Ash Hollow is a focal point for understanding the geologic history of the Central Great Plains prior to the onset of the Great Ice Age. It is the type locality of the Ash Hollow Formation, named by Henry Engelmann after a visit in 1858-1859. These sediments were deposited in ancient valley-systems that drained east from the Rocky Mountains.
Much of the ancient valley-fill is exposed in cross section in the cliff faces along Ash Hollow. The basal pebble-gravel forms the roof of Ash Hollow Cave in the exhibit area. Some of the overlying hard ledges or mortar-beds probably represent hard-pan or caliche soils, and others contain fossil grass seeds and root-casts of yucca, all indicative of a semi-arid climate and plants somewhat like today. But the animals found in these rocks were very different, including camels, rhinoceroses, and long-jawed mastodons, most of which became extinct before the Ice Age. The Ice Age animals were likewise mostly distinct from those now living here.
The earliest collections of fossils were made for E. D. Cope near here in 1879. Explorers who made geological contributions include John C. Fremont in 1843 and G. K. Warren in 1855.
Nebraska Geological Society
Nebraska State Historical Society
Ash Hollow State Park, Hwy. 26 southeast of Lewellen