Here is a question via Highly Allocthonous, via somebody else. I'm elaborating on my answer given in the comments.
The question is, why do the Great Lakes form this apparent line, which may be a great circle, or a small circle. We Canadian Geologists would hrmmph at such a question, but then wouldn't be able to answer it in a simple manner.
The answer is that it is a morph (combination) of the gross geology, and glacial processes. Here is the isostatic rebound map. Isostatic rebound is a measure of how much the earth is still popping up since the last glaciation. It forms a good average of the maximum ice thickness, since the thicker the ice, the more the continent was pushed down.
This is pretty close! It forms the same pattern as if you pushed your thumb on a beach ball, which is a global small circle. Now we throw in the simplified geology.
The yellow represents sedimentary rock, and the blotchy stuff is 'granitic'. Again, it correlates with the Great Lakes Line.
So, north of the line, we have very hard rock, which would not like to have a Great Lake carved out of it, and below the line, we have mounds of dirt pushed by the last glaciation, which has buried all potential great lakes. In the middle we have the ideal zone for Great Lakes.
Now, I caution you not to look at any given Great Lake, and say it simply got there by being scooped out by glaciation. If that were true, we'd have just one Super Giant Great Lake along that line. Each lake has it's own unique geology story, and over the years, I've tried to point out a few.