place. The water-worn appearance of those rocks, in every part of the State, which are undergoing no disintegration at their surface, must, it would seem, arrest the attention of a very careless observer : although I have been surprised to meet with so few men who have noticed the fact. In some cases, however, the rocks are not merely spioothed, but are grooved and furrowed, as if heavy and irregular bodies had been dragged over their surfaces. The following sketch exhibits a rock of this description near the turnpike, from Boston to Chelmsford, near theone between Bedford and Billerica, and not hv from the sixteenth mile stone from Boston. The rock is intermediate between gneiss and mica slate. Its strata seams run in the direction a, a; and the grooves and scratches in the direction b, b.The direction of these grooves is nearly north and south; and this is their general course in every part of the, State, east of Hoosic mountain. Commonly, however, they run a few degrees eastof south, and west of north. I shall first mention several localities where these furrows correspond in direction to this description, and then notice a few anomalous cases in the west part of the State.One hundred rods east of the village on Fall River, in Troy, are grooves and scratches on granite. Some of the bowlders lying on the surface here will weigh from 50 to 100 tons.Similar grooves occur on a road leading from the south part of Scituate to Hanover four corners. Tlie rock is granite.Also in Abington, Randolph, Canton, Sharon, Dedham, andDover ; on granite and sienite ; very common.Also on the conglomerate in Dorchester.In passing from Worcester to Berlin, through Boylston, the like appearances present themselves frequently on the surface of the gneiss and mica slate.Likewise in several places between Andover and Boston, ongranite and sienite.It may be well in this place to suggest a caution against mistaking the structure of the rock as revealed by disintegration, for these diluvial furrows. Some varieties of mica slate exhibit a surface extremely resembling one mechanically grooved. But in that rock, the direction of these pseudo-grooves always corresponds with that of ttie layers of the rock ; and thus the deception may be discovered. But sienite and greenstone, which contain segregated veins, sometimes present cases that are very perplexing.One of these may be seen on the top of mount Tom, a few rods north of the signal staff, erected for the Trigonometical Survey of the State. The prevailing direction of the apparent furrows there, is nearly north and south; and did they not run east and west within a rod or two of the spot, I should have put down this as a genuine case of diluvial grooves. But examination, after my sus})icion3 were excited by this circumstance, satisfied me that it is only the internal structure of the rock, that is here revealed by the unequal disintegration of the surface.Origin of the Diluvium of Massachusetts.It is maintained by those geologists who account for all geological changes by existing causes, acting with no greater intensity than at present, that most of the stratum which I have described as diluvium, has been produced and brought into its present state by the action of existing streams, rains, frost, and other agents now in operation. But the simple fact that the current must have had a southerly direction in every part of the State, and has left traces of its action on our highest mountains, renders such a supposition, it seems to me, altogether untenable. For how could rivers have risen so high ; or how, unless it were a single river, not less than 200 miles wide, could the waters have produced such eifects ?The same difficulty is in the way of supposing, as do some fluvial ists, that the land was once much lower than at present, having been gradually elevated by earthquakes. Admit, if it be wished, that the surface was once much lower than its present level : the difficulty will still be to find a current 200 miles wide.Other geologists, who perceive the utter insufficiency of such causes to account for diluvium, have imputed very much of it, and also diluvial grooves and furrows, to the retiring waters of the ocean, when first the solid strata were elevated. I doubt not that such was the origin of much of the diluvium that now covers the globe. But I think it quite obvious that all the diluvium in Massachusetts, which was produced by this and other causes, has beenmodified by a deluge long subsequent to the elevation of our continent from the ocean. For by examining the sections of our rockstrata, appenced to this Report, as well as the Map illustrative of the course of the same, it will be seen that their prevailing dip is easterly, and their general direction north and south. Hence the anticlinal line of these strata, that is, their axis of elevation, must be sought farther west than Massachusetts; and, consequently, the retiring waters must have rushed from the west at that epoch.But the actual current of the last deluge came from the north and northwest, as I have abundantly shown ; and therefore, it could not have resulted from the elevation of the strata.In the eastern part of the State, however, it will be observed that the strata of the graywacke formation run generally east and west. But they dip northerly ; and hence the current of water, which tlieir elevation produced, must have been towards the north ; though if we suppose it to have been southerly, this formation is loo limited in extent to account for diluvial action over the whole State.I may seem here, however, to be advancing opinions contradictory to the Mosaic chronology of the globe. But they are simply opposed to the prevaling interpretation of that record. If we only suppose, what many of the ablest theologians and philologists maintain, and what geological researches imperiously demand, thatMoses, after describing in the first verse of his history, the original creation of the universe ' in the beginning,' passes over in silence a long intervening period, before he gives us an account of the earth in its present state, and of the creation of its present inhabitants, all apparent collision between geology and revelation vanishes.Such an opinion I have adopted, not merely because facts in geology demand it, but because it seems equally required by a fair interpretation of the language of Moses.But to return from this digression ; it seems to me that the fair result of all the facts and reasonings which 1 have presented on the subject of diluvial action is, that a mighty deluge has swept from the north and northwest over every part of Massachusetts ;and that it cannot be accounted for by the original elevation of the strata of rocks ; nor can our diluvial phenomena be explained by the agency of rivers, rains, frosts, or any other causes acting with their present intensity. This deluge must, then have occurred since the earth's surface assumed essentially its present form : and was the last of those catastrophes to which this part of the globe has been subject ; and which cannot be referred to existing agencies. The inquiry naturally arises, whether this deluge was identical with that described by Moses. I have already remarked that this question can have no very great interest as bearing upon the veracity of the sacred historian ; since nearly all geologists agree that their science exihblts no evidence against the occurrence of such a deluge as he has described. Yet, as it is a characteristic of human nature to go from one extreme to another, and as it has been customary to impute almost every geological change to the deluge of Noah, is it not probable that philosopliers, disgusted with so much false reasoning on the subject, will be apt to overlook even creditable geological evidence of that event ? I have shown, if I mistake not, that the last deluge in Massachusetts was universal, and that it was comparatively recent. The deluge of Noah is described as universalover the globe ; and historical records give us no account of one more recent. Where then is the objection against considering them as identical ? Until some substantial reason can be given against such a conclusion, is it not unphilosophical to refuse to admit it ?I have thus far reasoned exclusively from diluvial action in Massachusetts. But there is evidence that the last deluge rushed from the north over all that part of iXorth America, between Nova Scotia and Lake Huron. Dr. Bigsby has stated facts in the sixth volume of the Geological Transactions, and the Messrs. Lapham,more recently, in the 22d volume of the Am. Jour, of Science, proving the truth of this statement in respect to the country about our western lakes; and Messrs. Jackson and Alger, in their recent, able memoir on the Mineralogy and Geology of Nova Scotia, have drawn the same inference from the present position of erratic bowlders in that country. Do not these facts, in connection with those stated in this Report, render it extremely probable, that over the whole breadth of North America, the current came from the north : although deflected in some places by local causes ?Nor is this all. The facts that have been observed in relation; to diluvial action in England, Scotland, Ireland, Sweden, Germany, Russia, and the Northern parts of Asia, seem to justify the inference, that the last deluge in those portions of the globe, came from the north ; though modified in its course by local causes. * Hence it would seem that this deluge, in all the northern parts of the globe, had this direction ; and may have been produced by the elevation of an extensive portion of the bottom of the Arctic ocean. De La Beche, in his recent able Geological Manual, f seems to regard the ' centre of disturbance ' as situated to the north of Europe; and leaves us to infer that diluvial action m America was merely the result of the mighty wave, proceeding from that centre. But so far as I can judge from the accounts which European geologists have given us of diluvial action, in that quarter of the globe, 1 doubt exceedingl}' whether it has left traces by any means as striking as in this country. As to grooves and furrows in the rocks, for example, the writer above quoted says, that ' Sir James Hall even considers that a rush over the land (in Scotland,) has left traces of its course in the shape of furrows, which the transported mineral substances, moving with great velocity, have cut into the solid rocks beneath.' Such language implies that these traces are by no means common, as in our country.Have we not then reason for supposing that the ' centre of disturbance ' might have been situated nearer to this continent than to Europe ? Although the general direction of the current on both continents seems to imply that its situation was not far from the north pole.I know of no instance in which organic remains of any interest have been found in our diluvium, with the exception, perhaps, of several species of recent shells in two or three places. The Messrs. Danas state, that in Cambridge, a common species of Mya was found, forming a stratum of three or four inches thick, in the side of a hill ; also strata of Mya, Mytilus, and Ostrea, several inches thick, and from five to ten feet below the surface, at Lechmere Point ; also fragments of Mya, 40 feet below the surface, at Jamaica Plains, in Roxbury, and the fragment of a similar shell 107 feet deep in the soil at Fort Strong on Dorchester Heights.3. TERTIARY FORMATIONS.For a long time these formations were confounded with alluvium and diluvium ; but they are clearly distinguished from both, by the much finer state of most of the materials that compose them; by the greater regularity of their stratification, by their relatively inferior position, and by containing peculiar organic remains. As appears from the Map attached to the recent geological work of Mr. Lyell,* tertiary strata occupy more than half of the surface of Europe ; yet geologists had paid very little attention to them till the publication of the work of Cuvier and Brongniart, on the Paris Basin, in 1811. In our country, although these formations occupy a vast extent of surface, particularly in the southern States ; embracing that broad tract along the coast marked on Mr.Maclure's Map as alluvial ; yet have they received but very little elucidation. Messrs. Morton, Vanuxem, and Conrad, have, however, recently devoted themselves successfully to this subject.After the tertiary beds around Paris and London had been described, it seemed for a long time to be taken for granted, that tertiary strata all over the world must be identical with these : as if those spots contained the types of the whole globe. But geologists now find that no formations are more independent than thetertiary ; and that it is very difficult to ascertain a precise identity of origin of any two basins, even when near to one another ; and as to those that are widely separated, it is no easy matter to determine whether they were deposited during the same geologicalepoch.I shall describe the tertiary rocks of Massachusetts under two divisions: 1. The most recent tertiary ; and 2. the Plastic Clay.These are distinguished from each other by their mineralogical characters, their organic remains, and the different position of their strata.The most recent Tertiary.The most extensive deposites of the beds of this class, are in the valley of the Connecticut ; where they are marked on the Map. They occur also, in small patches in many other places in the State : but they have been marked on the Map in no other place, except in Cambridge and Charlestown. The great resem blance in the mineralogical characters of these beds all over the State, their horizontal position, and the almost entire absence of organic remains in them, so far as they have been examined, have made it impossible to describe them as distinct deposites ; though I have little doubt, that many of them, at least, are such. Yet probably they do not differ much in age. But I leave to future observers to settle what I have no means of deciding.These newest tertiary strata consist of horizontal alternating layers of white siliceous sand and blue plastic clay. Along the Connecticut, the sand occupies the highest place in the series ; and covers most of the surface. Its upper portion is disturbed and piled up irregularly by diluvial action ; and sometimes mixed with transported gravel. But where the streams have worn passages from 10 to 15 feet deep, they have almost uniformly disclosed the stratum of clay. And not unfrequently tracts of considerable extent are entirely swept of sand, whereby the soil is rendered highly argillaceous. Generally the beds of sand and gravel appear to be several feet thick ; but sometimes I have found numerous alternations in the height of a few feet, or even a few inches—someof the layers not being more than half an inch thick. Some years since, I obtained the following rough sketch of a cliff, a few feet in height, in Deerfield ; the face of which had recently been laid bare by the sliding away of its outer portion. The beds a, a,.&:c. h, h, &1C. c, and d, represent different horizontal layers of sand and day ; the former becoming often very fine, and the latter sometimes approaching to loam. Some of the layers of claywere not more than half an inch thick ; and these in general, with the interstratified sand beds, appeared as if deposited from water perfectly at rest. But the stratum c, presented a most remarkable exception. It was composed of angular and rounded pieces of clay, mixed with sand, and obviously resulted from the breaking up of several thin beds oH clay and sand, by some unusual agitation of the waters. The stratum d, was still more remarkable.It consisted of sand and two layers of clay ; the latter being very irregularly bent, as if, when in a [plastic state, it had been acted on by opposing lateral forces.If I mistake not, this section throws light upon the manner in which some of the disturbances in the older rocks may have been produced. Let the stratum c, be only consolidated by heat, or otherwise, and we have a perfect conglomerated sandstone, or graywacke. Let the stratum d, be not only consolidated, but partially melted, so as to become in a good degree crystalline ; and we have that variety of mica slate or quartz rock, in which the planes of stratification do not correspond with the contorted schistose layers. The undisturbed beds of sand, by tiie same igneous action, might be converted into quartz rock, 'or mica slate ; and tlie interlaminated layers of clay, into argillaceous slate, or hornblende schist, or both. Thus from this thin tertiary formation,might result hornblende slate, mica slate, quartz rock, argillaceous slate, conglomerated grayvvacke, and sandstone : and these might present much of the regularity and irregularity peculiar to each rock. And to accomplish this, and also to give the strata an inclined position, we have only to suppose the same volcanic agency to be exerted, which we know has been a thousand times employed in the elevation of the strata, and in the protrusion of the unstratified rocks. Indeed, from some of the sections and descriptions given in the third volume of Lyell's Geology,* of the induration of the Newer Pliocene strata (newer tertiary) in the isle of Cyclops, near mount Etna, it appears that a considerable part of this transformation has there been accomplished.The surface of the tertiary formation in the vicinity of Boston has been so much acted upon by diluvial currents, that as already remarked, I have been at a loss whether to describe it astertiary or diluvial. But there is no doubt, I believe, that genuine clay beds, or layers of clay, do exist not far beneath the surface.This clay is represented by the well diggers as extremely hard ; and underneath it, are layers of sand and gravel. It is from 70 to 120 feet thick; and when [perforated, water rushes upwards with great violence. The only genus of organic remains found in the tertiary of the Connecticut valley, I have discovered also in the clay of Charlestown ; unless I have greatly misapprehended its character. But the same genus occurs also in the clay beds of Nantucket ; which I have been inclined to consider as belonging to the Plastic Clay ; so that this relic does not seem to afford much aid in determining the relative antiquity of these several beds
Dana, "Mineralogy and Geology of Boston" 1818
SPECIES II . — PETROSILEX . Petrosilex , Cleaveland , p . 242. Compact Feldspar , Jameson , vol . 1. p . 276 . Continuous Feldspar , Kirwan , vol . 1. p . 323. Compact Feldspar , Aikin , p . 213 . Chert of some mineralogists .
