Originally the Agate Springs Ranch, a working cattle ranch, was owned by Capt. James Cook.[citation needed] The monument's museum collection also contains more than 500 artifacts from the Cook Collection ofPlains Indians artifacts.
The national monument was authorized on June 5, 1965, but was not established until June 14, 1997. The Harold J. Cook Homestead (Bone Cabin Complex) was listed on theNational Register of Historic Places in 1977.[4] Agate Fossil Beds is maintained by theNational Park Service.[5]
The site is best known for a large number of well-preservedMiocenefossils, many of which were found at dig sites on Carnegie and University Hills. Fossils from theHarrison Formation and Anderson Ranch Formation, which date to theArikareean in theNorth American land mammal classification, about 20 to 16.3 million years ago, are among some of the best specimens of Miocenemammals.[6]