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?
In: Other
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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