A shallow sea once covered this land, leaving ammonites, baculites, and clams entombed in the fossil mud called the Pierre Shale, which is exposed in the gully below. After the sea drained away about 65 million years ago, a jungle grew here, and over a long time tree roots broke up the shale while chemicals from decaying plants produced a yellow soil. About 37 million years ago, sediment from the west washed over the jungle, which then rebounded and converted the new sediment into a red soil. Later sediments buried and fossilized both the yellow and red soils, now called the Yellow Mounds Paleosol and the Interior Paleosol.