The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from Pembina Valley in Canada to New Mexico.
A broken concretion with fossils inside; Late Cretaceous Pierre Shale near Ekalaka, Montana
Baculites from the Pierre Shale showing sutures and remnant aragonite; western South Dakota, Late Cretaceous.
Image: Hesperornis BW (white background)
Image: Ichthyornis restoration
The Niobrara Formation, also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member overlying the Fort Hays Limestone Member. The chalk formed from the accumulation of coccoliths from microorganisms living in what was once the Western Interior Seaway, an inland sea that divided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains of the US and Canada. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, pterosaurs, and several primitive aquatic birds. The type locality for the Niobrara Chalk is the Niobrara River in Knox County in northeastern Nebraska. The formation gives its name to the Niobrara cycle of the Western Interior Seaway.
The Smoky Hill Chalk badlands are in the Niobrara Formation, in Kansas.
Monument Rocks, Smoky Hill Chalk[clarification needed]
Cremnoceramus deformis is an index fossil of the Fort Hays Limestone Member.
Niobrara Chalk was weathered and opalized in the Valentine phase of the Ogallala Formation.