Big Clifty Sandstone

The Big Clifty Sandstone is a geologic formation in Kentucky. It is sandstone that is Pennsylvanian in age. At Mammoth Cave National Park it overlies the Girkin Formation, the Ste. Genevieve Limestone, and the St. Louis Limestone, providing a cap for the entire cave system. The existence of Big Clifty and similar sandstones at Mammoth Cave is the cause for the sheer size of the cave passages below it due to water flow through them and subsequent dissolution. Once this cap of the Big Clifty dissolves, the cave will begin to shrink in size.

The Big Clifty Sandstone also appears as sandstone knobs throughout south-central Kentucky, including the 200-foot tall Pilot Rock on the border of Todd County.