Some 3,000 acres of grassy plains surround a striking visitor center nestled beside the Niobrara River. Preserved are the 19-million-year-old fossil remains of rhinoceroses and other animals that roamed these plains more than 40 million years after dinosaurs disappeared. A life-size diorama describes an ancient waterhole, the remains of which are accessible via the Fossil Hills Trail (2mi), just outside. Another highlight is the James H. Cook Collection, an impressive display of over 100 Oglala Sioux artifacts. The pieces include a whetstone that once belonged to Crazy Horse. The Daemonelix Trail (1mi), off Route 29 near the west entrance to the monument, features preserved corkscrew-like burrows of a palaeocastor or prehistoric beaver.