Comment by ternaryoperator

7 hours ago

Is a 50-ton boulder rare or super-rare?

I thought the same thing. My back yard is full of huge rocks. Nothing quite that large, maybe 1/3 of that size. A few doors down there is a boulder twice as big as the O'Hare one.

I'm speaking in volume. I'm not sure of the mass of these boulders.

In an area that was covered by a glacier in the last 20k years, yes.

Chicago was covered by ice during the most recent glaciation, which means that boulder is probably a glacial erratic covered by till leftover from the movement of the glacier.