Agate Fossil Beds National Monument is on SR 29, 22 miles south of Harrison or 34 miles north of Mitchell in the Niobrara River Valley. Imbedded in the sedimentary rock are fossils formed some 20 million years ago. Named for James Cook's Agate Springs Ranch, this 3,000-acre site also features Cook's collection of 19th- and 20th-century American Indian artifacts. Learn More...