Sioux County is located in the state’s northwest corner. This county’s 2,067 square miles run from the South Dakota border all the way to Scotts Bluff County. Harrison, population 291, is the county’s largest community and its county seat.
The northern half of the county lies within the Oglala National Grassland. About 20 miles south of Harrison, the Agate Fossil Beds National Monument preserves a bone bed where Miocene Epoch mammal fossils have been discovered.