Physical Setting and Timing of Glaciation

Ice advanced into the northern USA after 26,000 14C years BP, yet the LGM extent was reached at different times in different places (Mickelson et al., 1983). Lobes in the Great Lakes and New England regions reached their maximum before 21,000 14C years BP. Rapid decay of the ice sheet began after 14,500 14C years BP. Although lobes to the west of the Great Lakes also advanced before 21,000 14C years BP, they reached their maximum extent at about 14,000 14C years BP, out of phase with the rest of the ice-sheet margin (Hallberg and Kemmis, 1986). Readvance of lobes, some perhaps as surges, are recorded all along the southern LIS margin at 13,000 14C years BP, 11,800 14C years BP and 9,800 14C years BP. After about 9,800 14C years BP ice retreated out of the northern USA (Mickelson et al., 1983).

The southern margin of the LIS stretched from Montana to Maine (Fig. 6.1). In the west the margin was highly lobate as it enlarged the bedrock-controlled lowlands of preglacial river valleys (Fig. 6.2). The Des Moines and James lobes advanced to 42°N into central Iowa and eastern South Dakota, respectively, after being split by the Prairie Couteau as ice moved south through the Red River lowland. The Superior and Chippewa lobes advanced southwestward out of the Superior lowland. The Green Bay lobe, Wisconsin Valley and Langlade sublobes were fed by ice advancing over the eastern end of the Superior lowland. The Lake Michigan lobe flowed down the axis of Lake Michigan, and into southern Illinois, to nearly 38°N. The Huron and Saginaw lobes advanced out of the Lake Huron lowland into Michigan and Indiana. In Indiana and

Figure 6.2 Shaded relief image showing the first-order topography of the northern USA. The major physiographic features of the study area discussed in the text are labelled. RRL = Red River Lowland, PC = Prairie Couteau, ML = Minnesota Lowland, MRL = Missouri River Lowland, D = Driftless Region, MSL = Mississippi River Lowland, SH = Superior Highlands, AP = Appalachian Plateau, AM = Adirondack Mountains, SLL = Saint Lawrence Lowland. The white line indicates the maximum extent of the Late Wisconsin Laurentide Ice Sheet. The grey line shows the maximum extent of pre-Wisconsin advances. (Data are from US Geological Survey ETOPO5 database.)

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Figure 6.2 Shaded relief image showing the first-order topography of the northern USA. The major physiographic features of the study area discussed in the text are labelled. RRL = Red River Lowland, PC = Prairie Couteau, ML = Minnesota Lowland, MRL = Missouri River Lowland, D = Driftless Region, MSL = Mississippi River Lowland, SH = Superior Highlands, AP = Appalachian Plateau, AM = Adirondack Mountains, SLL = Saint Lawrence Lowland. The white line indicates the maximum extent of the Late Wisconsin Laurentide Ice Sheet. The grey line shows the maximum extent of pre-Wisconsin advances. (Data are from US Geological Survey ETOPO5 database.)

Ohio, sublobes of the Lake Huron and the Erie lobes advanced to nearly 39°N. The ice margin in Pennsylvania and New York was less lobate and formed a major re-entrant as it encountered the Appalachian Plateau and the Ridge and Valley Province. Small tongues of ice did project down narrow river valleys, but these were smaller than the lowland lobes to the west. Farther to the east, the ice-sheet margin remained much less lobate with the exception of the Hudson River and the Cape Cod lobes. Ice in eastern New England advanced well into the Gulf of Maine and onto the exposed continental shelf.

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