Lake Michigan Lobe Research Articles

Palimpsest tunnel valleys: evidence for relative timing of advances in an interlobate area of the Laurentide ice sheet

AbstractDuring retreat from the lateWisconsinan maximum advance in the Great Lakes region of North America, the Laurentide ice sheet margin became distinctly lobate. The Lake Michigan, Saginaw, and Huron—Erie lobes converged in southern Michigan and Indiana, U.S.A. to form a complex interlobate region. Some time after the glacial maximum, the Lake Michigan lobe advanced over landscapes previously formed by the Saginaw lobe. This can be explained by an asynchronous advance of the Lake Michigan lobe during a Saginaw lobe retreat or by an increase in size of the Lake Michigan lobe relative to the Saginaw lobe during a synchronous readvance.Cross-cutting relationships in southwestern Michigan, including palimpsest tunnel valleys, document the overriding of Saginaw lobe terrain. Deep, generally straight trenches parallel to glacial flow lines with hummocky, irregular sides and bottoms are interpreted as tunnel valleys. Saginaw lobe tunnel valleys trend northeast—southwest and Lake Michigan lobe tunnel valleys generally trend east—west.At some time after a Saginaw lobe retreat in southern Michigan, the drumlinized landscape was overridden by an advance of the Lake Michigan lobe to an ice-marginal position at the Tekonsha moraine. Saginaw lobe tunnel valleys in the overridden area were completely filled with ice and debris from the Saginaw lobe retreat at the time of the Lake Michigan lobe advance. Supraglacial and proglacial sediments were deposited over the buried valleys by the Lake Michigan lobe, sometimes by meltwater streams that flowed at high angles to the trends of the valleys. After entrenchment of the Kalamazoo River valley, probably by a subglacial outburst flood, short tributaries were cut nearly at right angles across and through the debris and ice within several buried Saginaw lobe tunnel valleys. After the retreat of the Lake Michigan lobe, subsequent melting of ice in the palimpsest tunnel valleys exhumed the valleys, creating the cross-cutting relationships with the Lake Michigan lobe deposits.

Evidence at Lomax, Illinois, for Mid-Wisconsin (∼40,000 yr B.P.) Position of the Des Moines Lobe and for Diversion of the Mississippi River by the Lake Michigan Lobe (20,350 yr B.P.)

An abrupt change in environment from a wetland to a deeper slackwater lake at 20,345 ± 85 yr B.P. occurred in a second-order valley tributary to the Mississippi River near Lomax, Illinois. The age of this shift is associated with the overflow of glacial Lake Milan and diversion of the Mississippi River from the Princeton Valley (Rock Island to Peoria) to its present course (Rock Island to St. Louis). The diversion occurred due to blockage of the Princeton Valley segment of the ancient Mississippi River by the Lake Michigan Lobe, impoundment of glacial Lake Milan, and eventual overflow of the lake southwest of Rock Island across a former drainage divide near Andalusia, Illinois. Fossil ostracode assemblages in the slackwater sediment at Lomax indicate changes in the post-diversion, full-glacial paleohydrology and, based on multivariate analysis, hint at the area's paleoclimate. An older part of the succession at Lomax is consistent with glaciation in the upper Iowa River basin about 40,000 yr B.P. Aggradation of sediment rich in coarse silt is attributed to a response of the ancient Iowa River basin associated with deposition of the glacigenic Sheldon Creek Formation by the Des Moines Lobe.

Paleobiology of the Sand Beneath the Valders Diamicton at Valders, Wisconsin

Previously undescribed pollen, plant macrofossils, molluscs, and ostracodes were recovered from a 2.5-m-thick glaciolacustrine unit of silty sand and clay at Valders, Wisconsin. The interstadial sediment was deposited about 12,20014C yr B.P. after retreat of the Green Bay lobe that deposited diamicton of the Horicon Formation, and before advance of the Lake Michigan lobe that deposited the red-brown diamicton of the Valders Member of the Kewaunee Formation. Fluctuations of abundance ofCandona subtriangulata, Cytherissa lacustris,and three other species define four ostracode biozones in the lower 1.7 m, suggesting an open lake environment that oscillated in depth and proximity to glacial ice. Pollen is dominated byPiceaandArtemisia,but the low percentages of many other types of long-distance origin suggest that the terrestrial vegetation was open and far from the forest border. The upper part of the sediment, a massive sand deposited in either a shallow pond or a sluggish stream, contains a local concentration of plant macrofossils. The interpretation of a cold open environment is supported by the plant macrofossils of more than 20 species, dominated by those of open mineral soils (Arenaria rubella, Cerastium alpinumtype,Silene acaulis, Sibbaldia procumbens, Dryas integrifolia, Vaccinium uliginosumvar.alpinum, Armeria maritima,etc.) that in North America occur largely in the tundra and open tundra–forest ecotone of northern Canada. Ice-wedge casts occur in the sand.

