Agate Fossil Beds: The Geology Behind Nebraska's Miocene Bonebeds

About 20 million years ago, on what is now ranchland in northwest Nebraska, animals died in such numbers that their bones piled up into beds. Those bonebeds — the first great concentration of mammal fossils of their age found in North America — are why Agate Fossil Beds National Monument exists. The agates came later, and gave the place its name.

The monument's diorama: an omnivorous entelodont (Daeodon, once called Dinohyus) over a chalicothere (Moropus). NPS photo

The place

The rocks

The land is part of a raised tableland built by the Arikaree Group, which runs from the Pine Ridge escarpment, a few miles north of Harrison, south to the North Platte River valley. It is fine-grained sandstone and siltstone, laid down by rivers, by wind and by volcanic ash.

Every fossil in the monument comes from the Miocene part of the Arikaree Group — mostly the Harrison Formation and the informal "Anderson Ranch Formation," once called the Upper Harrison beds.

This corner of the High Plains escaped the glaciers of the last Ice Age. Because nothing scoured away the sediments of the past 12,000–15,000 years, the valley's landforms, layers and ancient soils still record how the climate swung since.

The agates

Agates are plentiful in a thin band of ash just above the Miocene bonebeds — enough to name both the nearby town and the monument.

• They are chalcedony, a kind of quartz.
• Colours run from green to amber to light grey, from traces of iron, manganese, chrome or aluminium.
• The most common is moss agate: milky quartz threaded with green mineral filaments that look like moss — or, sometimes, like blue cheese.

Collecting is not allowed — not agates, not fossils, not for a classroom. Every feature of the park is protected by federal law; research collecting needs a permit. Report illegal activity to a ranger or to NPS Investigative Services.

The fossils

The monument is one of the world's most important places for studying Miocene mammals and their world.

Protecting them is the monument's main geologic job. Beardog Hill and the Stenomylus Quarry are exposed to erosion, weathering and occasional vandalism; the historic quarries at Fossil Hills are easy for visitors to reach and are not monitored. A full list of the species from each site would help the people who look after them, and the east side of Carnegie Hill and one layer in the Stenomylus Quarry are candidates for new digs. All fossils on park land are protected by the Paleontological Resources Preservation Act of 2009.

Maps and reports

The NPS Geologic Resources Inventory has mapped the monument: a scoping summary, a digital geologic map (GIS files, with Google Earth views in newer products), a report on its setting, features, management issues and history, and a poster. A Soil Resources Inventory is complete too. Both are in the NPS DataStore — saved searches 2674 (geology) and 2687 (soils).

The Geologic Resources Inventory report for the monument. NPS

More

• The park's own pages on the Agate bonebed, mammal fossils, geologic formations and rivers
• NPS fossils and paleontology and geologic time
• The Nebraska Geological Survey

Sources

Written from "NPS Geodiversity Atlas—Agate Fossil Beds National Monument, Nebraska," Geologic Resources Division, National Park Service; a work of the United States government in the public domain. It cites Vicars (1979), Hunt (1981, 1990), LaGarry and others (2007), and a 2008 communication from Jason Kenworthy, NPS.