Pierre Shale outcrop along Rooney Road

This mile-thick, even, and well-bedded shale bed was deposited below wave disturbance on the floor of the Interior Seaway during the Cretaceous. Today Pierre Shale exists as eroded valley floors, dark with organic carbon. It is often the source rock for oil and gas deposits in the Denver Basin.

These shale beds may contain volcanic ash layers altered to clay. When wet, these clays expand and take on a soapy texture—bad news if you are trying to drive a car across them in a rainstorm.

Pierre Shale contains a wealth of fossils, particularly ammonites of the Interior Seaway. Researchers at the U.S. Geological Survey have put together ammonites’ evolutionary history by assembling a succession of species and calibrating the age of the beds using radiometric techniques. The results reveal one of the best-constrained evolutionary successions of marine invertebrates from anywhere in the world.