The Ash Hollow Formation is a geologic unit within the Ogallala Group in Nebraska and South Dakota that dates to the Neogene period. Known for preserving fossils, it helps document the region’s prehistoric environments and has yielded remains of mammals, reptiles, birds, amphibians, and plants. Named for Ash Hollow, Nebraska, the formation is visible at Ash Hollow State Historical Park and includes the area of Ashfall Fossil Beds State Historical Park, connecting it to one of the central Great Plains’ best-known fossil localities. It remains important both as a prominent landscape feature and as a source of paleontological evidence.