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Genesis of Boston Harbor drumlins, Massachusetts
SEDIMENTARY GEOLOGY ELSEVIER
Sedimentary Geology 91 (1994) 333-343
Genesis of Boston Harbor drumlins, Massachusetts William A. Newman
a, D a v i d M . M i c k e l s o n
b
a Department of Geology, Northeastern University, Boston, MA 02115, USA b Department of Geology and Geophysics, University of Wisconsin, Madison, W153706, USA
Received January 29, 1992; revised version accepted March 1, 1993
Abstract
Drumlins in Boston Harbor are composed of two superposed silty-sandy diamicton units and subordinate sand and gravel. The lower, compact, undifferentiated, diamicton is interpreted as a pre-Wisconsin till, based on truncated weathering profiles preserved below an erosion surface. The upper, less compact, diamicton contains numerous, discontinuous and poorly sorted lenses and stringers of sand and sandy gravel that tend to parallel the erosion surface. The upper diamicton and subordinate sandy gravel lenses are interpreted to date from the late Wisconsin glaciation. Along the exposed sections of the Long Island drumlin complex, the preserved weathering profile on the lower diamicton is thicker beneath the drumlin axes and is either thin or missing in the interdrumlin areas. This suggests that glacial erosion and streamlining of pre-Wisconsin deposits and their weathering profile took place after the weathering profile developed and before subsequent deposition of the overlying diamicton unit. The latter drapes the erosional drumlin form without contributing substantially to relief of the drumlin form. 1. Introduction
The origin of drumlins in different parts of the world has been discussed in numerous papers in recent years. A bibliography of drumlin research (Menzies, 1984) and papers published subsequently have included an even wider range of ideas about their formation. Current hypotheses can be grouped into three basic mechanisms. The first is that drumlins form by the accretion of basal till in the subglacial environment. This could take place either by the accumulation of frozen basal sediment at the base of the glacier, or by accumulation of wet subglacial sediment that had been deforming beneath the ice (e.g., Smalley and Unwin, 1968; Baranowski, 1969; Dardis, 1981; Menzies, 1982; Boulton, 1987; Boulton and Hindmarsh, 1987). In either case,
the basis of these hypotheses is that contemporaneous deposition, deformation, and shaping of diamicton beneath relatively thick ice into the well-known streamlined form creates the drumlin's morphology and internal structure. Another set of hypotheses focuses on the importance of erosion in the formation of drumlins (e.g., Whittecar and Mickelson, 1977; Stanford and Mickelson, 1985; Kriiger, 1987; Habbe, 1992). The presence of pre-existing, stratified and interbedded sand and gravel or streamlined bedrock in the cores of some drumlins demonstrates their erosional nature, but the importance of erosion in the formation of drumlins that are composed entirely of a single diamicton unit is difficult to demonstrate. A third hypothesis that has received much attention in the last few years is that of a fluvial
0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0037-0738(94)00019-Q
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W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
origin (Shaw and Kvill, 1984; Shaw and Sharpe, 1987; Shaw et al., 1989). This hypothesis suggests that huge flows of water beneath the ice created bedforms that we now interpret as drumlins, or that streamlined shapes were carved in the sole of the glacier by flowing water and these were subsequently in-filled with sediment. Upham (1894) argued that sand- and gravelcored drumlins in the Boston area and those in the Madison, Wisconsin area both had formed by the same process. He suggested that subglacial erosion of pre-existing sand and gravel produced the drumlin shape, followed by deposition of till and other sediment over this streamlined surface. Our observations from drumlins in Wisconsin and in the Boston basin support this conclusion. However, our interpretation in this paper is based primarily on diamicton-cored drumlins in which a truncated weathering profile strongly argues for a streamlined surface cut into a pre-existing landscape.
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Where drumlins contain sand and gravel in the core, at least two interpretations are possible. Either the sand and gravel was deposited before the drumlin-forming event and was later shaped by erosion, or the sand and gravel was deposited in a subglacial cavity as part of the drumlin forming process. In the absence of absolute dates or major lithologic differences, one can argue for one or the other interpretation with the use of sedimentology, cross-cutting relationships between sand and gravel and overlying diamicton, and aerial distribution. An angular unconformity between the gravel in the core and the overlying diamicton indicates that at least some erosion into the drumlin form took place after gravel deposition and before deposition of the overlying diamicton. This could also occur in cavity fillings. However, if the sand and gravel is extensive and has a top surface approximately at the same elevation in many drumlins, and if the sedimentology of the units indicates deposition in a
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W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
braided stream, then it is possible that the stratified sediments were deposited prior to the advance of ice that formed the drumlin. Therefore an erosional model of drumlin formation (Whittecar and Mickelson, 1977) would be most likely. Where two diamicton units occur in drumlins without an extensive gravel core, as they do in Boston Harbor, then the above arguments cannot be used. Although the diamictons cannot be dated, there is a weathering profile on the top of the lower diamicton unit in the Boston area. Therefore we suggest that the pattern of erosion on this weathering profile indicates that glacial erosion of the lower diamicton unit was an important process in drumlin formation.
2. Distribution of drumlins More than 200 drumlins occur in the Boston area. Those drumlins are located in Boston Har-
335
bor and in Massachusetts Bay to the east, where they are partly or entirely submerged (Rendigs and Oldale, 1990) and many of them are buried under glaciomarine sediments (Newman et al., 1990). Shaler (1870) suggested that these peculiar lenticular hills were remnants of a sheet of sediment that had been modified by post-glacial rivers and waves, while others were due to the topography of the underlying bedrock. Later Shaler (1888) interpreted drumlins to have formed by glacial erosion of older glacial sediment. In some parts of the basin the drumlins form clusters of more or less coalescent groups (e.g., at Hull and Worlds End; Figs. 1, 2); others occur in ill-defined rows 2 - 3 km long (e.g., Long Island and Peddocks Island) and elsewhere single drumlins are common. The average trend of the long axes of the Boston Basin drumlins is 125 ° (Crosby, 1934). The drumlins in the harbor itself trend about 110 ° and their emerged height and length range to about 30 m and about 0.4 km, respec-
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Drumlin (stippled) and its axial trend. ~ Eroded drumlin ~ ,LaabtrecW.isconsinice-flow directions inferred from till ~
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W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
336
tively, where their total length can be reconstructed. Drumlins resting on bedrock ridges are arranged in lines along the strike of the ridge (W.O. Crosby, 1903; I.B. Crosby, 1934; Kaye, 1976, 1982). In those drumlins containing a rock core there appears to be no systematic relationship between the position of the rock core and the center of the drumlin (LaForge, 1932). Drumlins on the mainland rarely reveal stratigraphic sections. However, frequent storm-wave erosion maintains the excellent drumlin sections that are exposed on some of the Boston Harbor islands. In particular, exposures on Long Island, Peddocks Island, Great Brewster Island, and Rainsford Island (Figs. 1, 2) were examined in detail during this study. The harbor drumlins consist mostly of two superposed diamictons with thin, discontinuous sand or sandy gravel layers between them (Fig. 3), but drumlins described in the vicinity by Upham (1879, 1889a, 1893a, b, 1894) contain considerable thicknesses of sand and gravel underlying an upper diamicton unit
and sometimes overlying diamicton or fossiliferous marine clay.
