Discover the Geology of Fossil Butte

Fossil Butte was established as a national monument on Oct. 23, 1972. Located approximately 11 miles west of the town of Kemmerer, it is famous for extraordinary plant and animal fossil specimens of early Eocene age (approximately 53–49 million years before present). The area is believed to have been a subtropical environment during Eocene time, resembling that of the current Gulf coast and southern Atlantic regions of the United States today.

During the Eocene three large lakes covered what is now southwestern Wyoming, northwestern Colorado, and northeastern Utah. Lake Uinta was the largest and most enduring of the lakes. The next largest was Lake Gosiute, and the smallest but deepest, Fossil Lake, covered the area where Fossil Butte now sits. Fossil Lake's freshwater system hosted large schools of fish similar to modern-day herring (like the Wyoming State Fossil, Knightia), catfish, perch, gar, mooneye, bowfin, paddlefish, and even stingrays.

The lake sediments were lithified into layers of rocks classified as the Fossil Butte Member of the Green River Formation. Most of the Fossil Butte Member consists of lacustrine limestone interbedded with numerous thin layers of organic-rich shale; thin beds of oil shale occur both above and below the most fossil-rich layers. Additionally, there was extensive volcanic activity in the region at the time, which added sediment to the 460-foot-thick Fossil Butte Member.

The landform, Fossil Butte, is an erosional remnant (essentially a large hill being slowly eroded into the valleys below) exposing the Fossil Butte Member prominently on its south and west sides, with subtle exposures to the east and north. In addition to the fish fossils, erosion of the butte has revealed reptile fossils such as crocodiles, turtles, snakes, and even lizards. Mammal fossils found in the area include bats, a horse, and other extinct herbivores. Bird fossils have also been discovered, including feathers and a large frigate bird, as well as a wide array of plant and insect fossils, such as palm fronds, cattails, several species of leaves, seeds, seed pods, and even flies and mosquitoes.

More information about Fossil Butte National Monument can be found on the National Park Service website.

Additional information about paleontology and Wyoming's fossil record can be found in Paleontology of the Green River Formation, With a Review of the Fish Fauna.

►References

Grande, Lance, 1984, Paleontology of the Green River Formation, with a review of the fish fauna (2d ed.): Geological Survey of Wyoming [Wyoming State Geological Survey] Bulletin 63, 333 p.

National Park Service, [n.d.], Fossil Butte National Monument, Wyoming, accessed February 2017, at https://www.nps.gov/fobu/index.htm.

National Park Service, [n.d.], Nature & Science–Geology fieldnotes, Fossil Butte National Monument, Wyoming, accessed February 2017, at https://www.nature.nps.gov/geology/parks/fobu/index.cfm.

Contact:
Christina George, christina.george@wyo.gov