External Characters . Its colours are white , red , grey , green and black . Of white , it occurs greenish white ; of red , brownish red , cochineal red , and brownish purple red ; of grey , yellowish grey ; of green , leek green and olive green , and of black , greyish black . All the col- ours are of various intensity , and frequently two or more of them occur in dots , or in stripes and zones , either parallel or in undu- lating curves . It is fusible without addition , before the blowpipe . Different specimens are fused with greater or less ease ; generally they must be subjected to the blowpipe flame for one or two minutes , and even then , the edges and angles only become converted into a whitish frit or enamel , filled with bubbles . Some specimens yield a blackish glass .
Petrosilex is one of the most abundant minerals in the vicinity of Boston . It enters into the composition , and forms the basis of Porphyry in Malden and Lynn , and of itself , sometimes forms hills and presents high mural precipices . Elegant striped varie- ties are found at Milton ; rolled masses and fragments are found in alluvial soil in Cambridge , West Cambridge , Medford , New- ton , Roxbury , & c . and is disseminated in Amygdaloid at Hing- ham . It is one of the most frequent pebbles on Nahant , Nan- tasket , and Chelsea beaches .
The external surface of this mineral often appears earthy ; this arises from its great tendency to decomposition . Epidote , Feldspar , Quartz and Sulphuret of iron are not unfrequently im- bedded in it , but not so abundantly as to render it porphyritick ; some specimens are covered with beautiful dendritick impressions of black oxide of manganese , which often gives a uniform dark colour to a large surface . No mineral in this vicinity has so often been confounded with Jasper and Porphyry , by mineralogical students , as Petrosi- lex ; but the infusibility of Jasper , even when the blowpipe flame is urged by oxygen gas , and the compound structure of Porphyry readily distinguish these from Petrosilex .
SPECIES IX . EPIDOTE Epidote , Cleaveland , p . 297 . Actinolite , Kirwan , vol . 1. p . 168 . Pistazite , Jameson , vol . 1. p . 530. Glassy- Thallite , Aikin , p . 277 .
External Characters . Its colour is green ; of which it occurs olive green , blackish green and siskin green ; all the colours are of various intensity , and the siskin green is doubtless owing to the intermixture of reddish white quartz . Its lustre is shining and glimmering , and is vitreo - resinous and vitreous . It is opaque ; some of the crystallized varieties approach to translucent . It occurs amorphous , and crystallized in , 1. Acicular six - sided prisms . 2. In four - sided prisms . The crystals are seldom well de- fined , and their forms are determined with difficulty ; they are often fascicularly and promiscuously aggregated ; they are small and very small ; and their surfaces are longitudinally striated , and the four - sided prisms are transversely rent The streak is similar , and in the lighter coloured specimens , whitish . It is hard . The fracture is fine splintery , fine grained uneven , and flat conchoidal ; promiscuous and fascicularly diverging radiated . It is brittle . The crystallized varieties are easily frangible ; the amor- phous are tough . The fragments are splintery , indeterminately angular , and not sharp edged . Its specifick gravity is about 3.368 . Epidote is found in most of the rocks in the vicinity of Bos- ton ; they owe their green colour chiefly to the presence of this mineral . It traverses Greenstone , Sienite and Petrosilex in veins , which are sometimes several inches thick ; it is disseminated in Amygdaloid at Brighton ; and it frequently lines cavities in Preh- nite . The finest specimens are found at Nahant , where it also occurs in the greatest abundance .
SPECIES XIX . CLAY . Variety I. - Potter's Clay . Potter's Clay , Cleaveland , p . 371. Earthy Clay , Aikin , p . 255. Earthy . Potter's Clay , Jameson , vol . 1. p . 304 .
External Characters . Its colours are yellow and green . Of yellow , it occurs light honey yellow , between honey and straw yellow ; and of green , it occurs pale asparagus green and greenish grey . The colours are liable to variation from admixture of various foreign sub- stances It is dull . It is opaque . It is amorphous . When moistened , it exhales a strong argillaceous odour .
The taste is earthy . It adheres strongly to the tongue . It soils . Its streak is similar but lighter . It is friable . It is very soft . 1 The fracture is from coarse to fine earthy . It is easily frangible . The fragments are indeterminately angular Chymical Character . Before the blowpipe it fuses into a black or dark green enamel . Geological Situation and Localities . Clay exists in vast quantities in the vicinity of Boston , as at Charlestown , Dorchester , Cambridge , Danvers , & c . Use . : The purest occurs at Danvers , where it is extensively manu- factured into the coarser kinds of pottery . The impurer varieties are employed for making bricks .
CLASS III . - INFLAMMABLE SUBSTANCES . SPECIES I. - HYDROGEN GAS . Subspecies I. - Carburetted Hydrogen Gas . Carburetted Hydrogen Gas , Cleaveland , p . 386. Idem , of Chymists . Fire damp of Miners . This gas is disengaged in abundance from wet marshes or ' from the bottoms of small pools , or ditches where vegetable matter is decomposing ; the air bubbles , so frequently observed rising through the water , consist of this gas . By filling a bell glass , or a tumbler with water and inverting it over these bubbles , the gas may be readily obtained by stirring the mud at the bottom of the pool , with a stick . When a flame is applied to it , it takes fire and burns with pale bluish light . It is composed of carbon and hydrogen ; oxygen gas and carbonic acid gas are sometimes mixed with it
SPECIES II . - PEAT . Two varieties of Peat are found in large quantities in this vicinity , viz . the Fibrous Peat and the Compact Peat . Variety I. - Fibrous Peat . Cleaveland , p . 416 . This variety has a brownish colour , and is composed of leaves and parts of plants in a state of partial decomposition ; some specimens have a very loose texture , and the different substances are readily detached from each other ; others are more firm and appear to be cemented together by some vegetable substance , in a state of more complete decomposition . This variety is very light and spongy , and swims on the surface of water ; it is found near the surface of the ground , in a stratum , from a foot to sev- eral feet in thickness , and generally covering the Compact Peat . It is not employed for fuel , but is separated from the next variety and thrown into the pits formed by the excavation of peat ; here it undergoes other changes , and is gradually converted into Com- pact Peat .
Variety II . - Compact Peat . Compact Peat , Cleaveland , p . 416 . This variety has a much darker colour than the preceding , and is nearly black ; it is more dense , firm and compact , and when dry exhibits an earthy fracture ; no remains of organized vegetable matter can be discovered in it , excepting a few fibrillæ , and small roots . When recently dug , it is soft and slimy , and easily cut into parallelopipedons 2 or 3 inches square and 18 or 20 inches long . These two varieties accompany and pass insensibly into each other , the more spongy and loose being found at the surface , but becoming more firm and compact as the distance from the surface increases . Trunks of trees are found in Peat , in a horizontal position several feet below the surface , and in some instances , small beds of fine silecious sand . Peat when burning gives but little flame , and emits a pungent and peculiar odour , similar to that of burning leather ; it produces a strong heat , and affords an abundance of ashes , which are employed for scouring and polishing brass , & c . When Peat is burnt in a furnace , the ashes vitrify and cake together , and if moistened in this state while hot , they emit the odour of sulphuretted hydrogen . Large quantities of both varieties of Peat are found in New- ton , Lexington , Cambridge , Danvers , & c .
CLASS I. - PRIMITIVE ROCKS . I. - Granite . Var . 1. Graphick Granite . 2. Porphyritick Granite . II . - Argillite . III . - Primitive Trap . Var . 1. Common Greenstone . 2. Greenstone Porphyry . 3. Green Porphyry . IV . - Porphyry . V. - Sienite . Var . 1. Sienitick Porphyry . 2. Porphyritick Sienite
I. The characters of Argillite have been already sufficiently detailed , page 57. It forms in Charlestown , Watertown , Chel- sea and Quincy gently undulating eminences ; but their height will not entitle them to the rank of hills . II . Argillite is stratified ; the strata are horizontal . It is inter- rupted by numerous parallel rents , which have a two - fold direc- tion and obliquely intersect each other ; hence Argillite appears to be cut into rhomboidal tables . At the upper portion of the ele- vations , the Argillite is often wholly composed of small regular forms . The rents sometimes pursue various directions , and di- vide the Argillite into forms as various ; the sides of the rents are sometimes separated a few inches from each other , and the inter- stice is filled with a kind of breccia , formed of angular fragments of Argillite , cemented by ferruginous Clay . This aggregate forms sometimes floorings to veins of Lime . III . Argillite is the oldest rock which is to be observed , in , situ , in this vicinity . It is subordinate to Greenstone , in Charles- town , Brighton and Newton , and to Sienite in Milton and Brain- tree . It passes into Novaculite , which forms an extensive bed in it , at Charlestown , and into Petrosilex , at Dorchester and Milton . IV . The hills where Argillite predominates are insulated , their bases being surrounded by an alluvion . V. Chlorite and Greenstone occur in Argillite in small beds ; Calcareous Spar and Quartz traverse it in small veins ; some- times an aggregate of Calcareous Spar and Quartz , with an Ar- gillaceous basis , is found in small veins in Argillite ; the Quartz is in small crystalline grains , and the Lime is intricately associated with it ; the mass is of a bluish grey colour and effervesces with acids ; by exposure to the atmosphere the surface becomes disin- tegrated and earthy ; it is found at Charlestown . We have ob- served no metals in Argillite , but the Sulphuret of Iron , which by its decomposition , frequently covers the rock with a coating of rust .
