Note: Noted for the excavation sites of fossils of Tyrannosaurus rex, Triceratops, and other animals in the Hell Creek Formation. Remains included the nearly intact T. rex skeletons named 'Sue' (discovered near the Ziebach-Perkins county line, now in the Field Museum, Chicago) and 'Stan' (a replica is on display at the Kirby Science Discovery Center, Sioux Falls).