3. Quaternary stratigraphy in Boston Harbor Upham (1879, 1889a, b) was one of the first to study the stratigraphy of the Boston Harbor area in detail. He and most subsequent workers recognised two diamicton units in the area that they interpreted as till. Where he found drumlins containing two diamicton units he initially interpreted the lower unit as basal till and the upper unit as supraglacial sediment let down from the surface of the same glacier that deposited the underlying till. Later, Upham (1894) argued that the lower unit was overrun and eroded into the drumlin form by a later ice advance. LaForge (1932) and Judson (1949) suggested that the two diamicton units in the drumlins were deposited during a single glaciation, but LaForge (1932) thought that both units were deposited
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Fig. 3. Stratigraphic section along the southeastern (i.e. lee) side of the Long Island drumlin complex.
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W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
during the late Wisconsin glaciation, while Judson (1949) associated the units with an early or middle Wisconsin glaciation. Kaye (1982) called the lower unit the "drumlin till". Newman et al. (1990) argued that the lower diamicton unit recognised by U p h a m and earlier workers is preWisconsin, based on clay mineralogy. If this is the case, then drumlins that have this lower unit in the core must either have been shaped by erosion or preserved during late Wisconsin glaciation. The unweathered lower diamicton contains fragments of mollusc shells derived from older deposits. Samples of Mercenaria mercenaria from the lower till unit at Peddocks Island (Fig. 1) yielded a 14C age older than 37 ka B.P. (H-1125; Kaye, 1976). Belnap (1979) estimated the age of the shells at between 200 and 214 ka B.P., based on amino acid racemisation. Later Belnap (1980) suggested that they were Sangamonian in age. N e w m a n et al. (1990) suggested that this lower diamicton unit was of Illinoian age or older and that the weathering profile contains a clay mineral sequence similar to sequences on Illinoian tills in the Midwest (e.g., Follmer, 1983). Many of the weathering profiles described in the Boston area are truncated at the top, presumably by late Wisconsin ice that deposited the u p p e r diamicton unit.
3.1. Descriptions o f sediments
The lower diamicton unit is light brown to olive (7.5YR 4 / 6 to 5YR 4 / 3 ) where weathered and olive-gray (5YR 4 / 2 ) where it is unoxidised, compact, and similar in grain size distribution to the overlying diamicton. The u p p e r diamicton is oxidised, light gray to pale yellow (5Y 4 / 3 - 5 / 4 ) , compact to loose, in places faintly stratified, and contains no marine shell fragments. The lower diamicton unit contains more low charge vermiculite than the upper unit, and the u p p e r unit also contains more illite than the diamicton unit below (Newman et al., 1990). Detailed description of the sections used for clay mineral studies and correlation between them is given by Newman et al. (1990). The 38 samples of the two diamicton units collected in this study show identical means of 37% sand, 45% silt and 19% clay. Magnetic susceptibility is slightly lower in the u p p e r till, but the difference is not statistically significant. In places, a thin sandy diamicton layer overlies the entire section. It contains lenses of sand and gravel, is oxidised and poorly consolidated, and appears to form part of the u p p e r diamicton unit. In some places the two diamicton units are separated by discontinuous beds of stratified sand, or
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Fig. 4. Stratigraphic section along the northwestern (i.e. stoss) side of the Long Island drumlin complex. The drumlin axis indicated is the same drumlin as the one at 330 m on Fig. 3. The same unit labels are used here as in Fig. 3, although some units are not exposed in this section.
338
lEA. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
sand and gravel, with clasts up to 0.3 m in diameter. Where exposed, this stratified unit is up to 4 m thick, but is absent in many places.
3. 2. Description of sections The stratigraphy and lithology in the sections is similar to that described from the harbor area. The drumlins of Long Island (Figs. 1, 2) are dissected at both ends by wave-eroded cliffs. Sections were also examined and samples taken on Peddocks Island, Moon Island, G r e a t Brewster Island and Rainsford Island, and at Prince Head. A section across the lee end of the drumlins (southeast) is shown in Fig. 3 and through the stoss (northwest) side of the drumlins in Fig. 4. The drumlin crest at 79 m in Fig. 4 is the stoss end of the drumlin at 330 m in Fig. 3. The oldest unit (units 1 and 2 in Figs. 3 and 4) is the lower diamicton ("drumlin till" of Kaye, 1982). It is unoxidised below (unit 1) and oxidised above (unit 2). Although the diamicton is not rich in carbonate, unit 1 contains scattered shells and shell fragments. The shells have been leached from unit 2. Note that unit 2 is missing on the stoss side of the drumlins where the section is exposed (Fig. 4). Grain size and other characteristics are similar in the Long Island section to those described for the units as a whole. O t h e r islands examined in Boston H a r b o r show exposures of either the u p p e r diamicton, the lower diamicton, or both, sometimes with intervening stratified sediment. The u p p e r diamicton cap along the crest of the drumlins is missing in exposures on Moon Island and Rainsford Island (Figs. 1, 2). At Moon Island, the lower part of the section is covered with slumped material, but 6 to 8 m of lower diamicton unit above the slumped sediment is leached of carbonate. The Rainsford Island section is more or less parallel to the drumlin axis and consists entirely of the lower diamicton based on its amount of consolidation and analysis of samples by Newman et al. (1990). The upper 8 m of section is leached, but the lower part of the section contains shells. Both of these sites suggest that the depth of leaching was over 8 m in the lower diamicton unit before the erosion event that removed part of the weather-
ing profile. Clearly some sediment was eroded even from these sites because the u p p e r part of the soil profile (A and B horizons) is not present even though the sediments are oxidised and carbonate has been leached. Several other exposures were examined, but it was unclear where a contact between the upper and lower diamictons should be drawn. Peddocks Island (Fig. 2) has an exposure nearly 1 km long that is parallel to and oblique to the ice-flow direction. The stratigraphy here is complex, with many sand lenses and boulder zones. Newman et al. (1990) show a mixed zone between what they interpret as the upper and lower diamicton units. Carbonate is unleached in the section, suggesting that as much as 8 m of sediment was eroded along the sides of this drumlin before deposition of the upper diamicton. Another exposure parallel to the drumlin orientation is at Prince H e a d (Fig. 2). Here, in a small drumlin about 10 m high with an eroded face parallel to the drumlin axis, both diamicton units are present and the lower unit is leached to the beach level (more than 4 m). Clast fabrics in this face are oriented at 115 ° to 140 ° in the lower diamicton and 90 ° in the u p p e r diamicton. The weathered portion of the lower diamicton (unit 2, Fig. 3) in some places contains vertical fractures 1-3 cm wide that contain silt and clay fillings of iron oxide and manganese oxide. Fractures extending downward into the unoxidised diamicton also contain secondary calcium carbonate. The amount of oxidation decreases downward gradually, and the boundary between units 1 and 2 shown on Fig. 3 should be considered as a zone 10-20 cm wide. Because the primary carbonate in the unleached diamicton (unit 1) is restricted to broken shells, the precise depth of leaching is difficult to determine. Textural and clay mineral data from one section near the center of a Long Island drumlin (Fig. 3, 125 m) shows that the upper diamicton (unit 5) is sandier than the diamicton beneath the weathering profile (unit 1) and significant differences occur in the low charge vermiculite and chlorite content (Newman et al., 1990). The lower diamicton is leached of carbonate to a depth of about 3.7 m at this location. Below the lower
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
boundary of carbonate leaching, fossil shell fragments occur but are not abundant. A thicker but similar buried weathering profile occurs beneath the distal crest of the more easterly drumlin (Fig. 3, 330 m along section). Here the contact between the upper and lower diamictons (units 2 and 3) is difficult to define precisely because there are no stratified sediments between and because both are oxidised and leached.
