The bedrock cliff exposed here is composed of limestone ("dolostone" is the exact term geologists use for this variety). This rock has been dated to 450 million years old - that's twice as old as the early dinosaurs! Newly forming limestone can only be found today in places like the Florida Keys, the Caribbean Ocean, and Australia's Great Barrier Reef. These facts suggest that 450 million years ago, what we now call Northern Illinois was part of a shallow sea!
Erosion is partly responsible for this exposure, or outcrop, but at the far left end of the cliff you can still see evidence that blocks of the dolostone were cut and removed in a stair-step pattern. Material for the stone house on the Severson Dells Farm, which dates to the 1840's, was quarried here where rock was readily available at the surface.