Of the Alluvion . ( Coloured on the map , gamboge yellow . ) The principal alluvion , though irregular , may be considered as having a triangular form . Its southwest boundary is Grey- wacke ; its southeast are the waters in the harbour of Boston , and its northwest is Greenstone principally , and Porphyry . It forms the peninsula which connects Nahant with the main , and pro . ceeding to the Porphyry formation in Lynn , it runs at its foot , southwesterly to Malden , and thence westerly , bounded by Pe- trosilex , to Medford near the banks of Mystic river ; here it turns northerly , and running in between Greenstone , it meets in Wil . mington and Reading , the great alluvion , which comes in through Chelmsford & c . from New Hampshire ; thence , having a south- west course generally , through part of West - Cambridge and Waltham ; it meets the Greywacke in Newton , and bounded by this formation , it runs southeast , through part of Brighton , Brook- line , and Roxbury to Dorchester , where it meets the ocean , and the southeast boundary is the coast , from this place to Nahant . + This deposition is interrupted in three places . At Malden and Charlestown by Argillite ; and at Watertown by Argillite and fine - grained Greywacke , which occurs here in small quantity ; these formations appear to be insulated in the alluvion . In the peninsulas of Boston and Charlestown , and in some parts of Dor- chester and Chelsea , are found the highest elevations of this dep- osition , as for example , Bunker's and Breed's hill , Dorchester heights , & c . and again , immense plains are formed by it , as at Cambridge and Waltham . Its greatest breadth is from four to five miles , and its narrowest portion a few rods only . At Sweet Auburn , in Cambridge , this deposition appears to have suffered some changes ; it is here formed into extensive ridges , of singular regularity , and which are separated from each other by deep ravines ; they bear great resemblance to ancient fortifications . In other parts , deep basons are shown , which contain small pools of water , and if conjecture may be allowed , it is probable , that the ravines were once outlets from these basons . * A small alluvion is found about the banks of Charles ' river , in Dedham and Needham , bounded by Petrosilex on the west , by Sienite on the east , and north by Greywacke ; this probably reposes on Sienite and Petrosilex , which are on each side of it . Another alluvial deposit , of considerable extent , is found stretching southeast from the Greywackein Dorchester , through part of Quincy and Weymouth to Hingham . Its northeast boundary is the waters of Boston harbour , and its southwest is Sienite . Argillite appears in it at Quincy . An alluvion begins in Lynn , and runs northerly through Danvers , between Sienite and Greenstone , beyond the compass of these observations .
FINAL REPROT ON GEOLOGY OF MASSACHUSETTS:1841. EDWARD HITCHCOCK,
ARGILLACEOUS oR CLay SLATE. This rock is evidently nothing but clay more or less indurated and divided into very- thin layers. When only moderately indurated, the rock is called shale, and exists in that state in the coal formation. But when hardened so as to become somewhat shining in its appearance, it is called clay slate.When found in connection with graywacke, it sometimes contains organicremains, and has been called transition clay slate. But when these all disappear and the surface has much lustre, it is called primary clay slate, andis associated with the newest of the primary stratified rocks, of which it forms one of the members. _ These terms, however, are fast getting out of use ; and probably the sooner they are gone the better.All the clay slate in Massachusetts belongs to the oldest varieties ; unless it be the narrow band which I have already described under graywacke, as occuring around Boston. This evidently occupies a lower place in the series than the graywacke; and fragments of it are sometimes seen in the conglomerates of the newer rocks: Hence I must regard it as an older formation thanthe graywacke. It is also entirely destitute of organic remains: as is all the clay slate in Massachusetts.It is well known that in Europe, most of the clay slates exhibit lamine of cleavage which do not coincide with those of original deposition. But I have sought in vain for such a distinction in the clay slate of Massachusetts. The rock has, indeed, very often a jointed structure, occasioned by oblique divis ional planes. And from the fact that the lamine which form roofing slates in the deposits are often more even than those resulting from deposition, and that they frequently are inclined at a larger angle than the adjoining rocks, I am disposed to believe that they are usually lamin of cleavage and not of deposition ; that is, produced by chemical agencies subsequent to deposition. Butprobably ina majority of instances, I have found these laminz abounding in minute undulations, and a slight difference of color was obvious in the different layers, which facts certainly lead to the supposition that they were produced by original deposition. Very probably, had I been able to spendmore time in the examination of our clay slate deposits, and had they been more fully developed in the state, I might have discovered planes of stratification and deposition differing from those of cleavage. But at present I canonly say, that when I speak of the strike and dip of the strata, I mean those divisional planes which form the slaty lamine.Evidence of Disturbances in the Argillaceous Slate.I do not here refer to those agencies by which the strata of this rock have been elevated; nor to those by which its usual flexures have been produced ; but to some movements that have taken place in certain anomalous directions. The instances which I shall refer to, all occur in the Franklin County range, and mostly in Guilford, Vt. :In some instances we find veins of quartz in the slate, as represented below. Here it is obvious, both from the curvatures in the undulating ridges of the slate, and from the wedge-form shape of the veins, that a force must have acted laterally on the edges of the laminz, while they were ina partially plastic state: and that an infiltration of quartz must have taken place subsequently.It is not perhaps difficult to conceive how such a lateral action might haye taken place, when the strata were originally elevated. The specimen from which the drawing was taken, (No. 411.) was found near the north line of Guilford, on the stage road.In the principal quarry of slate in that town, on the stage road to Brattleborough from Greenfield, are seen occasionally divisional planes perpendicular to the horizon, and to the laminz of the slate, which are nearly vertical, and run north and south, Not unfrequently, however, the slate at these cross fissures, when its edges are viewed from above, is bent as.in the following figures, which exactly represent the specimens, No. 417 and 418.2. DILUVIUM, OR DRIFT.I shall probably be thought by some, either ignorant of the present state of geology, or unreasonably tenacious of former opinions, by retaining the term Diluvium, to designate that coating of gravel, sand, and clay, covering the surface almost everywhere, and resulting from aqueous agency between the deposition of the tertiary and alluvial strata. By doing this, I do not intend to advocate the opinion that all this deposit was the result of one transient universal deluge. But in New England, the greater part of itcertainly appears as if the result of powerful currents of water, rushing over the surface in the manner of a deluge. So that in this country, diluvium can hardly be a misnomer. Yet this is not my reason for retaining the term.I retain it on the following grounds. Notwithstanding the efforts of some distinguished geologists to expunge the diluvial formation from the geological series, the decision of a large majority of geologists, as I apprehend, is, that for the present at least, it must be retained. ‘To strike it from the list of North American rocks, would be to dispense with the most remarkablemember of the series. 2. All the substitutes that have been proposed for that of Diluvium, such as “Erratic Block Group,” “Bowlder Formation,”“ Detrital Deposits,” “ Drift,” &c. seem to me to be as deficient in signification, and to convey as erroneous notions as the term Diluvium, which in euphony certainly takes the precedence. 3. Probably no part of geology isin a more unsettled state, or more imperfectly understood, than that of diluvium: and while it continues so, a designation for the formation is of little consequence, provided observers describe accurately what is includedunder it. When the exact limits and theory of the formation shall be settled, an appropriate name-can be easily applied to it.I have intimated that the limits of this deposit have not been certainly fixed. In general we find it spread irregularly over every other formation but the alluvial. But some deposits in Massachusetts, which I had formerly supposed to belong to the newest tertiary, I now place under diluvium ; and a similar disposition of some supposed tertiary beds has been made in Europe.On the other hand, I find it still more difficult to separate diluviumfrom alluvium in some cases; though the general characters of the two formations are very distinct.I do not include under diluvium the tearing up, rounding, smoothing, and comminution, of the bowlders, gravel and sand, that now compose it: although without doubt a part of these processes must have been performedduring the diluvial period. But probably a large proportion of this work was accomplished by aqueous agencies, previous to that more powerful one which we denominate the diluvial. That agency I suppose brought thesematerials, previously in a measure broken up and rounded, into their present state. And yet I do not suppose this agency to have been a very transient one. For I hope to present evidence soon of its long continuance aswell as great power.Of all the formations probably diluvium is the most difficult to study. A man may obtain some tolerable idea of the general geological features of a country by passing through it once or twice. But no transient traveler can tell us much of its diluvial phenomena. In order to do this, he must first become accurately acquainted with all the older formations and their limits.Else when he finds pebbles and bowlders drifted from their parent bed, how can he be certain of the direction in which they came’? It is now at least twenty years since I began to examine Massachusetts geologically ; and ten years since I commenced exploring the whole state. No year has passed in which I have not accumulated many facts respecting diluvium; for which I have always kept an eye open. The great number of these facts which I shall here present, will at least show I think, that the general conclusions at which I arrive, have not been formed hastily, and without broad premises. Those who will take the trouble to compare the present with my former reports, will see that those general conclusions have not been altered since they were first announced : and that the additional facts which I now give, only serve to render them firmer. Whatever corrections future observers maybe obliged to make in my statements, I feel quite sure that they will never doubt that the diluvial waters in Massachusetts took a direction between south and southeast ; and that they have left upon the solid ledges innumerable furrows and scratches, as proofs of their direction and great power.These are the most important results at which I havearrived; and they are entirely independent of hypothesis. When first announced, the latter statement especially, respecting grooves and scratches, was received I believe by the ablest geologists of our country with strong scepticism. But I doubt whether all of them have not ere this seen enough of such phenomena inother parts of the land, to be satisfied that [ have stated only the truth: if 1 may be allowed to form an opinion from the numerous annual GeologicalReports that have appeared in different parts of the country within a few years past; and from several articles in the scientific journals. In some instances the direction of these grooves varies considerably from those in Massachusetts: but I speak now only of their existence. And as I have beenobliged to bear the obloquy of stating what was thought to be erroneous, I trust I may claim the honor, if there be any, of first calling public attention in this country to an interesting geological phenomenon.In general terms, this deposit may be said to lie between the tertiary and alluvial strata. But it seems quite probable that some finer deposits of clay and sand, which were produced during the same geological period, have been referred to the tertiary strata. In my former Report I described certain horizontal deposits of blue clay, covered by layers of sand, occurring in limited deposits in the state, especially in the valley of Connecticut river, as belonging to the Newest Tertiary: because I had no evidence that the diluvium passed beneath it. But since that time, I have discovered several sections, recently made, which render it almost certain that this Newest Tertiary constitutes a part of the diluvial deposit :—usually its upper part: but sometimes interstratified with it: as in Fig. 65, which will soon be exhibited, and which is a section in diluvium. Wherever valleys of any considerable extent existed in the state at the period of diluvial action, and these were cut off from the ocean by some barrier, I find this clay and sand. I infer that it was deposited by the retiring diluvial waters ; though I undertake not here to decide whether those waters retired in consequence of thesubsidence of the ocean, or the elevation of the land. I shall present evidence that in some parts of the state, this draining of the waters occupied a considerable length of time: and probably left large expansions of the riyers or lakes, which were not drained for centuries, or until the streams at their outlets had worn down the barrier. This process I should call an alluvial agency: and I consider the diluvial agency to have ceased at the pointwhen the effects can be explained by existing agencies, operating with their present intensities.In general terms, this deposit may be said to lie between the tertiary and alluvial strata. But it seems quite probable that some finer deposits of clay and sand, which were produced during the same geological period, have been referred to the tertiary strata. In my former Report I described certain horizontal deposits of blue clay, covered by layers of sand, occurring in limited deposits in the state, especially in the valley of Connecticut river, as belonging to the Newest Tertiary: because I had no evidence that the diluvium passed beneath it. But since that time, I have discovered several sections, recently made, which render it almost certain that this Newest Tertiary constitutes a part of the diluvial deposit :—usually its upper part: but sometimes interstratified with it: as in Fig. 65, which will soon be exhibited, and which is a section in diluvium. Wherever valleys of any considerable extent existed in the state at the period of diluvial action, and these were cut off from the ocean by some barrier, I find this clay and sand. I infer that it was deposited by the retiring diluvial waters ; though I undertake not here to decide whether those waters retired in consequence of thesubsidence of the ocean, or the elevation of the land. I shall present evidence that in some parts of the state, this draining of the waters occupied a considerable length of time: and probably left large expansions of the riyers or lakes, which were not drained for centuries, or until the streams at their outlets had worn down the barrier. This process I should call an alluvial agency: and I consider the diluvial agency to have ceased at the pointwhen the effects can be explained by existing agencies, operating with their present intensities.In general terms, this deposit may be said to lie between the tertiary and alluvial strata. But it seems quite probable that some finer deposits of clay and sand, which were produced during the same geological period, have been referred to the tertiary strata. In my former Report I described certain horizontal deposits of blue clay, covered by layers of sand, occurring in limited deposits in the state, especially in the valley of Connecticut river, as belonging to the Newest Tertiary: because I had no evidence that the diluvium passed beneath it. But since that time, I have discovered several sections, recently made, which render it almost certain that this Newest Tertiary constitutes a part of the diluvial deposit :—usually its upper part: but sometimes interstratified with it: as in Fig. 65, which will soon be exhibited, and which is a section in diluvium. Wherever valleys of any considerable extent existed in the state at the period of diluvial action, and these were cut off from the ocean by some barrier, I find this clay and sand. I infer that it was deposited by the retiring diluvial waters ; though I undertake not here to decide whether those waters retired in consequence of thesubsidence of the ocean, or the elevation of the land. I shall present evidence that in some parts of the state, this draining of the waters occupied a considerable length of time: and probably left large expansions of the riyers or lakes, which were not drained for centuries, or until the streams at their outlets had worn down the barrier. This process I should call an alluvial agency: and I consider the diluvial agency to have ceased at the pointwhen the effects can be explained by existing agencies, operating with their present intensities.These have been partially stated: but the whole need to be given. 