Over time, Cedar Falls cut through vast layers of sandstone to create Cedar Creek Canyon, which reaches a depth of 400 feet. The rock layer capping Petit Jean Mountain is sandstone, an ancient sedimentary formation deposited as sand in a river system on a deltaic plain. The rock formation particular to Petit Jean State Park is called Hartshorne Sandstone. When the Hartshorne Sandstone formed, the Ozarks and most of the southern portion of the River Valley were exposed and not covered by sea water. This layer was deposited 315-307 million years ago during geological Middle Pennsylvanian time, also known as the Desmoinesian Stage.