I'm still working on my great lakes knol, although it is not a happy journey.
The Great Lakes look like ice lobes, and so they have always been explained as ice lobes.
The ice advances are a unique phenomenon, in that they totally erase all that has gone before. So, one can say that the Great Lakes were born only after the retreat of the last glaciation, simply because that's all we have!
But to apply a bit more intelligence to the problem, we have to conduct an Einsteinian thought experiment. What if the Great Lakes didn't give a hoot about glaciation? What if temperate glaciers can only do a 'wash and wax' to the land, and are not the massive steel bulldozers they are imagined to be?
Before glaciation, we had a hundred million years of everyday erosion, after the last great split of the continents. We can imagine what the land looked like before the first ice cube, and for that we take the geological map.
I'm simply going to take a huge bite out of every soft, erodible rock there is. Oh look! I get the Great Lakes!
Now, I find a bedrock topography map.
Oh look, the soft crap has been worn away, by 100 million years of erosion, and not the stupid ice sheets!
Finally, if the Great Lakes were scooped out by giant ice bulldozers, then we would expect a great hunk of dirt specifically at the end of each lake, but that doesn't happen, the dirt is evenly spread, and it really isn't that much. Something like an inch or two polished off the top of the whole glaciated area.
Oh, and I found this picture doing this.
I love it! This is the stuff they are going to use for our nuclear waste up a Bruce!