1.Bowlder Stones or Erratic Blocks. How large a rounded and transportedblock of stone must be in order to make it a bowlder, seems not to be exactly settled. But in Massachusetts the amount of them that are of great size is so large, that we need not reckon those of doubtful character. These bowlders form one of the most striking objects in the landscape in many parts of the state, and scarcely no part is free fromthem. 2 Gravel and Sand mixed together confusedly. 'These constitute the great body of diluvium; and are ° composed of every variety of rock found in the state, and of some varieties found in place only beyond its limits. 'The softest kinds of rock, however, have been mostly reduced to sand or clay ; and the great mass, both of bowlders and pebbles, consists of the most unyielding of our rocks ; such as quartz, porphyry, sienite, greenstone, gneiss, mica slate, and granite. Thus, in the region west of Connecticut river, while masses of the quartz rock of Berkshire county are met with at almost every step, it is very rare to meet witha fragment of limestone at the distance of more than half a dozen miles from the limestone deposits; although a glance at the geological map will show, that the latter occupy much more of the surface in Berkshire county than quartz rock. 3. Beds of Clay. This clay is usually of a blue color, but becoming nearly white in those parts of the state where a great deal of feldspar is contained in the detritus. In all cases, however, this diluvial clay abounds in the protoxide of iron. It is not of much use except for making bricks, and common red earthern ware. Ususually it occupies basins, ortrough shaped cavities, and must have been deposited in quiet waters ; as neither bowlders nor gravel are usually mixed with it. Frequently, however, as along the sea coast, its deposition is confused, and sometimes (ex. gr.at Fitchburg, west end of the village,) pebbles are mixed with it ; and sometimes (as on the rail road cut in the south part of Palmer,) even quite large bowlders. 4. Consolidated sand and pebbles. In some instances the hydrate of iron acts as a cement of diluvium ; but the rock thus produced is easily crumbled down. Carbonate of lime, however, in some localities, has formed a conglomerate of considerable tenacity. ‘The calcareous diluvium which is not uncommon in Springfield, West Springfield, and South Hadley; andwhich has been particularly described in the first part of my report, is sometimes very firm; though on exposure to the weather, it at length crumblesdown ; and therefore can hardly answer for any purpose of construction. No.1560 is an example of this rock.5. Beds of Sand. 'This sand is siliceous and varies from very fine to quite coarse ; the latter usually lying at the top. The beds of this sand are almost invariably spread over the beds of clay, not only in Massachusetts, but in other parts of the country which I have visited: and so common is the fact, that where we find clay, we expect to find over it layers of sand, unless alJuvial agency hasremoved them. I cannot but infer, therefore, that these siliceous deposits resulted from some general agency ; and [cannot conceive thatany cause now operating with its present intensity could have produced them, and therefore refer it to diluvial action. 6. Limonite, or Hydrated Oxide of Iron. This I have found among diluvium only 1i n Berkshire County and in the eastern part of New York. And there it is a diluvial product in no sense except that diluvial action has torn it from the beds of this ore that project from the strata of mica and talcose slate. I have not seen in the state any ex-: ampleo f a deposit of iron ore during the diluvial period worthy of notice, although we frequently meet with pebbles coated and sometimes cemented byhydrate of iron, and probably some of the deposits of yellow ochre belong to this period.That the clay and much of the sand which I have included in this formation, are stratified, will admit of no question by those who have seen goodsections through them. But some have an impression that what is usually called diluvium, viz. gravel and bowlders, was thrown together in such utter confusion, as to exhibit no sorting of the materials and no parallelism of arrangement.And the usual aspect of diluvial hills, seeming to be a merepromiscuous mass of gravel and sand, is apt to confirm this idea. But when a fresh section is made through such hills, and through the formation generally, I have rarely failed to discover as distinct marks of stratification as in the older fragmentary rocks equally coarse. In fact, I cannot conceive how detritus could be deposited in water the most violently agitated, without separating in some measure into portions more or less coarse: and this is nearlyall the stratification which diluvium exhibits. It is sometimes made more striking by some of the layers being a good deal more ferruginous than oth_ ers: but rarely by any thing like a seam separating them: and the same thing is usually true of the coarsest conglomerate. The sections which I shall shortly present show the extent to which it is stratified._ As a general fact, the stratification is horizontal; or as much so as in any deposit made from water.A formation deposited like diluvium must be supposed to have almostevery variety of thickness: and such is the case with this deposit. Wefind it in every place where such materials as compose it could find a lodgment, when acted upon by waters in violent agitation. We should not expectit upon the tops of narrow ridges and insulated peaks, nor upon steepescarpments, except in small quantity, even though carried there by the currents.But wherever narrow valleys or gorges existed, through which thediluvial waters might have rushed, we should expect the detritus to be accumulated in the greatest abundance: and there in fact we usually find it of thegreatest thickness, and its characters most strikingly developed. Yet its maximum thickness, in such places, is extremely difficult to be ascertained : especially in a country like ours, where, until recently, no deep excavations have been made.It is chiefly because their greater size renders them more conspicuous, that the phenomena of bowlders are more striking than those of sand and gravel.For the origin and dispersion of both are essentially the same. Nevertheless, bowlders are by far the most instructive index of diluvial agency in Massachusetts.It seems difficult to conceive how running water should have been able to remove such enormous masses. Yet it is certain that all of them have been transported quite a distance. ‘Those in Fall River and Troy for instance, must have been brought from the opposite side of Taunton river, or from several miles north. ‘That in Plymouth correspondsto ledges in the Blue Hills,near Boston: but I know of no porphyry ledge south of those Hills: andthey are 30 miles north of Manomet Hill. Is it possible that such a bowlder could have been carried so far? The bowlder in Fitchburg, that has been discribed, must be have been brought several miles and across deep valleys. ‘Thesame is true of that in Adams; and the valley which it must have crossed, isa very deep one. In contemplating this Sisyphean labor, however, weought to recollect that a rock in water loses nearly half its weight.know of no currents in our existing seas of sufficient velocityand power to produce a moiety of the effects which the diluvial currents have produced. The present currents could, indeed, transport icebergs loaded with bowlders and pebbles, at the rate of 3 or 4 miles per hour, and along some narrow friths, at the rate of 10 or 12 miles per hour; and these bowlders might be dropped from time to time along the bottom. But this is a very different thing from urging over that bottom such a mass of detritus, and with such violence, as to wear down the surface, and score it over so deeply as we find to have been done upon our rocks. But it may be said, that the breakers upon. an exposed shore might have accomplished the work. They mighthave worn away the rocks: but they could not have produced those parallel and continuous grooves and scratches, which the more exposed surface of New England every where presents, and which hold on their course obliquely across those lofty ridges, which must have formed the shores of the ocean, as the continent gradually rose from the waters. The diluvial phenomena of our country indicate a current that swept over the entire surface, and not waves breaking against cliffs and rolling back. ‘The latter produce 5. It is difficult to explain all the phenomena of diluvial action on this continent, without calling in the aid of icebergs, or some other agent, by which masses of rock could be floated along the surface of the waters; yet after all, those icebergs will explain but a small part of the effects of this action.I cannot conceive of any other way,except by icebergs, by which those bow]ders, found many miles from their native beds, and perched upon high and almost perpendicular ridges, or dropped in the midst of sandy plains, could have been transported. But to suppose that the great mass of sand, gravel, and blocks, composing diluvium, could have been borne along by icebergs, is an hypothesis which any man would abandon as soon as he should examine that deposit, and see its amount. ‘Then again, he would find that usually the largest and most numerous bowlders lie nearest to the rock from which they have been detached ; and that the train decreases in size and number as we proceed southeasterly, except the few straggling bowlders already alluded to. Now I cannot conceive that an iceberg, lifted up by the watersfrom a particular spot, with bowlders frozen into its lower parts, should begin - to let those bowlders fall out within a few rods from the place where it started.I should expect that in general they would not begin to fall, till the iceberg began to melt. In short, the great mass of diluvium appears as if driven pell mell along the surface by violent currents; and so does the surface over which it has been urged, whenever it can be examined. It is difficult, therefore, to explain some of the phenomena of our diluvium without icebergs ; but still more difficult thus to explain the whole.6. The phenomena of diluvial action in this country far transcend theeffects of any agencies now operating, without admitting a great increase of intensity in their action. So much has been said of late years, and so ably, respecting the sufficiency of the present operations of nature to explain all past phenomena, that one cannot but feel a diffidence in resorting to extraordinary agencies in past times. But in vain have I searched for any agencynow at work, that produces effects at all comparable to the effects of diluvial action. ‘The statements of Professor John Phillips, one of the ablestliving geological writers, concerning diluvial action generally, is eminently applicable to that portion of it that has fallen under my notice, in America.“Such effects,” says he, “are not at this day in progress ; nor, in general, can we conceive the possibility of their being produced by the operation of existing agencies, operating with their present intensities, or in their present directions.” (Treatise on Geology, Vol. I. p. 296.) I have frequently examined the rocky shores of the ocean, and the beds of rivers where theyform cataracts, or force their way through narrow defiles and gorges, to ascertain whether the effects of the powerful currents and waves of the presenttime are comparable to those of diluvial action: and I have always been struck with the vastly greater force of the latter. On exposed shores, and at the bottom of gorges and cataracts, we do, indeed, find the rocks worn away; but their surfaces are usually smooth, and almost wanting in those deep and extended grooves and scratches, which are the result of diluvial action. Deep gorges do, indeed, show the process of extensive excavations.But this work is generally not so much the effect of direct erosion,as of the expansion of water in the fissures of rocks, by freezing ; in consequence of which, large blocks are gradually lifted up, while the stream wearsoff their angles, and at length has force enough to move them from their beds, as well formed bowlders. In this and other modes of an analogous kind, I presume the largest part of bowlders have been formed, or got ready to be driven southerly by diluvial currents. But one sees at once that the two agencies have operated in a very different manner. While the excavations of gorges and the wearing away of the sea coast, have been the combined and slow result of disintegration and moderate erosion, diluvial phenomena seem to have been the direct result of mighty currents of water, loaded with detritus, and rushing over the surface with such violence as to wear down the rocks much more powerfully and rapidly: a violence that did not permit the detritus to find its way around obstacles, but forced it directly over them in a manner altogether different from the comparatively puny action of water at the present day. But after all, it is impossible so to present this subject as to make one feel it, who is not extensively familiar with diluvial phenomena.7. The facts that have been presented respecting diluvial action, furnish a probable reason why the diluvium of this country is so destitute of organic remains. In New England I scarcely know of any of these remains, except perhaps a single example of bones of the mastodon found in Sharon,Berlin, and Cheshire in Connecticut ; anda few shells in some of the diluvial clays in Maine: and not improbably in both these instauces the deposits may have been alluvial. The same may be said of all the instances of organic remains in the diluvium of this country. Its almost entire want of these, so far as discoveries have yet been made, is a wellknown fact. And if the diluvial waters were poured down from the arctic regions, loaded with ice, wesee the reason of it. Animals did not live in such waters; and we ought to expect to find in them only those animals which were then the inhabitants of this continent, and were destroyed by the diluvial waters. Whether themastodon and other extinct races discovered in this country were then destroyed, or subsequently, does not seem to be well determined. Even the morequiet period that succeeded the early periods of diluvial action, seem to have been almost destitute of organic existence. Hence it is, that the blue clay of New England contains almost no relics of animals or plants.It will be seen that in the preceding inferences I have not attempted to form any general theory of diluvial action, but only to state the more important independant conclusions to which the facts, with