4. Interpretation Uniformity of grain size distribution, strength and uniformity of fabric and consolidation of the lower diamicton (units 1, 2) and part of the upper diamicton (unit 3) suggests that these units are basal till. Genesis by lodgement or basal meltout cannot be ascertained applying the criteria used here. Similarly, distinction cannot be made based on observations from the sedimentary record, between deposition either from basal-rich ice or
339
from a subglacially deforming wet bed. Part of the upper diamicton (unit 5) may represent a mixture of supraglacial flows, sediment let down from melting ice in cavities or subaerially, or possibly post-glacial slopewash sediment. An important aim of this paper is to point out that the shape of drumlins is determined to a great extent by the shape of an unconformity (erosion surface) developed on the lower till (units 1 and 2) by erosion at the base of glacier ice. Units 3, 4, and 5 were deposited above this unconformity after streamlining by subglacial erosion. On the stoss (northwest) side of the Long Island drumlins (Fig. 4) the upper diamicton (unit 3) is fairly thin (3-4 m) and overlies the older diamicton unit which contains shells throughout. This indicates that the leached section of the weathering profile was eroded before unit 3 was deposited. The loose, slightly stratified unit 5 seen on the lee side of the drumlins is missing in this section.
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Diamictoninterpreted to be supraglacial till, flow and slope#ash sediment
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Ice flow direction inferred from fabric measurements
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Fig. 5. Interpretation of sections in the Long Island drumlin complex, based on observations from the stoss and lee sides. Note that no section of exposure is available parallel to the drumlin long axis.
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
340
1
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Unconformity developed beneath ice during the late Wisconsin glacial advance
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Diamicton interpreted to be basal till of late Wisconsin advance
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Ice flew direction inferred from fabric measurements
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Fig. 6. Sketch of formation of the Long Island drumlin complex. (A) Pre-Wisconsin ice sheet, flowing southeastward across Boston Harbor, deposited till (1) on the northeast-trending bedrock ridge that underlies the Long Island drumlin complex. Following recession of the pre-Wisconsin ice sheet the till deposit was subjected to subaerial weathering. During this time a deep weathering profile developed, with leaching of carbonates and oxidisation of sediments (2). (B) During the advance of the late Wisconsin ice sheet the pre-Wisconsin surface was eroded by ice flowing eastward, forming the drumlin core and completely removing the weathering profile from the stoss side of the drumlin. (C) Subsequently, the same ice mass deposited till on the underlying unconformity, producing the final drumlin form. Subglacial or ice-marginal fluvial deposits (4) and supraglacial till and flow sediment now cover much of the land surface.
W..A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
The significance of the buried weathering profile and, in particular, the depth of carbonate leaching is that it can be traced laterally in section, where it is thicker in the lower diamicton beneath the drumlin axes relative to beneath interdrumlin troughs (Fig. 3). In the low area to the south of the main drumlin on Long Island (Fig. 3, 24 m) the thickness of the lower till unit that is leached is only about 0.3 m. However, in the interdrumlin areas the top of the lower diamicton drops to beach level (Fig. 3, 30 m and 275 m) and rises again at the southern end and beneath the more northern drumlin crest (Fig. 3, 330 m). At the southern end of the section, no weathering profile is present on the lower diamicton, and carbonate is present immediately below its u p p e r contact. The abrupt change in clay minerals and the presence of pedogenic and weathering features just below the contact between the u p p e r and lower diamictons indicate that this weathering profile is not an extension of the modern one from above, but is the truncated portion of a weathering profile that developed on the land surface before the last glaciation (Newman et al., 1990). Although in some cases the absence of a weathering profile could result from a locally high water table, the gradual thinning of the profile moving away from the drumlin crests suggests truncation. In addition, sand and gravel layers interbedded with the upper diamicton (Figs. 3, 5) in the Long Island section parallel the erosion surface on the lower unit, indicating that the form was there when these sediments were deposited. The orientation of long axes of pebbles measured at several locations in the lower diamicton on Long Island are strongly developed and oriented at 140 ° (Figs. 3, 5). At G r e a t Brewster Island fabrics in the lower diamicton indicate an early flow toward 170 ° , later flow toward 110 ° and the latest flow toward 140 ° . At Prince Head, southeast of Long Island, ice flow near the base of the lower till was toward 115 ° . Later it became more southerly. T h e r e are not enough measurements to compile a flow history for the lower unit, but it seems likely that local topography influenced ice flow.
341
Fabric m e a s u r e m e n t s from the upper till (unit 3) suggest that in places ice flow was southward (e.g., fabrics on T h o m p s o n Island, Peddocks Island and Allerton Hill; striations on Moon Island). Elsewhere, flow was eastward (e.g., Spectacle Island, Long Island, and G r e a t Brewster Island). The wide range in drumlin orientations in the harbor is probably a result of the irregular topography of the harbor.
5. Discussion
The Long Island drumlin complex is interesting because wave erosion has exposed extensive sections normal to the drumlin axes and at opposite ends of the drumlins (Fig. 5). The lower, thick till and associated sediment was deposited by a pre-Wisconsin ice sheet (Fig. 6). Differences between pre-Wisconsin landforms and present landforms are indicated by the irregular pattern of the truncated weathering profile on the lower till. Selective erosion of the lower till by eastward flowing late Wisconsin ice sheet shaped the asymmetrical cores of the present harbor drumlins (Figs. 5, 6). This erosion selectively reduced the thickness of the leached and oxidized soil profiles to 0 to 4 m on the lower till surface. On Long Island, this weathering profile was most extensively eroded on the stoss side and along the drumlin flanks. In the lee of the drumlins, especially at their axis, glacial erosion was minimal. The upper till unit drapes the drumlinised lower till unit and is thinner in places near drumlin crests. This pattern is distinctly different than in many gravel-cored drumlins in Wisconsin where thicker till is present right on the drumlin crest and the till thins to the drumlin edges (Stanford and Mickelson, 1985). In this case, it was suggested that migration of till into the drumlin axis took place, but there appears to be no evidence of this mechanism in the present study. It might be expected that fabric orientations would converge on the down-ice side of the drumlin, but the current m e a s u r e m e n t s do not demonstrate this, and no sections lie close to the drumlins crests. The internal structure of these drumlins is
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less complex than those described in Nova Scotia (cf. Stea and Brown, 1989). T h e paucity of striated rock surfaces and fabric data in the study area prevent detailed reconstructions of changes in ice-flow direction. It is difficult to explain the patterns of d r u m lins and interdrumlin areas in the study area. O n e possibility is that this is related to parts of the glacier bed which were frozen and parts which were u n f r o z e n at the time of their formation. It could also be related to variation in the strength of materials at the bed, which may reflect differences in consolidation and hydraulic conductivity. A l t h o u g h it is possible to evaluate this quantitatively, there are no differences in grain size or c o m p a c t i o n b e t w e e n the lower till in the drumlins and the lower till in the interdrumlin areas. Alternatively, drumlin location may have b e e n influe n c e d by the preglacial topography, w h e r e b y areas that were hills after deposition, weathering, and subaerial erosion of the lower till unit, were simply a c c e n t u a t e d and streamlined into the drumlin shape. T h e clustered distribution suggests at least that these islands were uplands before the last glaciation.
