The maximum extent of the Laurentide Ice Sheet, or Last Glacial Maximum (LGM), occurred approximately 18,000 years ago (Figure 2). During peak glacial conditions, the ice sheet was about 2 km thick over Eastern Ontario and as much as 3 km thick to the north over what is now Hudson Bay. At this time, in the east, ice extended almost as far south as present-day New York City, and throughout northern Pennsylvania (Gilbert 1994). The eastern part of the ice margin was constrained by the Appalachian plateau (extending across several of the present-day eastern States), whereas its southward extent in the west was greater: the ice sheet had by about 20 ka BP reached south-central Illinois (Hansel and Johnson 1992).
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