more or less of probability, conduct us. Let us now see whether these facts will sustain any ofthose numerous diluvial theories that have been proposed in recent times by ingenious men. I may pass by the hypothesis to which some have clungeven in our own times, which imputes diluvial action to the shock of a comet; since it seems now quite certain that these bodies are in general composed of matter thinner and lighter than air.Another theory, which has long been a favorite one, imputes diluvial action to the deluge of Noah. The freshness and apparent recency of theeffects of this action and its apparent universality, give at first view a strong probability to this supposition, if we understand the language of scripture in its most literal sense. But many distinguished biblical writers regard the description of the Noachian deluge as an example of the use of universal terms with a limited meaning, and hence regard that deluge as not universal over the globe, but only over the region inhabited by man. Again, if the diluvial action of geology resulted from the deluge of Noah, why are the organic remains found in diluvium mostly of extinct animals? and why is not manamong the number? Finally, the diluvial action of geology must have occupied a much longer period than the 150 days, or at longest, the year, ofthe Noachian deluge. It is difficult, if not impossible, to make any one feel the force of this objection, who is not familiar with diluvial phenomena. But he who has seen where the hardest rocks have been worn away many feet at least, and probably sometimes many hundred feet, by diluvial action, cannot but see that many years must have been required for the work, even though the waters were driven over the surface with the greatest violence.These objections, are very conclusive against this hypothesis. As bearing upon the veracity of the sacred historian, however, this question I conceive can have little or no importance, since it does not prove the non-occurrence of the Noachian deluge, even though no traces be now remaining upon the earth’s surface of that eventA theory has lately been started to explain diluvial phenomena, founded on the action of glaciers in the Alps. Though not originally proposed, it has been chiefly elucidated and defended by the distinguished naturalist, Agassiz, of Neuchatel in Switzerland. That it may explain the movements of detritus in the Alps, which for the most part appears to have been carried outward from the axis of the mountain, I am not disposed to deny : and of course it would explain equally well all similar cases where slopes exist, down which glaciers may have descended. But I am unable to see how this agency could have transported detritus in a southerly direction several hundred miles, over nearly all the most elevated ridges of this country, almost without reference to the direction of those ridges, and even have driven it upward along slopes considerably inclined, as appears to have been done on the western side of New England. Nor does it give any reason for the immense accumulation ofsand, pebbles and bowlders, piled up in a fantastic manner, at great distances from any existing mountains, as in Plymouth and Barnstable counties. In the case of the Alps, it is supposed that the ridges have been considerably elevated since diluvial agency commenced; and this vertical movementwould tend to scatter the detritus over a wider space. But we have no evidence that any of the higher ridges of this country have been raised sensibly so recently. Certainly there is no chain of mountains on this continent whose elevation would send bowlders and gravel southeasterly over hundreds of miles. Even though the valleys were filled by ice, yet where is the force to be found to urge the detritus so far in that direction. I confess I have as yet seen only a very brief and evidently imperfect developement of this theory, in one or two short notices in the scientific journals. But as I understand it, it seems to me inadequate to explain the tout ensemble of diluvialphenomena.Another theory of diluvial action supposes it to be merely ancient alluvial action. Its advocates suppose that the same causes of change, which are now in operation on the globe, have always operated as they do at present with no more variation of intensity than they now exhibit. Diluvial action they suppose to have resulted partly from ancient rivers, which at some remote period may be supposed to have flowed at any elevation, and subsequently to have lowered their beds to their present levels. Another cause may have been the bursting of lakes: another, landslips: another, the action of breakers upon the coast, as the continents gradually emerged from the ocean: another, the tides and currents of the ocean: another, icebergs, such as now‘float from arctic regions and drop bowlders and sand along their track : so that when the bottom of the ocean was afterwards raised above the waters, the surface would be strewed over with detritus, as we now find it.{fT have not entirely mistaken the phenomena of diluvial action in this country, especially in New England, it willbe exceedingly difficult to reconcile them to this hypothesis. The difficulties are numerous. In the firstplace, neither rivers, nor the bursting of lakes, nor breakers upon the coast, although they may prepare bowlders to be transported, will account for a current from 1500 to 2000 miles wide, which has swept southerly over this continent: Nor could they produce those grooves and scratches which are socommon and uniform in their direction in New England. In the secondplace, as I have attempted to show in another place, the relative levels of the surface have not very essentially altered since the period of diluvialaction; as they must have been by this theory. In the third place, oceanic currents have not sufficient velocity to transport any thing coarser than sand along the bottom, and therefore diluvial grooves and scratches could not have been thus produced. In the fourth place, although icebergs might have transported some bowlders, they could not have conveyed the great mass of diluvial detritus into its present situation: Finally, diluvial action, as I have attenipted to show, did not take place till this continent had attained nearly its present elevation above the ocean.Nearly the same difficulties encumber another theory, which imputes diluvial action to the retiring waters of the ocean; as chains of mountains were suddenly thrown up. If the continent was already above the ocean when it took place, this would be impossible. Or if it were not, as it rose, the waters would rush each way from the central axis: but in fact they moved only ina southerly direction. Again, the direction of the current was not at right angles to any system of elevation in this country with which we are acquainted: And finally, the effects vastly transcend the alledged cause.The only remaining theory deserving notice, supposes that an extensiveelevation of the bottom of the Arctic Ocean threw its waters, loaded with ice and detritus, southerly over the American and also the Eastern Continent.It supposes that theremay have been a succession of elevatory movements, producing successive waves, so that the waters may have risen andfallen, and during their ebb, they may have frozen to the surface; so that upon rising again new loads of detritus may have been lifted up, and driven onwards, and even up somewhat steep declivities, scouring and abrading the surface.This theory certainly quadrates better than any other with the diluvial phenomena of this country. Still it is very difficult to conceive how an oceanic current, however violent at its commencement, should have sufficient force, after passing southerly from the arctic region to the 45th, or even the 40th, degree of north latitude, to accomplish what the diluvial current has accomplished in this country. And at the longest, such an upheaving of the arctic ocean as is here supposed, judging from analogous movements in modern times, could not have occupied more than two or three years. But onecannot become familiar with diluvial phenomena without feeling a conviction that a much longer period must have been occupied in this work ; although compared with many other geological periods it was short. .To my mind, therefore, no theory of diluvial action hitherto proposed, is so free from objections that I feel satisfied with it. That remarkable and very powerful currents of water have swept over this continent from the north and northwest, F cannot doubt. ‘That this took place at a comparatively recent period, and probably since the continent assumed its present levels, and reached essentially its present height above the ocean, and that no existing agency, without a great increase of intensity, could have accomplished the work, seem to me almost equally certain. But I do not feel prepared to propose any regular theory of this agency in which I place much confidence.Yet the subject is one of the deepest interest, and the facts arefast accumulating concerning it ; so that erelong I doubt not every theoretical difficulty will vanish.In the clay beds of New England, which I have described as diluvial, two distinct and peculiar varieties of concretion occur, which deserve a particular description. One of them is composed of carbonate of lime and clay,and the other of hydrate of iron and clay. ‘Though familiar with the former from my earliest days, under the name of claystones, as is every boy brought up on the banks of Connecticut river, yet it is only since my last report to the Government that my attention has been particularly turned to thesubject. They are, indeed, described briefly in my Report of 1835: asare also the other variety produced by the hydrate of iron; the latter I then regarded as organic remains. ‘This opinion, upon the whole, I think must be given up: though the resemblance to organic remains is strong. Theclaystones are almost universally referred by the community to the action of water upon indurated clay. And since they are always rounded and generally found in the beds of streams, this is a very natural, though an entirely erroneous conclusion. ‘They are undoubtedly concretions, formed by laws somewhat analogous to those of crystalization ; probably while the clay was in a soft state, by the action of water.I freely confess, however, that so far as my means of information extend, the subject of concretions is involved in great obscurity. Thatthey are formed by segregation, in consequence of elective affinity, seems to be generally admitted. But why the particles should arrange themselves in curves, rather than in straight lamin, it seems difficult to determine: and we are quite ignorant whether any of the laws of crystalography will apply to the formationof concretions. Prof. Alexander Brongniart has, indeed, given the world a valuable paper on siliceous concretions. (Mnnales des Sciences Naturelles, Tome Vingi— Troisieme, p, 166.) And he has rendered it probable that silica takes the form of a concretion when in a gelatinous state, and of a crystal when in a state of solution. But this gives us not even a glimpse of the laws that regulate the arrangement of the particles: nor explains the reason why they should assume one form when in a gelatinous condition, and another Claystones.I shall first mention the localities and situation of the claystones of Massachusetts.They are more abundant in the valley of Connecticut river, than in any other part of the state.In general they are found beneath the waters of the river, where the bank consists of fine blue diluvial clay, in horizontal layers, about half an inch thick. Sometimes the water has removed them several rods; but rarely far enough to wear them much. The most abundant locality, is in Hadley, on the east side of Connecticut river, half a mile south of the village. In South Hadley (or perhaps within the limits of Springfield,) they occur half a mile south of the village, at the Canal, in a clay bankby the road side, some 50 feet above the river. In West Springfield they are found at the mouth of Agawam river; and also a mile up the stream, near the bridge leading to Suffield. In Amherst they are washed from a bed of clay at some distance from any stream. In Deerfield, they occur on the bank of Connecticut river, a few rods north of Sunderland Bridge. In Montague, they are seen at low water on the east bank of Connecticut river, ina clay bed, which underlies a meadow. Specimens will be found also in the State collection from Windsor and Wethersfield in Connecticut : where they are common in the clay beds, and differ from those in Massachusetts only in having the reddish tinge which is there common in the clay. Although I have seen imperfect claystones from the vicinity of Boston, and have heard of them in other parts of the State, yet I have met with good specimens out of the valley of the Connecticut, only in North Adams, where they are common ina clay bed which has been excavated for making. brick. Along Hudson river they are quite common: and I have met with them atthe slide on Presumpscut river near Portland in Maine. Indeed, they are found in nearly every part of our country, where diluvial clay exists.At most of the localities in Massachusetts that have here been described, thousands of specimens can be obtained.7. Argillaceous Slate. The argillaceous slate in the eastern part of the State is so intimately connected with the varieties of rock above noticed, that it ought in justice to be described as one of the members of the graywacke group ; although marked as a distinct deposit on the Map.And I doubt not but it is older: for fragments of this slate occur in one of the varieties of conglomerate that have been described ; and this not only shows the posterior production of the latter, but renders it doubtful whether both rocks were produced during the same geological epoch.In general its color is dark gray, passing to blue: It is rarely fissile enough to be employed for roofing. The laminz, as on Rainsford island, in Boston harbor are sometimes very tortuous.Not unfrequently it passes into an imperfect novaculite ; as in Charlestown, Roxbury There can be little doubt that the peninsula of Boston has a foundation of argillaceous slate.This is, indeed, the only rock that has ever been found therein place. And from the occurrence of argillaceous slate in South Boston, and in Charlestown, with a northerly dip in both places, it would be very surprising if any other rock should be found in Boston; unless it were an intruding mass of trap rock. But this slate on the peninsula is buried deep by clay, gravel, and sand.I have colored the peninsula as a diluvial deposit. Artesian wells bored there a few years ago, 260 feet deep, did not reach its bottom.Varioloid Wacke. The rock which I thus designate, has generally been regarded by those who have described the geology of Boston and its vicinity, as amygdaloid. But it seems to me that there are insuperable objections against the supposition that the nodules in general were introduced by infiltration, or even sublimation ; the only modes by which geologists suppose the cavities of amygdaloid were filled. For they consist generally of rounded masses of compact feldspar ; a substance which must certainly have been the result of igneous fusion. On the other hand, the rounded form of these nodules, and their non-crystaline structure in general, forbid the arrangement of this rock along with the porphyries. But some writers regard variolites as rather intermediate between porphyry and amygdaloid, ( Traite de Mineralogie, Par. T. S. Beudant, ( Paris, 1830,) Vol. 1. p. 569.) and such I suppose to be the character of the rock under consideration. By the term varioloid, 1 intend merely to designate the external aspect of the rock ; since the mode of its formation seems involved in much obscurity : but its variolous appearance none can deny.Professor Webster in his account of the geology of the region around Boston, states that the veins of greenstone in the graywacke conglomerate of that vicinity, run about 10° W. of South, and 10° E. of North. All such veins are probably of nearly cotemporary origin : their parallelism being explicable only on the supposition of their having been produced by the same cause.