Absence of Glaciation in Illinois during Marine Isotope Stages 3 through 5

A10Be inventory and14C ages of material from a core from northernmost Illinois support previous interpretations that this area was ice free from ca. 155,000 to 25,000 yr ago. During much of this period, from about 155,000 to 55,000 yr ago,10Be accumulated in the argillic horizon of the Sangamon Geosol. Wisconsinan loess, containing inherited10Be, was deposited above the Sangamon Geosol from ca. 55,000 to 25,000 yr ago and was subsequently buried by late Wisconsinan till deposited by the Lake Michigan Lobe of the Laurentide Ice Sheet. The Sangamonian interglacial stage has been correlated narrowly to marine oxygen isotope substage 5e; our data indicate instead that the Sangamon Geosol developed during late stage 6, all of stages 5 and 4, and early stage 3.

Numerical modeling of subglacial sediment deformation: Implications for the behavior of the Lake Michigan Lobe, Laurentide Ice Sheet

We apply a numerical model of the late Wisconsin (circa 20,000 years B.P.) Lake Michigan Lobe (LML), Laurentide Ice Sheet, to investigate how fine‐grained subglacial sediment might influence lobe behavior, particularly rapid millennial‐scale marginal oscillations observed in the geologic record. Over the Canadian Shield, we assume a rigid bed (“hard bedded”) basal boundary condition. In areas overlain by fine‐grained sediment (“soft bedded”), the base of the ice is coupled to a deformable sediment layer, using a rate‐dependent stress‐strain law. Geotechnical tests of clay‐rich till deposited by the LML provide control for sediment rheologic parameters. Simulated cross‐sectional profiles are consistent with reconstructions from geologic evidence. Time‐dependent simulations suggest that a soft‐bedded lobe could have reached steady state in about 20,000 years or less, in contrast to 50,000 to 60,000 years for an otherwise identical hard‐bedded lobe. A soft‐bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial‐scale shifts in accumulation or ablation as a nonsliding hard‐bedded lobe, but in both cases the response is slower than that indicated by the geologic record. Results suggest that while strong millennial‐scale changes in accumulation and ablation can produce responses in hard‐bedded or soft‐bedded ice that are consistent with the geologic record, changes in subglacial sediment viscosity, even relatively modest changes (whether independent or in conjunction with climate change), might more readily account for millennial‐ and submillennial‐scale fluctuations of the lobe margin. These observations do not exclude a role for sliding, but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior.

Numerical modeling of advective transport of saturated deforming sediment beneath the Lake Michigan Lobe, Laurentide Ice Sheet

We investigated the dynamics of advective transport of saturated, fine-grained deforming sediment beneath the Late Wisconsinan Lake Michigan Lobe (LML) of the Laurentide Ice Sheet using a numerical flowline model. Sediment rheology is modeled using a general constitutive law for mass movement of fine-grained material. Geotechnical tests on selected tills deposited by the LML in northern Illinois suggest an effective viscosity of about 5 × 10 9 Pa-s for fully saturated sediment and mild nonlinearity in the stress-strain relation. Numerical calculations of sediment flux, based on conservative parameters and assumptions, yield estimates on the order of 100 m 3/y/meter width of the lobe, suggesting that advective transport could account directly for at least 25% of the total sediment flux estimated on the basis of field evidence. Less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice-velocity-dependent sediment transport processes. Modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment. These results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine-grained glacial till, and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity.

