natural “depository” of an extinct animal community on the Niobrara River in northwestern Nebraska, U.S., 40 miles (64 km) north of Scottsbluff. The beds were laid down as sedimentary deposits about 20 million years ago ( Miocene Epoch) and bear the remains of prehistoric mammals including Menoceras (two-horned rhinoceros), Moropus (7 feet 【2 metres】 at the shoulders with a horselike head), and 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 5 square miles (13 square km). James H. Cook was the first white person to discover the fossil bones, about 1878, and he settled in the region in the 1880s.