Fossilized oyster shells, coral, fish bones, shark teeth, and other marine organisms can be found in the ‘Chalk Pyramids’ or Monument Rocks that rise up from the flat plains of western Kansas. These ancient chalky spires and buttes, 70 feet (21.34 meters) tall, are the remains of a massive, warm and tropical, inland sea called the Western Interior Seaway. From Wikipedia:
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… 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.