Both of those maps show a small glacier free island west of Chicago.
What’s with that?
I never noticed that before. That is curious, huh? Maybe a huge magma chamber bubbled to the surface? JK
I’ve always been more curious about the thickness above our old house in Ithaca, NY. We had glacial evidence everywhere — striations, moraines, Finger Lakes, erratics. I didn’t realize it at the time, but studying geology in Jr High was pretty cool.