I have always thought Lake Michigan/Huron look mighty round if you follow from the north end down through Green Bay and the Fox river to the west, and cut into Canada along the Niagara Escarpment [1] on the East. There seem to be signs of a round structure all the way down in Ohio. But while this does form a roundish structure, the history is quite different than an impact:

[1] https://en.wikipedia.org/wiki/Niagara_Escarpment

Still I wonder, what if this other stuff happened in this shape because of a giant impact crater billions of years ago before all that?