EDWARD HITCHCOCK
ARGILLACEOUS oR CLay SLATE. This rock is evidently nothing but clay more or less indurated and divided into very- thin layers. When only moderately indurated, the rock is called shale, and exists in that state in the coal formation. But when hardened so as to become somewhat shining in its appearance, it is called clay slate.When found in connection with graywacke, it sometimes contains organicremains, and has been called transition clay slate. But when these all disappear and the surface has much lustre, it is called primary clay slate, andis associated with the newest of the primary stratified rocks, of which it forms one of the members. _ These terms, however, are fast getting out of use ; and probably the sooner they are gone the better.All the clay slate in Massachusetts belongs to the oldest varieties ; unless it be the narrow band which I have already described under graywacke, as occuring around Boston. This evidently occupies a lower place in the series than the graywacke; and fragments of it are sometimes seen in the conglomerates of the newer rocks: Hence I must regard it as an older formation thanthe graywacke. It is also entirely destitute of organic remains: as is all the clay slate in Massachusetts.It is well known that in Europe, most of the clay slates exhibit lamine of cleavage which do not coincide with those of original deposition. But I have sought in vain for such a distinction in the clay slate of Massachusetts. The rock has, indeed, very often a jointed structure, occasioned by oblique divis ional planes. And from the fact that the lamine which form roofing slates in the deposits are often more even than those resulting from deposition, and that they frequently are inclined at a larger angle than the adjoining rocks, I am disposed to believe that they are usually lamin of cleavage and not of deposition ; that is, produced by chemical agencies subsequent to deposition. Butprobably ina majority of instances, I have found these laminz abounding in minute undulations, and a slight difference of color was obvious in the different layers, which facts certainly lead to the supposition that they were produced by original deposition. Very probably, had I been able to spendmore time in the examination of our clay slate deposits, and had they been more fully developed in the state, I might have discovered planes of stratification and deposition differing from those of cleavage. But at present I canonly say, that when I speak of the strike and dip of the strata, I mean those divisional planes which form the slaty lamine.Evidence of Disturbances in the Argillaceous Slate.I do not here refer to those agencies by which the strata of this rock have been elevated; nor to those by which its usual flexures have been produced ; but to some movements that have taken place in certain anomalous directions. The instances which I shall refer to, all occur in the Franklin County range, and mostly in Guilford, Vt. :In some instances we find veins of quartz in the slate, as represented below. Here it is obvious, both from the curvatures in the undulating ridges of the slate, and from the wedge-form shape of the veins, that a force must have acted laterally on the edges of the laminz, while they were ina partially plastic state: and that an infiltration of quartz must have taken place subsequently.It is not perhaps difficult to conceive how such a lateral action might haye taken place, when the strata were originally elevated. The specimen from which the drawing was taken, (No. 411.) was found near the north line of Guilford, on the stage road.In the principal quarry of slate in that town, on the stage road to Brattleborough from Greenfield, are seen occasionally divisional planes perpendicular to the horizon, and to the laminz of the slate, which are nearly vertical, and run north and south, Not unfrequently, however, the slate at these cross fissures, when its edges are viewed from above, is bent as.in the following figures, which exactly represent the specimens, No. 417 and 418.2. DILUVIUM, OR DRIFT.I shall probably be thought by some, either ignorant of the present state of geology, or unreasonably tenacious of former opinions, by retaining the term Diluvium, to designate that coating of gravel, sand, and clay, covering the surface almost everywhere, and resulting from aqueous agency between the deposition of the tertiary and alluvial strata. By doing this, I do not intend to advocate the opinion that all this deposit was the result of one transient universal deluge. But in New England, the greater part of itcertainly appears as if the result of powerful currents of water, rushing over the surface in the manner of a deluge. So that in this country, diluvium can hardly be a misnomer. Yet this is not my reason for retaining the term.I retain it on the following grounds. Notwithstanding the efforts of some distinguished geologists to expunge the diluvial formation from the geological series, the decision of a large majority of geologists, as I apprehend, is, that for the present at least, it must be retained. ‘To strike it from the list of North American rocks, would be to dispense with the most remarkablemember of the series. 2. All the substitutes that have been proposed for that of Diluvium, such as “Erratic Block Group,” “Bowlder Formation,”“ Detrital Deposits,” “ Drift,” &c. seem to me to be as deficient in signification, and to convey as erroneous notions as the term Diluvium, which in euphony certainly takes the precedence. 3. Probably no part of geology isin a more unsettled state, or more imperfectly understood, than that of diluvium: and while it continues so, a designation for the formation is of little consequence, provided observers describe accurately what is includedunder it. When the exact limits and theory of the formation shall be settled, an appropriate name-can be easily applied to it.I have intimated that the limits of this deposit have not been certainly fixed. In general we find it spread irregularly over every other formation but the alluvial. But some deposits in Massachusetts, which I had formerly supposed to belong to the newest tertiary, I now place under diluvium ; and a similar disposition of some supposed tertiary beds has been made in Europe.On the other hand, I find it still more difficult to separate diluviumfrom alluvium in some cases; though the general characters of the two formations are very distinct.I do not include under diluvium the tearing up, rounding, smoothing, and comminution, of the bowlders, gravel and sand, that now compose it: although without doubt a part of these processes must have been performedduring the diluvial period. But probably a large proportion of this work was accomplished by aqueous agencies, previous to that more powerful one which we denominate the diluvial. That agency I suppose brought thesematerials, previously in a measure broken up and rounded, into their present state. And yet I do not suppose this agency to have been a very transient one. For I hope to present evidence soon of its long continuance aswell as great power.Of all the formations probably diluvium is the most difficult to study. A man may obtain some tolerable idea of the general geological features of a country by passing through it once or twice. But no transient traveler can tell us much of its diluvial phenomena. In order to do this, he must first become accurately acquainted with all the older formations and their limits.Else when he finds pebbles and bowlders drifted from their parent bed, how can he be certain of the direction in which they came’? It is now at least twenty years since I began to examine Massachusetts geologically ; and ten years since I commenced exploring the whole state. No year has passed in which I have not accumulated many facts respecting diluvium; for which I have always kept an eye open. The great number of these facts which I shall here present, will at least show I think, that the general conclusions at which I arrive, have not been formed hastily, and without broad premises. Those who will take the trouble to compare the present with my former reports, will see that those general conclusions have not been altered since they were first announced : and that the additional facts which I now give, only serve to render them firmer. Whatever corrections future observers maybe obliged to make in my statements, I feel quite sure that they will never doubt that the diluvial waters in Massachusetts took a direction between south and southeast ; and that they have left upon the solid ledges innumerable furrows and scratches, as proofs of their direction and great power.These are the most important results at which I havearrived; and they are entirely independent of hypothesis. When first announced, the latter statement especially, respecting grooves and scratches, was received I believe by the ablest geologists of our country with strong scepticism. But I doubt whether all of them have not ere this seen enough of such phenomena inother parts of the land, to be satisfied that [ have stated only the truth: if 1 may be allowed to form an opinion from the numerous annual GeologicalReports that have appeared in different parts of the country within a few years past; and from several articles in the scientific journals. In some instances the direction of these grooves varies considerably from those in Massachusetts: but I speak now only of their existence. And as I have beenobliged to bear the obloquy of stating what was thought to be erroneous, I trust I may claim the honor, if there be any, of first calling public attention in this country to an interesting geological phenomenon.In general terms, this deposit may be said to lie between the tertiary and alluvial strata. But it seems quite probable that some finer deposits of clay and sand, which were produced during the same geological period, have been referred to the tertiary strata. In my former Report I described certain horizontal deposits of blue clay, covered by layers of sand, occurring in limited deposits in the state, especially in the valley of Connecticut river, as belonging to the Newest Tertiary: because I had no evidence that the diluvium passed beneath it. But since that time, I have discovered several sections, recently made, which render it almost certain that this Newest Tertiary constitutes a part of the diluvial deposit :—usually its upper part: but sometimes interstratified with it: as in Fig. 65, which will soon be exhibited, and which is a section in diluvium. Wherever valleys of any considerable extent existed in the state at the period of diluvial action, and these were cut off from the ocean by some barrier, I find this clay and sand. I infer that it was deposited by the retiring diluvial waters ; though I undertake not here to decide whether those waters retired in consequence of thesubsidence of the ocean, or the elevation of the land. I shall present evidence that in some parts of the state, this draining of the waters occupied a considerable length of time: and probably left large expansions of the riyers or lakes, which were not drained for centuries, or until the streams at their outlets had worn down the barrier. This process I should call an alluvial agency: and I consider the diluvial agency to have ceased at the pointwhen the effects can be explained by existing agencies, operating with their present intensities.In general terms, this deposit may be said to lie between the tertiary and alluvial strata. But it seems quite probable that some finer deposits of clay and sand, which were produced during the same geological period, have been referred to the tertiary strata. In my former Report I described certain horizontal deposits of blue clay, covered by layers of sand, occurring in limited deposits in the state, especially in the valley of Connecticut river, as belonging to the Newest Tertiary: because I had no evidence that the diluvium passed beneath it. But since that time, I have discovered several sections, recently made, which render it almost certain that this Newest Tertiary constitutes a part of the diluvial deposit :—usually its upper part: but sometimes interstratified with it: as in Fig. 65, which will soon be exhibited, and which is a section in diluvium. Wherever valleys of any considerable extent existed in the state at the period of diluvial action, and these were cut off from the ocean by some barrier, I find this clay and sand. I infer that it was deposited by the retiring diluvial waters ; though I undertake not here to decide whether those waters retired in consequence of thesubsidence of the ocean, or the elevation of the land. I shall present evidence that in some parts of the state, this draining of the waters occupied a considerable length of time: and probably left large expansions of the riyers or lakes, which were not drained for centuries, or until the streams at their outlets had worn down the barrier. This process I should call an alluvial agency: and I consider the diluvial agency to have ceased at the pointwhen the effects can be explained by existing agencies, operating with their present intensities.In general terms, this deposit may be said to lie between the tertiary and alluvial strata. But it seems quite probable that some finer deposits of clay and sand, which were produced during the same geological period, have been referred to the tertiary strata. In my former Report I described certain horizontal deposits of blue clay, covered by layers of sand, occurring in limited deposits in the state, especially in the valley of Connecticut river, as belonging to the Newest Tertiary: because I had no evidence that the diluvium passed beneath it. But since that time, I have discovered several sections, recently made, which render it almost certain that this Newest Tertiary constitutes a part of the diluvial deposit :—usually its upper part: but sometimes interstratified with it: as in Fig. 65, which will soon be exhibited, and which is a section in diluvium. Wherever valleys of any considerable extent existed in the state at the period of diluvial action, and these were cut off from the ocean by some barrier, I find this clay and sand. I infer that it was deposited by the retiring diluvial waters ; though I undertake not here to decide whether those waters retired in consequence of thesubsidence of the ocean, or the elevation of the land. I shall present evidence that in some parts of the state, this draining of the waters occupied a considerable length of time: and probably left large expansions of the riyers or lakes, which were not drained for centuries, or until the streams at their outlets had worn down the barrier. This process I should call an alluvial agency: and I consider the diluvial agency to have ceased at the pointwhen the effects can be explained by existing agencies, operating with their present intensities.These have been partially stated: but the whole need to be given. 1.Bowlder Stones or Erratic Blocks. How large a rounded and transportedblock of stone must be in order to make it a bowlder, seems not to be exactly settled. But in Massachusetts the amount of them that are of great size is so large, that we need not reckon those of doubtful character. These bowlders form one of the most striking objects in the landscape in many parts of the state, and scarcely no part is free fromthem. 2 Gravel and Sand mixed together confusedly. 'These constitute the great body of diluvium; and are ° composed of every variety of rock found in the state, and of some varieties found in place only beyond its limits. 