NIOBRARA LIMESTONE

geologic division of rocks in the United States during the Late Cretaceous Period (97.5 to 66.4 million years ago). Named for exposures studied along the Missouri River near the mouth of the Niobrara River, Knox county, Neb., the Niobrara occurs over a wide area including Nebraska, Kansas, North and South Dakota, Minnesota, Montana, Wyoming, Colorado, and New Mexico. The Niobrara Limestone varies in thickness from about 60 m (200 feet) to more than 270 m and consists of chalks, shales, limestones, and many thin layers of bentonite (altered volcanic-ash deposits that appear like soapy clays). The Niobrara marks the withdrawal of the Cretaceous seas from the region of the Rocky Mountain geosyncline. Aquatic reptiles such as the mosasaur Clidastes, which was about 4.5 m long, and flying reptiles such as Pteranodon , possessing a 7.5-metre wingspread, have been found in the Niobrara.