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The Great Lakes ended up having very high levels, and then a moraine dam somewhere would break and that'd drain the lake down several dozen feet, and then that would happen again, and again.

Each time the lake dropped you got a new beach, and additional indentations in the very large terminal moraines at the margins.

BTW, Lake Erie's last big drop was only 4,000 years ago. Virtually NO American Indian settlements were built in the muck area left behind that extended from Ohio through North Central Indiana. That area is still drying out!

Most of the big changes we can see today are a consequence of the movement of water, not ice. The Ice was mostly gone 14,000 years ago but came back in the Younger Dryas ~ which is a totally separate topic ~ but that happened AFTER humans had started moving to America, and may well have wiped almost all of them out ~

Have a look at THIS to see if we're discussing the same thing. Off the southern end of Lake Michigan are what appear to be and I've seen described as terminal moraines; lots of them. We would need a topographic map to check elevations that would help determine if these are beaches or moraines. Also, next door in Iowa there appears to be two or three terminal moraines. At least they have the classic appearance of moraines. Similar situation to the SSW of Lake Erie.

BTW, I've got a huge version of this map on my computer for the whole of North America; all 54Meg of it that I can't locate on the web any more...