They resembled modern sheep or pigs and ranged in height from less than a foot to more than 3 feet. Water cascades over resistant beds of sandstone and siltstone of Tertiary age in a number of tributary valleys of the Niobrara River valley. Large logs weighing several tons each have been found. Various kinds of petrified wood are found throughout Nebraska. They described and named many fossils and rock units, including the Cretaceous-age Dakota Group for Dakota County, Nebraska.
Some burrows contain the fossil skeleton of the beaver.
The largest known specimen was collected in Sioux County, Nebraka. It is an erosional feature composed of Oligocene rocks of the Gering Formation overlying the Brule Formation. Chalk was once mined here from rocks of the Miocene-age Ogallala Group. The presence of many subtropical species indicates that the climate of ancient Nebraska was very different from the climate of our time. Drilled in 1979, it is 9,971 feet deep.