If you could see the underlying bedrock map of the Earth, there would be circular blotches that are the remnants of buried astroblemes – the scars caused by meteor or asteroid hits that puckered the surrounding bedrock.
Looking at the bedrock map of Michigan, it is obviously a perfectly circular bullseye roughly 400 miles across.

Whatever caused the original circular scar, it has been modified by the ancient glaciers that scooped out depressions around the northern three sides. The reason the southern side was not scooped out also, was the glaciers stopped right around there.

Could it be tied to the Siberian Traps? And the Permian extinction?