The Great Lakes were covered by ice that was 1-2 miles thick during the Ice Age. I believe it was the movement of these glaciers that 'carved-out' the Great Lakes.
Yeah, I know that. Growing up in Chicago that point was drilled into our little heads almost daily that our surrounding Geography was carved out by Glaciers (Hills, Moraine Valley, Boulders in Chicago that 'shouldn't be there', etc).
But I was referring specially to the map that shows the Great Lakes disappearing if the sea level dropped 330 ft. That doesn't make sense.