The Great Lakes cont'd
6. After 25,000 yr BPT, the ice margin advanced from both the north and east to cover all of the watershed as far south as the Ohio River and as far west as northern Wisconsin and east-central Minnesota. After 18 ka the ice margin began to retreat, which was interrupted by several major readvances. After 10.0 ka the ice margin retreated northward and by 9.0 ka started to completely withdrew from the watershed for the last time.
7. During the retreat of the ice margin large glacial lakes developed within the basins of the Great Lakes. Their surface elevation and extent varied considerably over time as outlets were either blocked or uncovered by glacier ice.
8. Outlets were also subject to isostatic rebound as well as by channel downcutting, which likewise affected the level of the glacial lakes and the lakes that followed. On several occasions lake levels were affected by catastrophic influx of meltwater from glacial Lake Agassiz which developed outside of the Great Lakes basin
9. With thr complete recession of the Continental Ice Cap and the removal of Ice Dams and changing elevation most dominant in the north, the drainage patterns of the Great Lakes including the St. lawrence River, was established in its present day form.
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