Some 50 million years ago, during the Eocene Epoch of the Cenozoic Era, Fossil Butte National Monument in southwestern Wyoming was submerged under a lake 50 miles long and up to 20 miles wide. Palms, figs, cypress, and other subtropical trees and shrubs grew along Fossil Lake's shorelines, while willows, beeches, oaks, maples, and ferns covered the lower slopes of nearby mountains. Six-foot gars, paddlefish, bowfins, mooneyes, stingrays, turtles, and more than 20 other freshwater species lived in the tributaries, shallows, and deep water of the lake. Crocodiles patrolled the lakeshore, and birds and bats flew overhead. As these creatures died, they sank to the bottom of the lake.

Fossil Butte National Monument
©National Park Service
More than 80 fossils are displayed at the Fossil Butte National Monument visitor
center including a 13-foot-crocodile.

Eventually the lake dried up, leaving behind a flat-topped remnant of rock that stands where the center of the lake once was. Embedded in the butte are the fossils of more than 20 kinds of fish, 100 varieties of insects, and an unknown number of plants. These fossils are among the most perfectly preserved remains of ancient plant and animal life animal life in the world. Not just the skeletons of fish are visible, but their teeth, delicate scales, and skin as well. Among the fossils preserved here are the fragile bones of a bat, the oldest ever found in North America, and a remarkably complete fossil snake. While most of Fossil Lake's plants and animals died natural deaths, occasionally huge numbers of fish were killed suddenly. These die-offs are recorded in "mass mortality" layers.

A sudden change in temperature or an invasion of blue-green algae may have contributed to these die-offs. A large "mass mortality" slab is on display at the visitor center, along with many other fossils. Several short hiking trails with exhibits allow visitors to see the fossils in their natural condition and learn about some of the history of fossil-collecting in the area. The Fossil Lake Trail winds through the aspen groves and high desert landscape that surround the butte.

Fossil Butte National Monument Information

Address: 9 miles west of Kemmerer on U.S. Hwy 30, Kemmerer, WY
Telephone: 307/877-4455
Hours of Operation: Monument grounds are open sunrise to sunset.
Visitor center hours are:

  • Daily 8 a.m. - 7 p.m. (summer)
  • Daily 8 a.m. - 4:30 p.m. (rest of the year)

Admission: Free

Learn about these other national monuments:

­Agate Fossil Beds National Monument

Colorado National Monument

George Washington Birthplace National Monument

Natural Bridges National Monument

Scotts Bluff National Monument

Alibates Flint Quarries National Monument

Devils Postpile National Monument

George Washington Carver National Monument

Ocmulgee National Monument

Sunset Crater Volcano National Monument

Aniakchak National Monument and Preserve

Dinosaur National Monument

Gila Cliff Dwellings National Monument

Oregon Caves National Monument

Timpanogos Cave National Monument

Booker T. Washington National Monument

Effigy Mounds National Monument

Grand Portage National Monument

Organ Pipe Cactus National Monument

Tonto National Monument

Buck Island Reef National Monument

El Malpais National Monument

Hohokam Pima National Monument

Petroglyph National Monument

Tuzigoot National Monument

Cape Krusenstern National Monument

El Morro National Monument

Homestead National Monument

Pinnacles National Monument

Walnut Canyon National Monument

Capulin Volcano National Monument

Florissant Fossil Beds National Monument

Hovenweep National Monument

Pipe Spring National Monument

Wupatki National Monument

Casa Grande Ruins National Monument

Fort Frederica National Monument

Jewel Cave National Monument

Pipestone National Monument


Castle Clinton National Monument

Fort Matanzas National Monument

John Day Fossil Beds National Monument

Poverty Point National Monument


Cedar Breaks National Monument

Fort Stanwix National Monument

Lava Beds National Monument

Rainbow Bridge National Monument


Chiricahua National Monument

Fossil Butte National Monument

Montezuma Castle National Monument

Russell Cave National Monument


To learn more about national monuments, memorials, and historic sites, and other travel destinations in North America, visit:

ABOUT THE AUTHOR:

Eric Peterson is a Denver-based author who has contributed to numerous guidebooks about the Western United States.