The Niobrara Formation, also called the Niobrara Chalk is a geologic formation in North America that was laid down between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian ages 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, and pterosaurs as well as several primitive aquatic birds. The type localit