Crushed limestone trail. Doable by road bike so if that's what you have don't let it stop you (or, at least, I've done many many miles on the Katy on mine, same surface) but watch for the occasional eroded rough spot. Use dry lube...white gravel dust abounds. The west central IL trails I have done are well kept. I'm too far away from this one to play.

Map and more info...
http://www.tunnelhilltrail.org/index.html