AGATE FOSSIL BEDS NATIONAL MONUMENT

natural depository of an extinct animal community, on the Niobrara River, Sioux County, northwestern Nebraska, U.S., 40 mi (64 km) north of Scottsbluff. The beds were laid down as sedimentary deposits about 20,000,000 years ago (Miocene Epoch) and bear the remains of prehistoric mammals including the Diceratherium (two-horned rhinoceros), the Moropus (7 ft at the shoulders with a horselike head), and the Dinohyus (a large piglike beast). The site, named because of its proximity to rock formations containing agates, lies in the Carnegie and University hills. Established as a national monument in 1965, it covers an area of 2,269 ac (918 ha). Capt. James W. Cook was the first white man to discover the fossil bones c. 1878.