'The softest kinds of rock, however, have been mostly reduced to sand or clay ; and the great mass, both of bowlders and pebbles, consists of the most unyielding of our rocks ; such as quartz, porphyry, sienite, greenstone, gneiss, mica slate, and granite. Thus, in the region west of Connecticut river, while masses of the quartz rock of Berkshire county are met with at almost every step, it is very rare to meet witha fragment of limestone at the distance of more than half a dozen miles from the limestone deposits; although a glance at the geological map will show, that the latter occupy much more of the surface in Berkshire county than quartz rock. 3. Beds of Clay. This clay is usually of a blue color, but becoming nearly white in those parts of the state where a great deal of feldspar is contained in the detritus. In all cases, however, this diluvial clay abounds in the protoxide of iron. It is not of much use except for making bricks, and common red earthern ware. Ususually it occupies basins, ortrough shaped cavities, and must have been deposited in quiet waters ; as neither bowlders nor gravel are usually mixed with it. Frequently, however, as along the sea coast, its deposition is confused, and sometimes (ex. gr.at Fitchburg, west end of the village,) pebbles are mixed with it ; and sometimes (as on the rail road cut in the south part of Palmer,) even quite large bowlders. 4. Consolidated sand and pebbles. In some instances the hydrate of iron acts as a cement of diluvium ; but the rock thus produced is easily crumbled down. Carbonate of lime, however, in some localities, has formed a conglomerate of considerable tenacity. ‘The calcareous diluvium which is not uncommon in Springfield, West Springfield, and South Hadley; andwhich has been particularly described in the first part of my report, is sometimes very firm; though on exposure to the weather, it at length crumblesdown ; and therefore can hardly answer for any purpose of construction. No.1560 is an example of this rock.5. Beds of Sand. 'This sand is siliceous and varies from very fine to quite coarse ; the latter usually lying at the top. The beds of this sand are almost invariably spread over the beds of clay, not only in Massachusetts, but in other parts of the country which I have visited: and so common is the fact, that where we find clay, we expect to find over it layers of sand, unless alJuvial agency hasremoved them. I cannot but infer, therefore, that these siliceous deposits resulted from some general agency ; and [cannot conceive thatany cause now operating with its present intensity could have produced them, and therefore refer it to diluvial action. 6. Limonite, or Hydrated Oxide of Iron. This I have found among diluvium only 1i n Berkshire County and in the eastern part of New York. And there it is a diluvial product in no sense except that diluvial action has torn it from the beds of this ore that project from the strata of mica and talcose slate. I have not seen in the state any ex-: ampleo f a deposit of iron ore during the diluvial period worthy of notice, although we frequently meet with pebbles coated and sometimes cemented byhydrate of iron, and probably some of the deposits of yellow ochre belong to this period.That the clay and much of the sand which I have included in this formation, are stratified, will admit of no question by those who have seen goodsections through them. But some have an impression that what is usually called diluvium, viz. gravel and bowlders, was thrown together in such utter confusion, as to exhibit no sorting of the materials and no parallelism of arrangement.And the usual aspect of diluvial hills, seeming to be a merepromiscuous mass of gravel and sand, is apt to confirm this idea. But when a fresh section is made through such hills, and through the formation generally, I have rarely failed to discover as distinct marks of stratification as in the older fragmentary rocks equally coarse. In fact, I cannot conceive how detritus could be deposited in water the most violently agitated, without separating in some measure into portions more or less coarse: and this is nearlyall the stratification which diluvium exhibits. It is sometimes made more striking by some of the layers being a good deal more ferruginous than oth_ ers: but rarely by any thing like a seam separating them: and the same thing is usually true of the coarsest conglomerate. The sections which I shall shortly present show the extent to which it is stratified._ As a general fact, the stratification is horizontal; or as much so as in any deposit made from water.A formation deposited like diluvium must be supposed to have almostevery variety of thickness: and such is the case with this deposit. Wefind it in every place where such materials as compose it could find a lodgment, when acted upon by waters in violent agitation. We should not expectit upon the tops of narrow ridges and insulated peaks, nor upon steepescarpments, except in small quantity, even though carried there by the currents.But wherever narrow valleys or gorges existed, through which thediluvial waters might have rushed, we should expect the detritus to be accumulated in the greatest abundance: and there in fact we usually find it of thegreatest thickness, and its characters most strikingly developed. Yet its maximum thickness, in such places, is extremely difficult to be ascertained : especially in a country like ours, where, until recently, no deep excavations have been made.It is chiefly because their greater size renders them more conspicuous, that the phenomena of bowlders are more striking than those of sand and gravel.For the origin and dispersion of both are essentially the same. Nevertheless, bowlders are by far the most instructive index of diluvial agency in Massachusetts.It seems difficult to conceive how running water should have been able to remove such enormous masses. Yet it is certain that all of them have been transported quite a distance. ‘Those in Fall River and Troy for instance, must have been brought from the opposite side of Taunton river, or from several miles north. ‘That in Plymouth correspondsto ledges in the Blue Hills,near Boston: but I know of no porphyry ledge south of those Hills: andthey are 30 miles north of Manomet Hill. Is it possible that such a bowlder could have been carried so far? The bowlder in Fitchburg, that has been discribed, must be have been brought several miles and across deep valleys. ‘Thesame is true of that in Adams; and the valley which it must have crossed, isa very deep one. In contemplating this Sisyphean labor, however, weought to recollect that a rock in water loses nearly half its weight.know of no currents in our existing seas of sufficient velocityand power to produce a moiety of the effects which the diluvial currents have produced. The present currents could, indeed, transport icebergs loaded with bowlders and pebbles, at the rate of 3 or 4 miles per hour, and along some narrow friths, at the rate of 10 or 12 miles per hour; and these bowlders might be dropped from time to time along the bottom. But this is a very different thing from urging over that bottom such a mass of detritus, and with such violence, as to wear down the surface, and score it over so deeply as we find to have been done upon our rocks. But it may be said, that the breakers upon. an exposed shore might have accomplished the work. They mighthave worn away the rocks: but they could not have produced those parallel and continuous grooves and scratches, which the more exposed surface of New England every where presents, and which hold on their course obliquely across those lofty ridges, which must have formed the shores of the ocean, as the continent gradually rose from the waters. The diluvial phenomena of our country indicate a current that swept over the entire surface, and not waves breaking against cliffs and rolling back. ‘The latter produce 5. It is difficult to explain all the phenomena of diluvial action on this continent, without calling in the aid of icebergs, or some other agent, by which masses of rock could be floated along the surface of the waters; yet after all, those icebergs will explain but a small part of the effects of this action.I cannot conceive of any other way,except by icebergs, by which those bow]ders, found many miles from their native beds, and perched upon high and almost perpendicular ridges, or dropped in the midst of sandy plains, could have been transported. But to suppose that the great mass of sand, gravel, and blocks, composing diluvium, could have been borne along by icebergs, is an hypothesis which any man would abandon as soon as he should examine that deposit, and see its amount. ‘Then again, he would find that usually the largest and most numerous bowlders lie nearest to the rock from which they have been detached ; and that the train decreases in size and number as we proceed southeasterly, except the few straggling bowlders already alluded to. Now I cannot conceive that an iceberg, lifted up by the watersfrom a particular spot, with bowlders frozen into its lower parts, should begin - to let those bowlders fall out within a few rods from the place where it started.I should expect that in general they would not begin to fall, till the iceberg began to melt. In short, the great mass of diluvium appears as if driven pell mell along the surface by violent currents; and so does the surface over which it has been urged, whenever it can be examined. It is difficult, therefore, to explain some of the phenomena of our diluvium without icebergs ; but still more difficult thus to explain the whole.6. The phenomena of diluvial action in this country far transcend theeffects of any agencies now operating, without admitting a great increase of intensity in their action. So much has been said of late years, and so ably, respecting the sufficiency of the present operations of nature to explain all past phenomena, that one cannot but feel a diffidence in resorting to extraordinary agencies in past times. But in vain have I searched for any agencynow at work, that produces effects at all comparable to the effects of diluvial action. ‘The statements of Professor John Phillips, one of the ablestliving geological writers, concerning diluvial action generally, is eminently applicable to that portion of it that has fallen under my notice, in America.“Such effects,” says he, “are not at this day in progress ; nor, in general, can we conceive the possibility of their being produced by the operation of existing agencies, operating with their present intensities, or in their present directions.” (Treatise on Geology, Vol. I. p. 296.) I have frequently examined the rocky shores of the ocean, and the beds of rivers where theyform cataracts, or force their way through narrow defiles and gorges, to ascertain whether the effects of the powerful currents and waves of the presenttime are comparable to those of diluvial action: and I have always been struck with the vastly greater force of the latter. On exposed shores, and at the bottom of gorges and cataracts, we do, indeed, find the rocks worn away; but their surfaces are usually smooth, and almost wanting in those deep and extended grooves and scratches, which are the result of diluvial action. Deep gorges do, indeed, show the process of extensive excavations.But this work is generally not so much the effect of direct erosion,as of the expansion of water in the fissures of rocks, by freezing ; in consequence of which, large blocks are gradually lifted up, while the stream wearsoff their angles, and at length has force enough to move them from their beds, as well formed bowlders. In this and other modes of an analogous kind, I presume the largest part of bowlders have been formed, or got ready to be driven southerly by diluvial currents. But one sees at once that the two agencies have operated in a very different manner. While the excavations of gorges and the wearing away of the sea coast, have been the combined and slow result of disintegration and moderate erosion, diluvial phenomena seem to have been the direct result of mighty currents of water, loaded with detritus, and rushing over the surface with such violence as to wear down the rocks much more powerfully and rapidly: a violence that did not permit the detritus to find its way around obstacles, but forced it directly over them in a manner altogether different from the comparatively puny action of water at the present day. But after all, it is impossible so to present this subject as to make one feel it, who is not extensively familiar with diluvial phenomena.7. The facts that have been presented respecting diluvial action, furnish a probable reason why the diluvium of this country is so destitute of organic remains. In New England I scarcely know of any of these remains, except perhaps a single example of bones of the mastodon found in Sharon,Berlin, and Cheshire in Connecticut ; anda few shells in some of the diluvial clays in Maine: and not improbably in both these instauces the deposits may have been alluvial. The same may be said of all the instances of organic remains in the diluvium of this country. Its almost entire want of these, so far as discoveries have yet been made, is a wellknown fact. And if the diluvial waters were poured down from the arctic regions, loaded with ice, wesee the reason of it. Animals did not live in such waters; and we ought to expect to find in them only those animals which were then the inhabitants of this continent, and were destroyed by the diluvial waters. Whether themastodon and other extinct races discovered in this country were then destroyed, or subsequently, does not seem to be well determined. Even the morequiet period that succeeded the early periods of diluvial action, seem to have been almost destitute of organic existence. Hence it is, that the blue clay of New England contains almost no relics of animals or plants.It will be seen that in the preceding inferences I have not attempted to form any general theory of diluvial action, but only to state the more important independant conclusions to which the facts, with more or less of probability, conduct us. Let us now see whether these facts will sustain any ofthose numerous diluvial theories that have been proposed in recent times by ingenious men. I may pass by the hypothesis to which some have clungeven in our own times, which imputes diluvial action to the shock of a comet; since it seems now quite certain that these bodies are in general composed of matter thinner and lighter than air.Another theory, which has long been a favorite one, imputes diluvial action to the deluge of Noah. The freshness and apparent recency of theeffects of this action and its apparent universality, give at first view a strong probability to this supposition, if we understand the language of scripture in its most literal sense. But many distinguished biblical writers regard the description of the Noachian deluge as an example of the use of universal terms with a limited meaning, and hence regard that deluge as not universal over the globe, but only over the region inhabited by man. Again, if the diluvial action of geology resulted from the deluge of Noah, why are the organic remains found in diluvium mostly of extinct animals? and why is not manamong the number? Finally, the diluvial action of geology must have occupied a much longer period than the 150 days, or at longest, the year, ofthe Noachian deluge. It is difficult, if not impossible, to make any one feel the force of this objection, who is not familiar with diluvial phenomena. But he who has seen where the hardest rocks have been worn away many feet at least, and probably sometimes many hundred feet, by diluvial