Niobrara Formation

The Niobrara Formation, also called the Niobrara Chalk, is a geologic formation across the Great Plains of the United States and Canada, deposited between about 87 and 82 million years ago during the Coniacian, Santonian and Campanian stages of the Late Cretaceous. It consists of two members, the Smoky Hill Chalk overlying the Fort Hays Limestone, composed mainly of chalk with some shale, formed from the accumulated coccoliths of microorganisms that lived in the Western Interior Seaway, the shallow inland sea that divided North America for much of the Cretaceous. The formation's type locality is the Niobrara River in Knox County, in northeastern Nebraska.