Deglaciation, lake levels, and meltwater discharge in the Lake Michigan basin

The deglacial history of the Lake Michigan basin, including discharge and routing of meltwater, is complex because of the interaction among (1) glacial retreats and re-advances in the basin (2) the timing of occupation and the isostatic adjustment of lake outlets and (3) the depositional and erosional processes that left evidence of past lake levels. In the southern part of the basin, a restricted area little affected by differential isostasy, new studies of onshore and offshore areas allow refinement of a lake-level history that has evolved over 100 years. Important new data include the recognition of two periods of influx of meltwater from Lake Agassiz into the basin and details of the highstands gleaned from sedimentological evidence. Major disagreements still persist concerning the exact timing and lake-level changes associated with the Algonquin phase, approximately 11,000 BP. A wide variety of independent data suggests that the Lake Michigan Lobe was thin, unstable, and subject to rapid advances and retreats. Consequently, lake-level changes were commonly abrupt and stable shorelines were short-lived. The long-held beliefs that the southern part of the basin was stable and separated from deformed northern areas by a hinge-line discontinuity are becoming difficult to maintain. Numerical modeling of the ice-earth system and empirical modeling of shoreline deformation are both consistent with observed shoreline tilting in the north and with the amount and pattern of modern deformation shown by lake-level gauges. New studies of subaerial lacustrine features suggest the presence of deformed shorelines higher than those originally ascribed to the supposed horizontal Glenwood level. Finally, the Lake Michigan region as a whole appears to behave in a similar manner to other areas, both local (other Great Lakes) and regional (U.S. east coast), that have experienced major isostatic changes. Detailed sedimentological and dating studies of field sites and additional development of geophysical models offer hope for reconciling the field data with our understanding of earth rheology.

REINTERPRETATION OF THE NORTHWEST PORTION OF THE TEKONSHA MORAINE, SOUTH-CENTRAL MICHIGAN

The northwest portion of the Tekonsha Moraine has traditionally been considered to be a product of, and composed of sediments from, the Lake Michigan Lobe. A recent study has also correlated till from the moraine with an exposure (Ganges till) along the shore of Lake Michigan. The results of this paper contradict those interpretations. Multiple lines of evidence—till fabrics, 7-A/10-A peak height ratios of clay minerals, indicator stones, morphology, and elevation relationships—indicate a Saginaw Lobe origin for the till and glaciofluvial sediments. Moreover, aerial photographs reveal the presence of ice-stagnation topography. The moraine is herein reinterpreted as a hummocky and pitted (collapsed) outwash plain/kame moraine. Given that this feature has long been considered a moraine, similar landforms in Michigan and elsewhere may be more common than heretofore recognized. [Key words: glacial geomorphology, Michigan, Quaternary, Tekonsha Moraine.]

Glacial-Lake Outburst Erosion of the Grand Valley, Michigan, and Impacts on Glacial Lakes in the Lake Michigan Basin

Geomorphic and sedimentologic evidence in the Grand Valley, which drained the retreating Saginaw Lobe of the Laurentide Ice Sheet and later acted as a spillway between lakes in the Huron and Erie basins and in the Michigan basin, suggests that at least one drainage event from glacial Lake Saginaw to glacial Lake Chicago was a catastrophic outburst that deeply incised the valley. Analysis of shoreline and outlet geomorphology at the Chicago outlet supports J H Bretz's hypothesis of episodic incision and lake-level change. Shoreline features of each lake level converge to separate outlet sills that decrease in elevation from the oldest to youngest lake phases. This evidence, coupled with the presence of boulder lags and other features consistent with outburst origin, suggests that the outlets were deepened by catastrophic outbursts at least twice. The first incision event is correlated with a linked series of floods that progressed from Huron and Erie basin lakes to glacial Lake Saginaw to glacial Lake Chicago and then to the Mississippi. The second downcutting event occurred after the Two Rivers Advance of the Lake Michigan Lobe. Outbursts from the eastern outlets of glacial Lake Agassiz to glacial Lake Algonquin are a possible cause for this period of downcutting at the Chicago outlets.

Genesis of diamicton in the Oak Creek Formation of south‐east Wisconsin, USA

ABSTRACTThe clayey diamicton of the Oak Creek Formation was deposited by the Lake Michigan Lobe of the Laurentide Ice Sheet during the late Wisconsin deglaciation. Rapid changes in the position of the ice margin may have been facilitated by a subglacial deforming bed. However, although it is difficult to find proof either for or against a subglacial deforming bed, most observations from this investigation are more easily explained by transportation within and deposition from the ice itself. Good exposures of the diamicton units, especially of the lower contacts, along the shore of Lake Michigan, are the basis for the interpretation of basal till genesis. Basal deposition occurred mainly by stagnation and stacking block by block followed by melting out, as opposed to lodgement grain by grain. This interpretation of basal deposition is applicable in other areas and in most topographic environments.

