Ash Hollow is a focal point for understanding the geologic history of the Central Great Plains before the Great Ice Age and 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, and much of that ancient valley fill is exposed in cross section in the cliff faces along 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, indicating a semi-arid climate and plants somewhat like those of today. The animals found in these rocks were very different, including camels, rhinoceroses, and long-jawed mastodonts, most of which became extinct before the Ice Age, and the Ice Age animals were also mostly distinct from those now living here. The earliest fossil collections near here were made for F.D. Cope in 1879, and explorers who made geological contributions include John C. Fremont in 1843 and G.K. Warren in 1855.