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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If all the ground was of the same density would be evenly eroded. But the ground isn’t. Different areas of the earth have different density to some areas take less effort to be worn away then areas with higher density materials. Not to mention that even before the ice ages earth was never a perfectly smooth ball to begin with so areas always had different depths to start with.
Besides glacial action, there’s the failed Midcontinent Rift. Lake Superior sits inside of the ancient rift valley. The massive structure also extends into the lower peninsula of Michigan.
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.
most of the lakes are formed because they overlay very old geology. they basically correspond to stuff that happened roughly 1 billion years ago down to stuff that happened 150 million years ago.
Lakes Superior, Erie and Ontario are basically old rift valleys. Rift Valleys are formed when the [continental crust is being pulled apart](https://volcanodiscovery.de/uploads/pics/rift.jpg). Lake Superior is the Mid-continent Rift System, and Erie and Ontario are the St Lawrence Rift System.
Michigan and Huron are basically an old subsidence area. Subsidence is when the continental crust sinks. in this case cased caused by [being near an area being pushed up](https://pastglobalchanges.org/sites/default/files/download/docs/magazine/2019-1/images/image001.jpg). this is the Michigan Basin.
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