Surface form of the southern Laurentide Ice Sheet and its implications to ice-sheet dynamics

Reconstructions of the ice-surface morphology of several lobes of the southern Laurentide Ice Sheet reinforce previous arguments that this sector of the ice sheet was thin and low sloping. Driving stresses, estimated from the geometry of the reconstructed ice surfaces, are 0.7-4.3 kPa for the 14 ka Des Moines Lobe, 0.9-1.2 kPa for the 14 ka James Lobe, 0.9-1.7 kPa for the 18-20 ka Lake Michigan Lobe, 1.8-2.9 kPa for the 15-18 ka Chippewa Sublobe, and 17-22 kPa for the 15-18 ka Green Bay Lobe. Previous estimates of rates of ice-margin advance (450-2,000 m/yr) indicate moderate-to-fast ice velocities for the ice lobes. Reconstructed driving stresses and velocity estimates of the Des Moines, James, and Lake Michigan Lobes are analogous to the distal ends (ice Plains) of low-sloping (0.4 X 10 -3 ) but fast moving (500 m/yr) West Antarctic ice streams, whose dynamics have been attributed to sliding and/or subglacial sediment deformation by pervasive shear. These reconstructions support recent models of the Laurentide Ice Sheet which include movement by sliding or by subglacial sediment deformation along its southern, western, and northwestern sectors; evidence for either mechanism should be represented in the sedimentologic and geomorphic records. Thin ice in these regions indicates that the Laurentide Ice Sheet contained less ice volume and represented less of an orographic obstacle to atmospheric circulation than has been considered in models of the ice sheet on a rigid bed with steep profiles.

Subglacial floods and the origin of low-relief ice-sheet lobes

AbstractThe likely genesis of low-relief ice-sheet lobes is episodic subglacial floods which occur in the form of water sheets hundreds of kilometers wide. During the few weeks prior to termination, when the flood discharge is large, the average thickness of a water sheet beneath the Lake Michigan lobe likely exceeded 1 m and reached thicknesses in excess of 6 m. The release of a part of the basal shear stress during this period produced an elongation of the lobe of tens of kilometers.

Subglacial floods and the origin of low-relief ice-sheet lobes

AbstractThe likely genesis of low-relief ice-sheet lobes is episodic subglacial floods which occur in the form of water sheets hundreds of kilometers wide. During the few weeks prior to termination, when the flood discharge is large, the average thickness of a water sheet beneath the Lake Michigan lobe likely exceeded 1 m and reached thicknesses in excess of 6 m. The release of a part of the basal shear stress during this period produced an elongation of the lobe of tens of kilometers.

Evidence for the intra-Glenwood (Mackinaw) low-water phase of glacial Lake Chicago

A 14C age estimate of 13 470 ± 130 BP (ISGS-1378) from organic material at the base of transgressive lake deposits exposed in a southern Lake Michigan shore bluff near Riverside, Michigan, confirms that an intra-Glenwood low-water phase occurred in the Lake Michigan basin during the Mackinaw (Cary – Port Huron) Interstade. The altitude of organic material at Riverside suggests that the water plane was at or below modern lake level during the intra-Glenwood low-water phase. This observation indicates that drainage from the Lake Michigan basin was eastward, probably through the Straits of Mackinac into the Lake Huron basin. Such a drainage pattern implies that the correlative lake occupying the Lake Huron basin (Arkona low-water phase) must have had a level either congruent with or below that of the Lake Michigan basin. This lake system ultimately drained from the Lake Huron basin through a low eastern outlet (probably the Trent River lowland). The existence of low-level lakes at this time also indicates that the ice margin of the Lake Michigan lobe must have retreated at least as far north as the Straits of Mackinac region during the Mackinaw Interstade.

