The Pierre Shale is a Late Cretaceous geologic formation lying east of the Rocky Mountains across the Great Plains, extending from the Pembina Valley in Canada to New Mexico, and dating to the Campanian and Maastrichtian stages. It consists mainly of dark gray, fossiliferous shale with veins and seams of gypsum and concretions of iron oxide, reaching about 3,138 feet thick at its type locality, and was named for an exposure near Fort Pierre on the Missouri River in South Dakota, formally described by Meek and Hayden in 1862. It overlies the Niobrara formation and underlies the Fox Hills beds, formed from marine sediment deposited in the Western Interior Seaway, and correlates with other marine shales including the Bearpaw, Mancos and Lewis Shales and Alberta's Lea Park Formation; it hosts commercial petroleum in Colorado and has yielded abundant ammonites, marine reptiles and other invertebrate fossils documenting the ancient seaway.
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