6. Conclusions Observations on the drumlins in Boston Harbor suggest the following sequence of events (Fig. 6): (1) A d v a n c e of ice into the h a r b o r area before the Wisconsin glaciation. This ice advanced over marine silt and clay or older sediments that contained shells of Mercenaria mercenaria. (2) Following glacial recession, a long period of weathering took place w h e n clay minerals were altered ( N e w m a n et al., 1990) and w h e n the lower till unit was oxidised and leached of c a r b o n a t e to depths in excess of 6 to 8 m. (3) Ice advanced during late Wisconsin time across the study area and carved drumlins from the underlying w e a t h e r e d till. This p r o d u c e d thickness variations in the leached and oxidized till in the cross-section at L o n g Island. Evidently this glacial event deposited less consolidated upper till, flow sediments and sand and gravel lenses
as a blanket over the drumlinised unconformity that had developed b e n e a t h the ice. Drumlins probably f o r m e d by differential erosion of pre-existing till, probably u n d e r wet or partly frozen bed conditions. Finally, till was deposited on the sculptured drumlin cores during the retreat phase of glaciation, when the orientation of the drumlin forms was c h a n g e d as a result of late changes in ice-flow direction.
Acknowledgements W e thank Kirsten Lindquist and H a r a l d Newm a n for drafting the figures and A l a n Saiz for producing the c o m p u t e r graphics used in the figures. Nelson H a m , Lars R o n n e r t and Kent Syverson provided critical c o m m e n t s on the manuscript, as did two a n o n y m o u s reviewers. G e o r g e Dardis contributed substantial helpful criticism.
References Baranowski, S., 1969. Some remarks on the origin of drumlins. Geogr. Pol., 17: 197-208. Belknap, D.F., 1979. Application of Amino Acid Geochronology to Stratigraphy of Late Cenozoic Marine Units of the Atlantic Coastal Plain. Ph.D. Thesis, University of Delaware, Newark (unpublished). Belknap, D.F., 1980. Amino acid geochronology and the Quaternary of New England and Long Island. Am. Quat. Assoc., Abstr. Prog., pp. 16-17. Boulton, G.S., 1987. A theory of drumlin formation by subglacial sediment deformation. In: J. Menzies and J. Rose (Editors), Drumlin Symposium. A.A. Balkema, Rotterdam, pp. 25-80. Boulton, G.S.and Hindmarsh, R.C.A., 1987. Sediment deformation beneath glaciers: rheology and geological consequences. J. Geophys. Res., 92: 9059-9082. Crosby, I.B., 1934. Evidence from drumlins concerning the glacial history of Boston Basin. Geol. Soc. Am. Bull., 45: 135-158. Crosby, W.O., 1903. A study of the geology of the Charles River estuary and Boston Harbor, with special reference to the building of the proposed dam across the tidal portion of the river. Tech. Quart., 16: 64-92. Dardis, G.F., 1981. "Stagnant-ice" topography and its relation to drumlin genesis, with reference to south-central Ulster. Ann. Glaciol., 2: 183-184. Follmer, L.R., 1983. Sangamon and Wisconsinan pedogenesis in the midwestern United States. In: H.E. Wright (Editor),
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343 Late Quaternary Environments of the United States, Vol. 1. The Late Pleistocene (S.C. Porter, Editor.). University of Minnesota Press, Minneapolis, pp. 138-145. Habbe, K.A., 1992. On the origin of the drumlins of the south German alpine foreland, II. The sediments underneath. Geomorphology, 6: 69-78. Judson, S.S., 1949. The Pleistocene stratigraphy of Boston, Massachusetts, and its relation to the Boylston Street Fishweir. In: F. Johnson (Editor), The Boylston Street Fishweir, II. Peabody Found. Archaeol. Pap., 4: 7-48. Kaye, C.A., 1976. The geology and early history of the Boston, Massachusetts area; a Bicentennial approach. U.S. Geol. Surv. Bull., 1476, 78 pp. Kaye, C.A., 1982. Bedrock and Quaternary geology of the Boston area, Massachusetts. Geol. Soc. Am., Rev. Eng. Geol., 5: 25-40. Kriiger, J., 1987. Relationship of drumlin shape and distribution to drumlin stratigraphy and glacial history, Myrdalsj6kull, Iceland. In: J. Menzies and J. Rose (Editors), Drumlin Symposium. A.A. Balkema, Rotterdam, pp. 257-266. LaForge, L., 1932. Geology of the Boston area, Massachusetts. U.S. Geol. Surv. Bull., 839: 50-60. Menzies, J., 1982. A till hummock (proto-drumlin) at the ice-glacier bed interface. In: R. Davidson-Arnott, W. Nickling and B.D. Fahey (Editors), Research in Glacial, Glaciofluvial, and Glaciolacustrine Systems. GeoBooks, Norwich, pp. 33-47. Menzies, J., 1984. Drumlins: A Bibliography. GeoBooks, Norwich, 117 pp. Newman, W.A., Berg, R.C., Rosen, P.S. and Glass, H.D., 1990. Pleistocene stratigraphy of the Boston Harbor drumlins, Massachusetts. Quat. Res., 34: 148-159. Rendigs, R.R. and Oldale, R.N.. 1990, Maps showing the results of a subbottom acoustic survey of Boston Harbor, Massachusetts. U.S. Geol. Surv. Misc. Field Studies, Map MF-2124, 2 sheets. Shaler, N.S., 1870. On the parallel ridges of glacial drift in eastern Massachusetts, with some remarks on the glacial period. Proc. Boston Soc. Nat. Hist., 13: 196-204.
