Exposed on these slopes are two Tertiary age sedimentary rock formations, the Brule and the Ogallala. The Brule, the lower and older, is a brown siltstone containing volcanic ash blown into Nebraska from the western United States during a period of volcanism 28 to 32 million years ago. The Ogallala, the upper and younger, is a 6- to 12-million-year-old sequence of limy ledges and silty sandstone layers, some containing fossil grass roots, capped by a ledge of cherty limestone. Many of these ledges, in particular the cherty limestone, represent ancient soils that formed in a semiarid climate. Rocks representing about 16 million years of geologic history are missing at this site between the Brule and the Ogallala but are present elsewhere in Nebraska. Water for this rest area comes from fracture zones in the Brule. Without such fractures, the Brule would yield very little water. Oil and gas, discovered in this area in 1949, is produced from Cretaceous age sandstone around 90 million years old, at depths of about 5,000 feet below the land surface.