This is an outcrop at Franklin Creek State Park in Lee County, IL composed of the New Richmond Sandstone overlain by the Shakopee Dolomite which are early Ordovician in age. The "cave" is eroded out of the New Richmond Sandstone. This is the oldest exposed bedrock in the state of Illinois.
~70 photos taken with a Sony NEX-5T with kit lens (SEL 16-50mm).