Age, Stratigraphic Position, and Significance of the Lemont Drift, Northeastern Illinois

The Lemont drift was recognized early as a distinctive glacigenic unit in northeastern Illinois. Leached, clay-enriched zones in the upper part and below the calcareous Wadsworth Till are not remnants of a paleosol; they result from modern weathering processes extending into the subsurface below the solum of the modern soil. The Lemont is late Wisconsinan (Woodfordian), not Illinoian or early Wisconsinan (Altonian) as had been suggested. It correlates in part with the Haeger Till Member in Illinois and other units in Wisconsin and Michigan and is the result of a significant fluctuation of the Lake Michigan Lobe ice margin during Woodfordian deglaciation. Diamicton in the lower part of the unit is the result of earlier (pre-Haeger) glacial sedimentation; lacustrine sediment between tills within the Lemont drift records the first phase of ancestral Lake Michigan during Woodfordian deglaciation. The latter event occurred during ice margin retreat following deposition of Yorkville Till, and the interval is na...

Modeling the influence of till theology on the flow and profile of the Lake Michigan lobe, southern Laurentide ice sheet, U.S.A.: discussion

Modeling the influence of till theology on the flow and profile of the Lake Michigan lobe, southern Laurentide ice sheet, U.S.A.: discussion

Reply to: “Modeling the influence of till rheology on the flow and profile of the Lake Michigan lobe, southern Laurentide ice sheet. U.S.A.: discussion”

Reply to: “Modeling the influence of till rheology on the flow and profile of the Lake Michigan lobe, southern Laurentide ice sheet. U.S.A.: discussion”

Quaternary Glaciations in the Northern Hemisphere. Report of the International Geological Correlation Programme: Project 24 (International Union of Geological Sciences and UNESCO)

(partial) Introduction, V Sibrava. Quaternary glaciations in the United States of America: Introduction, G M Richmond & D S Fulle ton. Statigraphy and correlation of the glacial deposits of the Lake Michigan Lobe prior to 14 ka BP, W H Johnson. Correlation of Quaternary glacial deposits in Alaska, T D Hamilton. Summation of Quaternary glaciations in the United States of America, G M Richmond & D S Fulle ton. Quaternary glaciations in Mexico: Quaternary glacial statigraphy and chronology of Mexico, S E White. Quaternary glaciations in Canada: Quaternary statigraphy of Canada, R J Fulton. Quaternary correlations in Arctic Canada, J T Andrews et al. Quaternary glaciations in Europe: Statigraphy of the central area of the Scandinavian glaciation, J Lundqvist. Glaciations in the United Kingdom, F W Shotton. Correlation of Quaternary glaciations in England, Ireland, Scotland and Wales, D Q Bowen et al. Quaternary glaciations in the French Massif Central, Y Veyret. Pleistocene glaciations in Austria, H Kohl. Quaternary glaciations in Eurasia: Methodological approaches in the investigation of the development of glaciation in Northern Eurasia, A A Velichko. Ice-sheet reconstructions, S A Arkhipov et al. Glacial stratigraphy in Japan, S Horie. Quaternary glaciation in China, Yafeng Shi et al. Correlation of Quaternary glaciation in the Northern hemisphere, D Q Bowen et al.

Acanthocephala from Lake Fishes in Wisconsin: Host and Seasonal Distribution of Species of the Genus Neoechinorhynchus Hamann, 1892