343
Shaler, N.S., 1888. Report on the geology of Martha's Vineyard, Massachusetts. U.S. Geol. Surv., 7th Annu. Rep., pp. 297-363. Shaw, J. and Kvill, D., 1984. A glaciofluvial origin for drumlins of the Livingstone Lake area, Saskatchewan. Can. J. Earth Sci., 21: 1442-1459. Shaw, J. and Sharpe, D.R., 1987. Drumlin formation by subglacial water erosion. Can. J. Earth Sci., 24: 2316-2322. Shaw, J., Kvill, D. and Rains, B., 1989. Drumlins and catastrophic subglacial floods. In: J. Menzies and J. Rose (Editors), Subglacial B e d f o r m s - - D r u m l i n s , Rogen Moraine and Associated Subglacial Bedforms. Sediment. Geol., 62: 177-202. Smalley, I.J. and Unwin, D.J., 1968. The formation and shape of drumlins and their distribution and orientation in drumlin fields. J. Glaciol., 7: 377-390. Stanford, S.D. and Mickelson, D.M., 1985. Till fabric and deformational structures in drumlins near Waukesha, Wisconsin, U.S.A.J. Glaciol., 31: 220-228. Stea, R.R. and Brown, Y., 1989. Variation in drumlin orientation, form and stratigraphy relating to successive ice flows in southern and central Nova Scotia. In: J. Menzies and J. Rose (Editors), Subglacial Bedforms--Drumlins, Rogen Moraine and Associated Subglacial Bedforms. Sediment. Geol., 62: 223-240. Upham, W., 1879. Glacial drift in Boston and its vicinity. Proc. Boston Soc. Nat. Hist., 20: 220-234. Upham, W., 1889a. Marine shells and fragments of shells in the till near Boston. Am. J. Sci., 37: 359-372. Upham, W., 1889b. The structure of drumlins. Proc. Boston Soc. Nat. Hist., 24: 228-242. Upham, W., 1893a. Drumlins near Boston. Appalachia, 7: 39-48. Upham, W., 1893b. The origin of drumlins (with discussion by W.M. Davis and H. Barton). Proc. Boston Soc. Nat. Hist., 26: 2-25. Upham, W., 1894. The Madison type drumlins. Am. Geol., 14: 69-83. Whittecar, G.R. and Mickelson, D.M., 1977. Sequence of till deposition and erosion in drumlins. Boreas, 6: 213-217.
Sedimentary Geology 91 (1994) 333-343
Genesis of Boston Harbor drumlins, Massachusetts William A. Newman
a, D a v i d M . M i c k e l s o n
b
a Department of Geology, Northeastern University, Boston, MA 02115, USA b Department of Geology and Geophysics, University of Wisconsin, Madison, W153706, USA
Received January 29, 1992; revised version accepted March 1, 1993
Abstract
Drumlins in Boston Harbor are composed of two superposed silty-sandy diamicton units and subordinate sand and gravel. The lower, compact, undifferentiated, diamicton is interpreted as a pre-Wisconsin till, based on truncated weathering profiles preserved below an erosion surface. The upper, less compact, diamicton contains numerous, discontinuous and poorly sorted lenses and stringers of sand and sandy gravel that tend to parallel the erosion surface. The upper diamicton and subordinate sandy gravel lenses are interpreted to date from the late Wisconsin glaciation. Along the exposed sections of the Long Island drumlin complex, the preserved weathering profile on the lower diamicton is thicker beneath the drumlin axes and is either thin or missing in the interdrumlin areas. This suggests that glacial erosion and streamlining of pre-Wisconsin deposits and their weathering profile took place after the weathering profile developed and before subsequent deposition of the overlying diamicton unit. The latter drapes the erosional drumlin form without contributing substantially to relief of the drumlin form. 1. Introduction
The origin of drumlins in different parts of the world has been discussed in numerous papers in recent years. A bibliography of drumlin research (Menzies, 1984) and papers published subsequently have included an even wider range of ideas about their formation. Current hypotheses can be grouped into three basic mechanisms. The first is that drumlins form by the accretion of basal till in the subglacial environment. This could take place either by the accumulation of frozen basal sediment at the base of the glacier, or by accumulation of wet subglacial sediment that had been deforming beneath the ice (e.g., Smalley and Unwin, 1968; Baranowski, 1969; Dardis, 1981; Menzies, 1982; Boulton, 1987; Boulton and Hindmarsh, 1987). In either case,
the basis of these hypotheses is that contemporaneous deposition, deformation, and shaping of diamicton beneath relatively thick ice into the well-known streamlined form creates the drumlin's morphology and internal structure. Another set of hypotheses focuses on the importance of erosion in the formation of drumlins (e.g., Whittecar and Mickelson, 1977; Stanford and Mickelson, 1985; Kriiger, 1987; Habbe, 1992). The presence of pre-existing, stratified and interbedded sand and gravel or streamlined bedrock in the cores of some drumlins demonstrates their erosional nature, but the importance of erosion in the formation of drumlins that are composed entirely of a single diamicton unit is difficult to demonstrate. A third hypothesis that has received much attention in the last few years is that of a fluvial
0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0037-0738(94)00019-Q
334
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
origin (Shaw and Kvill, 1984; Shaw and Sharpe, 1987; Shaw et al., 1989). This hypothesis suggests that huge flows of water beneath the ice created bedforms that we now interpret as drumlins, or that streamlined shapes were carved in the sole of the glacier by flowing water and these were subsequently in-filled with sediment. Upham (1894) argued that sand- and gravelcored drumlins in the Boston area and those in the Madison, Wisconsin area both had formed by the same process. He suggested that subglacial erosion of pre-existing sand and gravel produced the drumlin shape, followed by deposition of till and other sediment over this streamlined surface. Our observations from drumlins in Wisconsin and in the Boston basin support this conclusion. However, our interpretation in this paper is based primarily on diamicton-cored drumlins in which a truncated weathering profile strongly argues for a streamlined surface cut into a pre-existing landscape.
--42 020', . . ~ 2 ^ q, ~,~ .~' ~
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/
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~
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Where drumlins contain sand and gravel in the core, at least two interpretations are possible. Either the sand and gravel was deposited before the drumlin-forming event and was later shaped by erosion, or the sand and gravel was deposited in a subglacial cavity as part of the drumlin forming process. In the absence of absolute dates or major lithologic differences, one can argue for one or the other interpretation with the use of sedimentology, cross-cutting relationships between sand and gravel and overlying diamicton, and aerial distribution. An angular unconformity between the gravel in the core and the overlying diamicton indicates that at least some erosion into the drumlin form took place after gravel deposition and before deposition of the overlying diamicton. This could also occur in cavity fillings. However, if the sand and gravel is extensive and has a top surface approximately at the same elevation in many drumlins, and if the sedimentology of the units indicates deposition in a
~
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/ \
"~PrineeHead d s ~ ~ ~H or.
0
.
1
Fig. 1. Map of islands in Boston Harbor with drumlin exposures.
2 mi
--
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
braided stream, then it is possible that the stratified sediments were deposited prior to the advance of ice that formed the drumlin. Therefore an erosional model of drumlin formation (Whittecar and Mickelson, 1977) would be most likely. Where two diamicton units occur in drumlins without an extensive gravel core, as they do in Boston Harbor, then the above arguments cannot be used. Although the diamictons cannot be dated, there is a weathering profile on the top of the lower diamicton unit in the Boston area. Therefore we suggest that the pattern of erosion on this weathering profile indicates that glacial erosion of the lower diamicton unit was an important process in drumlin formation.
