Sand for glass and cement has to come from somewhere, and in flat landlocked states without beaches that means digging it out of sandstone deposits. Since sandstone is easy to carve but remains stable, it makes for large and extensive caverns in areas that sand mining has occurred. Many underground and open pit sandstone mines exist throughout the midwest, often marked inconsistently on topo maps as "sandpits" quarries, and mines.

  
A small spiral-shaped mine filled with hibernating bats. We left early to avoid disturbing the bats too much, as they can die from too much disrupted hibernation.

     
Some freaking huge mine entrances that didn't lead very far into the cliff. Make sure to open them full size to see the scale!

        
A really cool active sandstone mine next door to the huge short ones, with a tunnel leading all the way through the hill and out into an open pit mine in the next river valley. This mine had a lot of twisty confusing large passages, but only a few were lit and cleaned up for use. The abandoned sections may have been more extensive, but we didn't have time to explore and map them fully. We were a bit worried that the processing plant outside (first photo) would have security, but we never saw anyone.

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