r: Five species of Neoechinorhynchus are reported, not including Neoechinorhynchus robertbaueri Amin, 1985, which was described elsewhere. Of 14 species of fish infected with Neoechinorhynchus cylindratus (Van Cleave, 1913) Van Cleave, 1919, from Silver and Tichigan Lakes, Micropterus salmoides, Micropterus dolomieui and Esox lucius were the principal definitive hosts. Lepomis macrochirus may be a second intermediate host in its life cycle. Most recruitment occurred in late summer and in autumn, sexual development and breeding by spring, and growth and elimination of ripe eggs by summer. A partial overlap of generations was observed in the autumn. Neoechinorhynchus prolixoides mostly infected Erimyzon sucetta but was also found in M. salmoides, M. dolomieui, Pomoxis nigromaculatus, Stizostedion vitreum, Pimephalus promelas (all new host records) and Moxostoma erythrurum; new state record for Wisconsin. Measurements of Neoechinorhynchus prolixoides were provided and comparison is made with the original description. Extension of its binucleate lemniscus relative to anterior testis margin is not a good diagnostic character. The seasonal periodicity of N. prolixoides bore basic similarities to that of N. cylindratus but involved more juveniles, particularly in the spring, and its simultaneous growth and maturation produced the highest adult intensity index during the summer. The smaller land-locked Silver Lake was considerably more heavily populated by both species than the larger riverconnected Tichigan Lake. Both species showed significant but limited posterior migration in the intestine of their major hosts by the summer. Their intestinal distribution was compared with sites occupied by other helminth species in concurrent infections. Host sex did not affect worm burden, but size did. Neoechinorhynchus rutili (Muller, 1780) Hamann, 1892 and Neoechinorhynchus strigosus Van Cleave, 1949 were also recovered from Culaea inconstans and Catostomus commersoni, respectively. Neoechinorhynchus salmonis Ching, 1984 is reported for the first time in the U.S., from Lepomis cyanellus (new host species and family records) that was recovered earlier from the Pike River, which drains into Lake Michigan. The descriptions of Fessisentis tichiganensis from Tichigan Lake (Racine County) and of Neoechinorhynchus robertbaueri from Silver Lake (Kenosha County) by Amin (1980 and 1985, respectively) constitute the first 2 of this series of reports on acanthocephalans from lake fishes in southeastern Wisconsin. Four other species of Neoechinorhynchus are reported herein from the above 2 lakes with emphasis on the host and seasonal distribution of the more common 2, Neoechinorhynchus cylindratus (Van Cleave, 1913) Van Cleave, 1919 and Neoechinorhynchus prolixoides Bullock, 1963. The Neoechinorhynchus sp. reported by Amin (1975) from Lepomis cyanellus Raf. in the Pike River, which drains into Lake Michigan, is herein assigned to Neoechinorhynchus salmonis Ching, 1984 and discussed. A number of new host and state records are noted. Received 29 April 1985; revised 9 August 1985; accepted 21 August 1985. MATERIALS AND METHODS Seasonal parasitological collections were made from Silver Lake (Kenosha County) and Tichigan Lake and canal (Racine County) during the spring (April, early May), summer (June, July, early August) and autumn (late October, November) between 1976 and 1984. The 188-ha Silver Lake has a maximum depth of 13.4 m. It is an eutrophic land-locked lake of glacial origin lying within the lateral moraine of the Lake Michigan lobe of the Wisconsin glacier. A small outlet historically permitted the intermittent discharge of overflow waters (until dammed in 1932) into the Fox River, a tributary of the Mississippi River drainage system. Silver Lake was probably originally populated by fish and parasite faunas of Mississippi River origin. The size of the eutrophic Tichigan Lake, originally a natural lake on the above Fox River, increased to 458 ha as a result of an impoundment of the Fox River in 1830; it has a maximum depth of 19.2 m. Fishes were routinely collected and specimens processed and measured as described by Amin (1985). All measurements are in micrometers, with averages in parentheses, unless otherwise noted. Mean per host is the number of parasites recovered/number of fishes examined. The site of infection was determined in host alimentary regions as footnoted in Tables V and VIII. Juveniles are recently recruited young adults with in-

Modeling the Influence of Till Rheology on the Flow and Profile of the Lake Michigan Lobe, Southern Laurentide Ice Sheet, U.S.A.

AbstractThe late Wisconsin Shelbyville till was deposited in southern Illinoisc. 20 000–21 000 year B.P. and records the maximum southern advance of the Lake Michigan lobe of the Laurentide ice sheet. The yield strength calculated for a representative till debris flow found just south of the ice margin is 8 kPa (0.08 bar), and probably approximates yield strength of basal Shelbyville till. An ice-profile model assuming plastic behavior in basal till suggests the southern Lake Michigan lobe may have been unusually thin. Reconstructed Laurentide glacier profiles from the south-west and western Great Plains (South Dakota, Alberta, Minnesota, and Montana), and the MacKenzie Delta, N.W.T., are similar to those inferred for the southern Great Lakes area, and much thinner than those of most modern ice sheets. The Pleistocene Laurentide ice sheet may have been asymmetric: thicker in the east than in the west. Glaciers resting on weak sediments can move both by subglacial sediment deformation (creep) and sliding at the sediment–ice interface. Till rheology is complex; shearing of till by over-riding glaciers would increase porosity and further reduce yield strength.