2. Distribution of drumlins More than 200 drumlins occur in the Boston area. Those drumlins are located in Boston Har-
335
bor and in Massachusetts Bay to the east, where they are partly or entirely submerged (Rendigs and Oldale, 1990) and many of them are buried under glaciomarine sediments (Newman et al., 1990). Shaler (1870) suggested that these peculiar lenticular hills were remnants of a sheet of sediment that had been modified by post-glacial rivers and waves, while others were due to the topography of the underlying bedrock. Later Shaler (1888) interpreted drumlins to have formed by glacial erosion of older glacial sediment. In some parts of the basin the drumlins form clusters of more or less coalescent groups (e.g., at Hull and Worlds End; Figs. 1, 2); others occur in ill-defined rows 2 - 3 km long (e.g., Long Island and Peddocks Island) and elsewhere single drumlins are common. The average trend of the long axes of the Boston Basin drumlins is 125 ° (Crosby, 1934). The drumlins in the harbor itself trend about 110 ° and their emerged height and length range to about 30 m and about 0.4 km, respec-
BOSTON HARBOR
O ~-~ ~.
Drumlin (stippled) and its axial trend. ~ Eroded drumlin ~ ,LaabtrecW.isconsinice-flow directions inferred from till ~
---i,.-
Pre Wisconsin ice-flow directions inferred from till fabrics. Numbers refer to ascending till fabric station position within thick sections. Late Wisconsin glacial striae. Pre late Wisconsin glacial striae located on bedrock bl~neath eroded drumlin.
--•~
Axial trend of submarine valleys.
¢ )~'~/~ ? ~-U I
~
~.
J.~
0
1
2
3Am
I 0
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Fig. 2. Trends of Boston Harbor drumlins, till fabric and bedrock striations, and axes of submerged valleys. Orientation of drumlins is accurate, while shape and size are based on lowest closed contour on 1 : 24,000 quadrangles.
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
336
tively, where their total length can be reconstructed. Drumlins resting on bedrock ridges are arranged in lines along the strike of the ridge (W.O. Crosby, 1903; I.B. Crosby, 1934; Kaye, 1976, 1982). In those drumlins containing a rock core there appears to be no systematic relationship between the position of the rock core and the center of the drumlin (LaForge, 1932). Drumlins on the mainland rarely reveal stratigraphic sections. However, frequent storm-wave erosion maintains the excellent drumlin sections that are exposed on some of the Boston Harbor islands. In particular, exposures on Long Island, Peddocks Island, Great Brewster Island, and Rainsford Island (Figs. 1, 2) were examined in detail during this study. The harbor drumlins consist mostly of two superposed diamictons with thin, discontinuous sand or sandy gravel layers between them (Fig. 3), but drumlins described in the vicinity by Upham (1879, 1889a, 1893a, b, 1894) contain considerable thicknesses of sand and gravel underlying an upper diamicton unit
and sometimes overlying diamicton or fossiliferous marine clay.
3. Quaternary stratigraphy in Boston Harbor Upham (1879, 1889a, b) was one of the first to study the stratigraphy of the Boston Harbor area in detail. He and most subsequent workers recognised two diamicton units in the area that they interpreted as till. Where he found drumlins containing two diamicton units he initially interpreted the lower unit as basal till and the upper unit as supraglacial sediment let down from the surface of the same glacier that deposited the underlying till. Later, Upham (1894) argued that the lower unit was overrun and eroded into the drumlin form by a later ice advance. LaForge (1932) and Judson (1949) suggested that the two diamicton units in the drumlins were deposited during a single glaciation, but LaForge (1932) thought that both units were deposited
l"
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i
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[ ~
Diamicton interpreted to be supraglacial till, flow and slopewash sediment
w
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u
396m
Oxidized and ieached diamicton interpreted to be basal till of pre-Wisconsin advance Unoxidized and unleached diamicton interpreted to be basal NI of a pre-Wisconsin advance
Interbedded medium sand, coarse sand and sandy gravel [ ~
i
-- •--
Indistinct contact between weathered and unweathered diamicton
Fig. 3. Stratigraphic section along the southeastern (i.e. lee) side of the Long Island drumlin complex.
337
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
during the late Wisconsin glaciation, while Judson (1949) associated the units with an early or middle Wisconsin glaciation. Kaye (1982) called the lower unit the "drumlin till". Newman et al. (1990) argued that the lower diamicton unit recognised by U p h a m and earlier workers is preWisconsin, based on clay mineralogy. If this is the case, then drumlins that have this lower unit in the core must either have been shaped by erosion or preserved during late Wisconsin glaciation. The unweathered lower diamicton contains fragments of mollusc shells derived from older deposits. Samples of Mercenaria mercenaria from the lower till unit at Peddocks Island (Fig. 1) yielded a 14C age older than 37 ka B.P. (H-1125; Kaye, 1976). Belnap (1979) estimated the age of the shells at between 200 and 214 ka B.P., based on amino acid racemisation. Later Belnap (1980) suggested that they were Sangamonian in age. N e w m a n et al. (1990) suggested that this lower diamicton unit was of Illinoian age or older and that the weathering profile contains a clay mineral sequence similar to sequences on Illinoian tills in the Midwest (e.g., Follmer, 1983). Many of the weathering profiles described in the Boston area are truncated at the top, presumably by late Wisconsin ice that deposited the u p p e r diamicton unit.
3.1. Descriptions o f sediments
The lower diamicton unit is light brown to olive (7.5YR 4 / 6 to 5YR 4 / 3 ) where weathered and olive-gray (5YR 4 / 2 ) where it is unoxidised, compact, and similar in grain size distribution to the overlying diamicton. The u p p e r diamicton is oxidised, light gray to pale yellow (5Y 4 / 3 - 5 / 4 ) , compact to loose, in places faintly stratified, and contains no marine shell fragments. The lower diamicton unit contains more low charge vermiculite than the upper unit, and the u p p e r unit also contains more illite than the diamicton unit below (Newman et al., 1990). Detailed description of the sections used for clay mineral studies and correlation between them is given by Newman et al. (1990). The 38 samples of the two diamicton units collected in this study show identical means of 37% sand, 45% silt and 19% clay. Magnetic susceptibility is slightly lower in the u p p e r till, but the difference is not statistically significant. In places, a thin sandy diamicton layer overlies the entire section. It contains lenses of sand and gravel, is oxidised and poorly consolidated, and appears to form part of the u p p e r diamicton unit. In some places the two diamicton units are separated by discontinuous beds of stratified sand, or
Drumlin Axis
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1
18m 15m
12m 9m
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130
140
150
160 m
-~
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~-~
Diamietoninterpreted to be supraglacial till, flow and slopewash sediment
[~
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Interbedded medium sand, coarse sand and sandy gravel
r~
unoxidized and unleached diamicton interpreted to be basal till of a pre-Wisconsin advance
Diamicteninterpreteo~to be basal tiU of late Wisconsin advance
m e --
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[~
Fig. 4. Stratigraphic section along the northwestern (i.e. stoss) side of the Long Island drumlin complex. The drumlin axis indicated is the same drumlin as the one at 330 m on Fig. 3. The same unit labels are used here as in Fig. 3, although some units are not exposed in this section.