action, cannot but see that many years must have been required for the work, even though the waters were driven over the surface with the greatest violence.These objections, are very conclusive against this hypothesis. As bearing upon the veracity of the sacred historian, however, this question I conceive can have little or no importance, since it does not prove the non-occurrence of the Noachian deluge, even though no traces be now remaining upon the earth’s surface of that eventA theory has lately been started to explain diluvial phenomena, founded on the action of glaciers in the Alps. Though not originally proposed, it has been chiefly elucidated and defended by the distinguished naturalist, Agassiz, of Neuchatel in Switzerland. That it may explain the movements of detritus in the Alps, which for the most part appears to have been carried outward from the axis of the mountain, I am not disposed to deny : and of course it would explain equally well all similar cases where slopes exist, down which glaciers may have descended. But I am unable to see how this agency could have transported detritus in a southerly direction several hundred miles, over nearly all the most elevated ridges of this country, almost without reference to the direction of those ridges, and even have driven it upward along slopes considerably inclined, as appears to have been done on the western side of New England. Nor does it give any reason for the immense accumulation ofsand, pebbles and bowlders, piled up in a fantastic manner, at great distances from any existing mountains, as in Plymouth and Barnstable counties. In the case of the Alps, it is supposed that the ridges have been considerably elevated since diluvial agency commenced; and this vertical movementwould tend to scatter the detritus over a wider space. But we have no evidence that any of the higher ridges of this country have been raised sensibly so recently. Certainly there is no chain of mountains on this continent whose elevation would send bowlders and gravel southeasterly over hundreds of miles. Even though the valleys were filled by ice, yet where is the force to be found to urge the detritus so far in that direction. I confess I have as yet seen only a very brief and evidently imperfect developement of this theory, in one or two short notices in the scientific journals. But as I understand it, it seems to me inadequate to explain the tout ensemble of diluvialphenomena.Another theory of diluvial action supposes it to be merely ancient alluvial action. Its advocates suppose that the same causes of change, which are now in operation on the globe, have always operated as they do at present with no more variation of intensity than they now exhibit. Diluvial action they suppose to have resulted partly from ancient rivers, which at some remote period may be supposed to have flowed at any elevation, and subsequently to have lowered their beds to their present levels. Another cause may have been the bursting of lakes: another, landslips: another, the action of breakers upon the coast, as the continents gradually emerged from the ocean: another, the tides and currents of the ocean: another, icebergs, such as now‘float from arctic regions and drop bowlders and sand along their track : so that when the bottom of the ocean was afterwards raised above the waters, the surface would be strewed over with detritus, as we now find it.{fT have not entirely mistaken the phenomena of diluvial action in this country, especially in New England, it willbe exceedingly difficult to reconcile them to this hypothesis. The difficulties are numerous. In the firstplace, neither rivers, nor the bursting of lakes, nor breakers upon the coast, although they may prepare bowlders to be transported, will account for a current from 1500 to 2000 miles wide, which has swept southerly over this continent: Nor could they produce those grooves and scratches which are socommon and uniform in their direction in New England. In the secondplace, as I have attempted to show in another place, the relative levels of the surface have not very essentially altered since the period of diluvialaction; as they must have been by this theory. In the third place, oceanic currents have not sufficient velocity to transport any thing coarser than sand along the bottom, and therefore diluvial grooves and scratches could not have been thus produced. In the fourth place, although icebergs might have transported some bowlders, they could not have conveyed the great mass of diluvial detritus into its present situation: Finally, diluvial action, as I have attenipted to show, did not take place till this continent had attained nearly its present elevation above the ocean.Nearly the same difficulties encumber another theory, which imputes diluvial action to the retiring waters of the ocean; as chains of mountains were suddenly thrown up. If the continent was already above the ocean when it took place, this would be impossible. Or if it were not, as it rose, the waters would rush each way from the central axis: but in fact they moved only ina southerly direction. Again, the direction of the current was not at right angles to any system of elevation in this country with which we are acquainted: And finally, the effects vastly transcend the alledged cause.The only remaining theory deserving notice, supposes that an extensiveelevation of the bottom of the Arctic Ocean threw its waters, loaded with ice and detritus, southerly over the American and also the Eastern Continent.It supposes that theremay have been a succession of elevatory movements, producing successive waves, so that the waters may have risen andfallen, and during their ebb, they may have frozen to the surface; so that upon rising again new loads of detritus may have been lifted up, and driven onwards, and even up somewhat steep declivities, scouring and abrading the surface.This theory certainly quadrates better than any other with the diluvial phenomena of this country. Still it is very difficult to conceive how an oceanic current, however violent at its commencement, should have sufficient force, after passing southerly from the arctic region to the 45th, or even the 40th, degree of north latitude, to accomplish what the diluvial current has accomplished in this country. And at the longest, such an upheaving of the arctic ocean as is here supposed, judging from analogous movements in modern times, could not have occupied more than two or three years. But onecannot become familiar with diluvial phenomena without feeling a conviction that a much longer period must have been occupied in this work ; although compared with many other geological periods it was short. .To my mind, therefore, no theory of diluvial action hitherto proposed, is so free from objections that I feel satisfied with it. That remarkable and very powerful currents of water have swept over this continent from the north and northwest, F cannot doubt. ‘That this took place at a comparatively recent period, and probably since the continent assumed its present levels, and reached essentially its present height above the ocean, and that no existing agency, without a great increase of intensity, could have accomplished the work, seem to me almost equally certain. But I do not feel prepared to propose any regular theory of this agency in which I place much confidence.Yet the subject is one of the deepest interest, and the facts arefast accumulating concerning it ; so that erelong I doubt not every theoretical difficulty will vanish.In the clay beds of New England, which I have described as diluvial, two distinct and peculiar varieties of concretion occur, which deserve a particular description. One of them is composed of carbonate of lime and clay,and the other of hydrate of iron and clay. ‘Though familiar with the former from my earliest days, under the name of claystones, as is every boy brought up on the banks of Connecticut river, yet it is only since my last report to the Government that my attention has been particularly turned to thesubject. They are, indeed, described briefly in my Report of 1835: asare also the other variety produced by the hydrate of iron; the latter I then regarded as organic remains. ‘This opinion, upon the whole, I think must be given up: though the resemblance to organic remains is strong. Theclaystones are almost universally referred by the community to the action of water upon indurated clay. And since they are always rounded and generally found in the beds of streams, this is a very natural, though an entirely erroneous conclusion. ‘They are undoubtedly concretions, formed by laws somewhat analogous to those of crystalization ; probably while the clay was in a soft state, by the action of water.I freely confess, however, that so far as my means of information extend, the subject of concretions is involved in great obscurity. Thatthey are formed by segregation, in consequence of elective affinity, seems to be generally admitted. But why the particles should arrange themselves in curves, rather than in straight lamin, it seems difficult to determine: and we are quite ignorant whether any of the laws of crystalography will apply to the formationof concretions. Prof. Alexander Brongniart has, indeed, given the world a valuable paper on siliceous concretions. (Mnnales des Sciences Naturelles, Tome Vingi— Troisieme, p, 166.) And he has rendered it probable that silica takes the form of a concretion when in a gelatinous state, and of a crystal when in a state of solution. But this gives us not even a glimpse of the laws that regulate the arrangement of the particles: nor explains the reason why they should assume one form when in a gelatinous condition, and another Claystones.I shall first mention the localities and situation of the claystones of Massachusetts.They are more abundant in the valley of Connecticut river, than in any other part of the state.In general they are found beneath the waters of the river, where the bank consists of fine blue diluvial clay, in horizontal layers, about half an inch thick. Sometimes the water has removed them several rods; but rarely far enough to wear them much. The most abundant locality, is in Hadley, on the east side of Connecticut river, half a mile south of the village. In South Hadley (or perhaps within the limits of Springfield,) they occur half a mile south of the village, at the Canal, in a clay bankby the road side, some 50 feet above the river. In West Springfield they are found at the mouth of Agawam river; and also a mile up the stream, near the bridge leading to Suffield. In Amherst they are washed from a bed of clay at some distance from any stream. In Deerfield, they occur on the bank of Connecticut river, a few rods north of Sunderland Bridge. In Montague, they are seen at low water on the east bank of Connecticut river, ina clay bed, which underlies a meadow. Specimens will be found also in the State collection from Windsor and Wethersfield in Connecticut : where they are common in the clay beds, and differ from those in Massachusetts only in having the reddish tinge which is there common in the clay. Although I have seen imperfect claystones from the vicinity of Boston, and have heard of them in other parts of the State, yet I have met with good specimens out of the valley of the Connecticut, only in North Adams, where they are common ina clay bed which has been excavated for making. brick. Along Hudson river they are quite common: and I have met with them atthe slide on Presumpscut river near Portland in Maine. Indeed, they are found in nearly every part of our country, where diluvial clay exists.At most of the localities in Massachusetts that have here been described, thousands of specimens can be obtained.7. Argillaceous Slate. The argillaceous slate in the eastern part of the State is so intimately connected with the varieties of rock above noticed, that it ought in justice to be described as one of the members of the graywacke group ; although marked as a distinct deposit on the Map.And I doubt not but it is older: for fragments of this slate occur in one of the varieties of conglomerate that have been described ; and this not only shows the posterior production of the latter, but renders it doubtful whether both rocks were produced during the same geological epoch.In general its color is dark gray, passing to blue: It is rarely fissile enough to be employed for roofing. The laminz, as on Rainsford island, in Boston harbor are sometimes very tortuous.Not unfrequently it passes into an imperfect novaculite ; as in Charlestown, Roxbury There can be little doubt that the peninsula of Boston has a foundation of argillaceous slate.This is, indeed, the only rock that has ever been found therein place. And from the occurrence of argillaceous slate in South Boston, and in Charlestown, with a northerly dip in both places, it would be very surprising if any other rock should be found in Boston; unless it were an intruding mass of trap rock. But this slate on the peninsula is buried deep by clay, gravel, and sand.I have colored the peninsula as a diluvial deposit. Artesian wells bored there a few years ago, 260 feet deep, did not reach its bottom.Varioloid Wacke. The rock which I thus designate, has generally been regarded by those who have described the geology of Boston and its vicinity, as amygdaloid. But it seems to me that there are insuperable objections against the supposition that the nodules in general were introduced by infiltration, or even sublimation ; the only modes by which geologists suppose the cavities of amygdaloid were filled. For they consist generally of rounded masses of compact feldspar ; a substance which must certainly have been the result of igneous fusion. On the other hand, the rounded form of these nodules, and their non-crystaline structure in general, forbid the arrangement of this rock along with the porphyries. But some writers regard variolites as rather intermediate between porphyry and amygdaloid, ( Traite de Mineralogie, Par. T. S. Beudant, ( Paris, 1830,) Vol. 1. p. 569.) and such I suppose to be the character of the rock under consideration. By the term varioloid, 1 intend merely to designate the external aspect of the rock ; since the mode of its formation seems involved in much obscurity : but its variolous appearance none can deny.Professor Webster in his account of the geology of the region around Boston, states that the veins of greenstone in the graywacke conglomerate of that vicinity, run about 10° W. of South, and 10° E. of North. All such veins are probably of nearly cotemporary origin : their parallelism being explicable only on the supposition of their having been produced by the same cause.
EDWARD HITCHCOCK
Formation & Ancient History
Strata
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Paleogeographic reconstruction
This graphic by Fabio Crameri based on Scotese & Wright (2018) is available via the open-access s-ink.org repository.
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This animated graphic by Fabio Crameri based on Scotese & Wright (2018) is available via the open-access s-ink.org repository.
This graphic by Fabio Crameri based on Scotese & Wright (2018) is available via the open-access s-ink.org repository.
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This graphic by Fabio Crameri from Crameri and Pérez-Díaz (2025) is available via the open-access s-ink.org repository.
Faults & Folds
USGS, Kaye, 1980
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USGS, Kaye, 1980
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Geology of the Boston Basin (2011/2012)
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