Ash Hollow Geology

The rock named for this hollow turned out to stretch far beyond it. Geologists now treat the Ash Hollow Formation as the middle unit of the Ogallala Group in Nebraska and South Dakota, laid over the older Valentine and under the younger Kimball, and date it to the late Miocene. Its most famous exposure is some 220 miles to the northeast: Ashfall Fossil Beds State Historical Park in Antelope County lies within the same formation, where more than a hundred specimens of the rhinoceros Teleoceras major have come out of the volcanic ash.

What the plaque says

Ash Hollow Geology 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 over- lying 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 mastodonts, most of which became extinct before the Ice Age. The Ice Age animals were like- wise 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

Where it stands

Worth the stop nearby

• Ash Hollow State Historical Park — 0.5 mi
  The emigrants' spring below Windlass Hill, six miles from the Blue Water Creek camp where Harney's soldiers killed eighty-six Lakota in 1855, and where their belongings came home in 2025.
• Julesburg — 23 mi
  The town that moved four times: a trading post, a stage stop burned in 1865, the wickedest end-of-track town on the Union Pacific, and finally Denver Junction renamed.
• Lake McConaughy and Kingsley Dam — 25 mi
  Nebraska's largest reservoir, 22 miles of North Platte water and white sand beaches behind a 1941 hydraulic-fill dam, built so farms far downstream could water their crops.
• Pilgrim Holiness Church (Baled Hay Church) — 29 mi
  A church raised in 1928 from bales of hand-tied rye straw in treeless Arthur, the oldest known straw-bale church in North America and a survivor of a Sandhills building habit.

More markers nearby

• Ash Hollow — steps away
• Windlass Hill Pioneer Homestead — 2.6 mi
• The Battle of Blue Water — 4.6 mi
• John Hollman Grave — 13 mi