338
lEA. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
sand and gravel, with clasts up to 0.3 m in diameter. Where exposed, this stratified unit is up to 4 m thick, but is absent in many places.
3. 2. Description of sections The stratigraphy and lithology in the sections is similar to that described from the harbor area. The drumlins of Long Island (Figs. 1, 2) are dissected at both ends by wave-eroded cliffs. Sections were also examined and samples taken on Peddocks Island, Moon Island, G r e a t Brewster Island and Rainsford Island, and at Prince Head. A section across the lee end of the drumlins (southeast) is shown in Fig. 3 and through the stoss (northwest) side of the drumlins in Fig. 4. The drumlin crest at 79 m in Fig. 4 is the stoss end of the drumlin at 330 m in Fig. 3. The oldest unit (units 1 and 2 in Figs. 3 and 4) is the lower diamicton ("drumlin till" of Kaye, 1982). It is unoxidised below (unit 1) and oxidised above (unit 2). Although the diamicton is not rich in carbonate, unit 1 contains scattered shells and shell fragments. The shells have been leached from unit 2. Note that unit 2 is missing on the stoss side of the drumlins where the section is exposed (Fig. 4). Grain size and other characteristics are similar in the Long Island section to those described for the units as a whole. O t h e r islands examined in Boston H a r b o r show exposures of either the u p p e r diamicton, the lower diamicton, or both, sometimes with intervening stratified sediment. The u p p e r diamicton cap along the crest of the drumlins is missing in exposures on Moon Island and Rainsford Island (Figs. 1, 2). At Moon Island, the lower part of the section is covered with slumped material, but 6 to 8 m of lower diamicton unit above the slumped sediment is leached of carbonate. The Rainsford Island section is more or less parallel to the drumlin axis and consists entirely of the lower diamicton based on its amount of consolidation and analysis of samples by Newman et al. (1990). The upper 8 m of section is leached, but the lower part of the section contains shells. Both of these sites suggest that the depth of leaching was over 8 m in the lower diamicton unit before the erosion event that removed part of the weather-
ing profile. Clearly some sediment was eroded even from these sites because the u p p e r part of the soil profile (A and B horizons) is not present even though the sediments are oxidised and carbonate has been leached. Several other exposures were examined, but it was unclear where a contact between the upper and lower diamictons should be drawn. Peddocks Island (Fig. 2) has an exposure nearly 1 km long that is parallel to and oblique to the ice-flow direction. The stratigraphy here is complex, with many sand lenses and boulder zones. Newman et al. (1990) show a mixed zone between what they interpret as the upper and lower diamicton units. Carbonate is unleached in the section, suggesting that as much as 8 m of sediment was eroded along the sides of this drumlin before deposition of the upper diamicton. Another exposure parallel to the drumlin orientation is at Prince H e a d (Fig. 2). Here, in a small drumlin about 10 m high with an eroded face parallel to the drumlin axis, both diamicton units are present and the lower unit is leached to the beach level (more than 4 m). Clast fabrics in this face are oriented at 115 ° to 140 ° in the lower diamicton and 90 ° in the u p p e r diamicton. The weathered portion of the lower diamicton (unit 2, Fig. 3) in some places contains vertical fractures 1-3 cm wide that contain silt and clay fillings of iron oxide and manganese oxide. Fractures extending downward into the unoxidised diamicton also contain secondary calcium carbonate. The amount of oxidation decreases downward gradually, and the boundary between units 1 and 2 shown on Fig. 3 should be considered as a zone 10-20 cm wide. Because the primary carbonate in the unleached diamicton (unit 1) is restricted to broken shells, the precise depth of leaching is difficult to determine. Textural and clay mineral data from one section near the center of a Long Island drumlin (Fig. 3, 125 m) shows that the upper diamicton (unit 5) is sandier than the diamicton beneath the weathering profile (unit 1) and significant differences occur in the low charge vermiculite and chlorite content (Newman et al., 1990). The lower diamicton is leached of carbonate to a depth of about 3.7 m at this location. Below the lower
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
boundary of carbonate leaching, fossil shell fragments occur but are not abundant. A thicker but similar buried weathering profile occurs beneath the distal crest of the more easterly drumlin (Fig. 3, 330 m along section). Here the contact between the upper and lower diamictons (units 2 and 3) is difficult to define precisely because there are no stratified sediments between and because both are oxidised and leached.
4. Interpretation Uniformity of grain size distribution, strength and uniformity of fabric and consolidation of the lower diamicton (units 1, 2) and part of the upper diamicton (unit 3) suggests that these units are basal till. Genesis by lodgement or basal meltout cannot be ascertained applying the criteria used here. Similarly, distinction cannot be made based on observations from the sedimentary record, between deposition either from basal-rich ice or
339
from a subglacially deforming wet bed. Part of the upper diamicton (unit 5) may represent a mixture of supraglacial flows, sediment let down from melting ice in cavities or subaerially, or possibly post-glacial slopewash sediment. An important aim of this paper is to point out that the shape of drumlins is determined to a great extent by the shape of an unconformity (erosion surface) developed on the lower till (units 1 and 2) by erosion at the base of glacier ice. Units 3, 4, and 5 were deposited above this unconformity after streamlining by subglacial erosion. On the stoss (northwest) side of the Long Island drumlins (Fig. 4) the upper diamicton (unit 3) is fairly thin (3-4 m) and overlies the older diamicton unit which contains shells throughout. This indicates that the leached section of the weathering profile was eroded before unit 3 was deposited. The loose, slightly stratified unit 5 seen on the lee side of the drumlins is missing in this section.
N [~
Diamictoninterpreted to be supraglacial till, flow and slope#ash sediment
[~
Oxidizedand leached diamicton interpreted to be basal till of pre-Wisconsin advance
[~
Interbeddedmedium sand, coarse sand and sandy gravel
[~
Unoxidizedand unleached diamicton interpreted to be basal till of a pre Wisconsin advance
[~
biamictoninterpreted to be basal till of late Wisconsin advance
[]~
Bedrock
Ice flow direction inferred from fabric measurements
--o--
Indistinct contact between weathered and unweathered diamicton
Fig. 5. Interpretation of sections in the Long Island drumlin complex, based on observations from the stoss and lee sides. Note that no section of exposure is available parallel to the drumlin long axis.
W.A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
340
1
Pre-Wisconsin end moraine or drumlins
A
Unconformity developed beneath ice during the late Wisconsin glacial advance
t.
