The Geology of Fossil Butte National Monument, Wyoming

Fossil Butte National Monument, on the eastern flank of the Wasatch Range in Lincoln County, southwestern Wyoming, protects flat-topped ridges holding some of the best-preserved fossils in the world. This summary draws on the National Park Service's Geodiversity Atlas, which gathers what the NPS Geologic Resources Inventory knows about each park's rocks, sediments, landforms, fossils, soils and the processes that shape them.

A fossil Lagerstätte

Established on October 23, 1972, the monument covers 3,318 hectares (8,198 acres). Its fossil fish, insects, plants, reptiles, birds and mammals are exceptionally abundant and detailed — preservation so rare that the site is called a Lagerstätte. Even soft-bodied creatures and delicate structures such as skin survive, letting scientists study entire extinct ecosystems from 50 million years ago.

The 50-million-year-old rocks of the Green River Formation are exposed on Cundick Ridge in Fossil Butte National Monument. Image from the National Park Service's page.

An Eocene greenhouse

The rocks record the Eocene, when Wyoming was a warm-temperate place of palm trees and insects; paddlefish, stingrays, gar, pike, trout-perch and herring; turtles, snakes, lizards and crocodiles; birds such as parrots, bitterns, goatsuckers and oilbirds; and mammals. New finds continue to illuminate this ecosystem, which flourished during a global greenhouse climate among the warmest of the past 66 million years.

The fossils lie in the Green River Formation and the Wasatch Formation, deposited in lakes, rivers and floodplains — settings well suited to burying and preserving remains intact. The monument contains the type sections of two members of the Green River Formation: the Angelo Member and the Fossil Butte Member. While the monument's own rocks are all Eocene, the mountain ranges flanking Fossil Basin to the east and west expose rocks as old as the Devonian.

Fossils in national parks are protected under the Paleontological Resources Preservation Act of 2009.

Geologic Resources Inventory map. National Park Service.

Regional setting and inventories

Fossil Butte belongs to the Middle Rocky Mountains Physiographic Province and shares its geologic history and characteristic formations with a much larger region. The Geologic Resources Inventory has produced digital geologic maps and reports for the monument, and a Soil Resources Inventory has also been completed; both are available through the NPS DataStore.

Sources

• National Park Service, "NPS Geodiversity Atlas—Fossil Butte National Monument, Wyoming": https://www.nps.gov/articles/nps-geodiversity-atlas-fossil-butte-national-monument-wyoming.htm
• The report cover shown on the source page is not reproduced.