Geology

Both rock names on this slope come from Lakota bands. The geologist N. H. Darton named the Brule in 1898 for the Brulé, the Sicangu Lakota of the Pine Ridge country, and the Ogallala takes its name from the Nebraska town, itself named for the Oglala. The Brule holds early rhinoceroses, oreodonts and the small camel Poebrotherium. Current dating puts it at about 34 to 30 million years old, somewhat older than the plaque's volcanism of 28 to 32 million years ago. The Ogallala above it is the rock that holds the Ogallala Aquifer.

What the plaque says

Geology 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. Department of Roads Conservation and Survey Division Nebraska State Historical Society

Where it stands

Worth the stop nearby

• Fort Sidney — 6.2 mi
  Three surviving buildings of the post that guarded the Union Pacific at Sidney from 1867 to 1894, where a young army surgeon named Walter Reed found a ward full of ugly cases.

More markers nearby

• The Golden Link — steps away
• Sidney-Black Hills Trail — 4.7 mi
• Fort Sidney — 6.4 mi
• Nebraska's Earliest Documented Burial — 6.6 mi