Here is the clyffe at Ferne Clyffe, complete with a sandstone overhang and a small rock shelter.
Ferne Clyffe State Park is in a section of far southern Illinois called the Shawnee Hills, one of the only patches of real topography in a state sorely lacking. Like everything in Illinois, its recent geologic history involves glaciers, only these glaciers are a lot older than the glaciers I normally reference when talking about Illinois. The northern Illinois glacial topography formed during the last glacial maximum, which ended about 11,000 years ago. But those glaciers only made it about as far south as the current location of Interstate 80. You have to go back one glaciation further to get to the glaciers that affected southern Illinois. That glacial episode ended about 130,000 years ago. The glacier stopped just north of the Shawnee Hills, but it caused the same kind of traumatic flash flooding here that the later glaciers used to carve little canyons at places like Matthiessen and Starved Rock State Parks far to the north. The Shawnee Hills are full of nifty little sandstone canyons and seasonal waterfalls.