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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
Ash Hollow State Historical Park, Hwy 26
Garden County
Marker 209A


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Last updated 10 June 2004

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