I understand that the Great Lakes were formed by the glacial terminus of one of the Ice Ages. What I don’t understand is the extreme depth of Superior, Huron and Michigan. These lakes are over 500 ft deep, even over 1000 ft deep, which seems fantastical based on their other dimensions.
In comparison, Long Island and the Sound were also formed by a glacial terminus, and the Sound is only about 230 ft deep.
I’m having trouble visualizing how glaciers slowly spreading down from the north would leave a deep scar at the terminus. I would expect a large pile of agglomerate (like Long Island), not a deep scar.
What gives?
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There are moraines around the Great Lakes. When the glaciers retreated, they did so unevenly, forming several lobes. Sediment was deposited at the terminus, but deposition was temporarily prevented beneath the lobes. One place where this is well-illustrated is Lake Erie. A lobe of the glacier lingered longer in the eastern half of the basin, leaving that part of the lake deeper. The boundary between the eastern and central parts of the lake, the edge of that lobe, is marked by a submerged ridge.
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