A jungle grew here. Before that, a shallow sea covered the land. Both are gone now, but both left evidence of their passing.
The sea's signature is ammonites, baculites, and clams, pearly fossils entombed in a fossil mud called the Pierre Shale. This shale is exposed in the gully below you.
A jungle sprang up after the sea drained away about 65 million years ago. For a long time tree roots broke up the shale, and chemicals from decaying plants produced a yellow soil. About 37 million years ago sediment from the west washed over the jungle.
The jungle rebounded, converting the new sediment into a red soil. Buried by later sediments, both yellow and red soils were fossilized. We call them the Yellow Mounds Paleosol and the Interior Paleosol.