Subsequent deposition of units 3, 4, and 5 during the late Wisconsin glacial retreat
3,4,5
-~
Diamicton interpreted to be supraglacial till, flow and slopewash sediment
I~
Oxidized and leached diamicton interpreted to be basal till of pre-Wisconsin advance
Interbedded medium sand, coarse sand and sandy gravel
[~]
Unoxidized and un~eached diamicton interpreted to be basal till of a pre-Wisconsin advance
Diamicton interpreted to be basal till of late Wisconsin advance
~
Bedrock
Ice flew direction inferred from fabric measurements
i e
Indistinct contact between weathered and unweathered diamicton
Fig. 6. Sketch of formation of the Long Island drumlin complex. (A) Pre-Wisconsin ice sheet, flowing southeastward across Boston Harbor, deposited till (1) on the northeast-trending bedrock ridge that underlies the Long Island drumlin complex. Following recession of the pre-Wisconsin ice sheet the till deposit was subjected to subaerial weathering. During this time a deep weathering profile developed, with leaching of carbonates and oxidisation of sediments (2). (B) During the advance of the late Wisconsin ice sheet the pre-Wisconsin surface was eroded by ice flowing eastward, forming the drumlin core and completely removing the weathering profile from the stoss side of the drumlin. (C) Subsequently, the same ice mass deposited till on the underlying unconformity, producing the final drumlin form. Subglacial or ice-marginal fluvial deposits (4) and supraglacial till and flow sediment now cover much of the land surface.
W..A. Newman, D.M. Mickelson / Sedimentary Geology 91 (1994) 333-343
The significance of the buried weathering profile and, in particular, the depth of carbonate leaching is that it can be traced laterally in section, where it is thicker in the lower diamicton beneath the drumlin axes relative to beneath interdrumlin troughs (Fig. 3). In the low area to the south of the main drumlin on Long Island (Fig. 3, 24 m) the thickness of the lower till unit that is leached is only about 0.3 m. However, in the interdrumlin areas the top of the lower diamicton drops to beach level (Fig. 3, 30 m and 275 m) and rises again at the southern end and beneath the more northern drumlin crest (Fig. 3, 330 m). At the southern end of the section, no weathering profile is present on the lower diamicton, and carbonate is present immediately below its u p p e r contact. The abrupt change in clay minerals and the presence of pedogenic and weathering features just below the contact between the u p p e r and lower diamictons indicate that this weathering profile is not an extension of the modern one from above, but is the truncated portion of a weathering profile that developed on the land surface before the last glaciation (Newman et al., 1990). Although in some cases the absence of a weathering profile could result from a locally high water table, the gradual thinning of the profile moving away from the drumlin crests suggests truncation. In addition, sand and gravel layers interbedded with the upper diamicton (Figs. 3, 5) in the Long Island section parallel the erosion surface on the lower unit, indicating that the form was there when these sediments were deposited. The orientation of long axes of pebbles measured at several locations in the lower diamicton on Long Island are strongly developed and oriented at 140 ° (Figs. 3, 5). At G r e a t Brewster Island fabrics in the lower diamicton indicate an early flow toward 170 ° , later flow toward 110 ° and the latest flow toward 140 ° . At Prince Head, southeast of Long Island, ice flow near the base of the lower till was toward 115 ° . Later it became more southerly. T h e r e are not enough measurements to compile a flow history for the lower unit, but it seems likely that local topography influenced ice flow.
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Fabric m e a s u r e m e n t s from the upper till (unit 3) suggest that in places ice flow was southward (e.g., fabrics on T h o m p s o n Island, Peddocks Island and Allerton Hill; striations on Moon Island). Elsewhere, flow was eastward (e.g., Spectacle Island, Long Island, and G r e a t Brewster Island). The wide range in drumlin orientations in the harbor is probably a result of the irregular topography of the harbor.
5. Discussion
The Long Island drumlin complex is interesting because wave erosion has exposed extensive sections normal to the drumlin axes and at opposite ends of the drumlins (Fig. 5). The lower, thick till and associated sediment was deposited by a pre-Wisconsin ice sheet (Fig. 6). Differences between pre-Wisconsin landforms and present landforms are indicated by the irregular pattern of the truncated weathering profile on the lower till. Selective erosion of the lower till by eastward flowing late Wisconsin ice sheet shaped the asymmetrical cores of the present harbor drumlins (Figs. 5, 6). This erosion selectively reduced the thickness of the leached and oxidized soil profiles to 0 to 4 m on the lower till surface. On Long Island, this weathering profile was most extensively eroded on the stoss side and along the drumlin flanks. In the lee of the drumlins, especially at their axis, glacial erosion was minimal. The upper till unit drapes the drumlinised lower till unit and is thinner in places near drumlin crests. This pattern is distinctly different than in many gravel-cored drumlins in Wisconsin where thicker till is present right on the drumlin crest and the till thins to the drumlin edges (Stanford and Mickelson, 1985). In this case, it was suggested that migration of till into the drumlin axis took place, but there appears to be no evidence of this mechanism in the present study. It might be expected that fabric orientations would converge on the down-ice side of the drumlin, but the current m e a s u r e m e n t s do not demonstrate this, and no sections lie close to the drumlins crests. The internal structure of these drumlins is
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less complex than those described in Nova Scotia (cf. Stea and Brown, 1989). T h e paucity of striated rock surfaces and fabric data in the study area prevent detailed reconstructions of changes in ice-flow direction. It is difficult to explain the patterns of d r u m lins and interdrumlin areas in the study area. O n e possibility is that this is related to parts of the glacier bed which were frozen and parts which were u n f r o z e n at the time of their formation. It could also be related to variation in the strength of materials at the bed, which may reflect differences in consolidation and hydraulic conductivity. A l t h o u g h it is possible to evaluate this quantitatively, there are no differences in grain size or c o m p a c t i o n b e t w e e n the lower till in the drumlins and the lower till in the interdrumlin areas. Alternatively, drumlin location may have b e e n influe n c e d by the preglacial topography, w h e r e b y areas that were hills after deposition, weathering, and subaerial erosion of the lower till unit, were simply a c c e n t u a t e d and streamlined into the drumlin shape. T h e clustered distribution suggests at least that these islands were uplands before the last glaciation.
6. Conclusions Observations on the drumlins in Boston Harbor suggest the following sequence of events (Fig. 6): (1) A d v a n c e of ice into the h a r b o r area before the Wisconsin glaciation. This ice advanced over marine silt and clay or older sediments that contained shells of Mercenaria mercenaria. (2) Following glacial recession, a long period of weathering took place w h e n clay minerals were altered ( N e w m a n et al., 1990) and w h e n the lower till unit was oxidised and leached of c a r b o n a t e to depths in excess of 6 to 8 m. (3) Ice advanced during late Wisconsin time across the study area and carved drumlins from the underlying w e a t h e r e d till. This p r o d u c e d thickness variations in the leached and oxidized till in the cross-section at L o n g Island. Evidently this glacial event deposited less consolidated upper till, flow sediments and sand and gravel lenses
as a blanket over the drumlinised unconformity that had developed b e n e a t h the ice. Drumlins probably f o r m e d by differential erosion of pre-existing till, probably u n d e r wet or partly frozen bed conditions. Finally, till was deposited on the sculptured drumlin cores during the retreat phase of glaciation, when the orientation of the drumlin forms was c h a n g e d as a result of late changes in ice-flow direction.
Acknowledgements W e thank Kirsten Lindquist and H a r a l d Newm a n for drafting the figures and A l a n Saiz for producing the c o m p u t e r graphics used in the figures. Nelson H a m , Lars R o n n e r t and Kent Syverson provided critical c o m m e n t s on the manuscript, as did two a n o n y m o u s reviewers. G e o r g e Dardis contributed substantial helpful criticism.
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