The Ashfall Fossil Beds of Antelope County in northeastern Nebraska are among the rare preservation sites called lagerstätten, which preserve ecological "snapshots" from a moment in time, due to extraordinary local conditions that have preserved a range of fossilized organisms undisturbed. The Ashfall Fossil Beds are especially famous for fossils of mammals from the middle Miocene geologic epoch. The Ashfall Fossil Beds are stratigraphically part of the Serravallian-age Ogallala Group.