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Thermopolis Hot Springs
Hell's Half Acre
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 buttte 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
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 on the WSGS website and in Paleontology of the Green River Formation, With a Review of the Fish Fauna.
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.
Christina George (307) 766-2286 Ext. 231