Three great lakes existed in the area of Wyoming, Utah, and Colorado 50 million years agoLake Gosiute, Lake Uinta, and Fossil Lake, the smallest. All are gone today, but they left behind a wealth of fossils in lake sediments that turned into the rocks known as the Green River Formation, made up of laminated limestone, mudstone, and volcanic ash. The fossils are among the world's most perfectly preserved remains of ancient plant and animal life. Some of these extraordinary fossils come from Fossil Lake, seen today as a flat-topped rock butte that stands near the center of the ancient lake. In 1972 Congress designated Fossil Butte National Monument to preserve the butte and its invaluable, fascinating record of the past.
The fossils of Fossil Lake are remarkable for their abundance and the broad spectrum of species found hereplants, insects, reptiles, birds, mammals, and over 20 kinds of fish. Paleontologists, scientists who study fossils, and private collectors have unearthed millions of specimens since the mid-1800s. Many billions more lie buried in the butte and surrounding ridges, protected and preserved for future paleontologists to study.
The fossils have striking detail. Many fish retain entire skeletons, teeth, delicate scales, and skin. Perhaps most remarkable is the story the fossils tell of an ancient life and landscape. The scene, during the Eocene Epoch of the Cenozoic Era, was quite different from today. Fossil Lake, 50 miles long and 20 miles wide, nestled among mountains in a lush forest of palms, figs, cypress, and other subtropical trees and shrubs. Willows, beeches, oaks, maples, and ferns grew on the lower slopes; forests of spruce and fir grew on the cool mountainsides. In and around the warm waters of the lake, animal life was diverse and abundant.
A variety of fish inhabited the tributaries, shallows, and deep water of the lake during its life of over two million years. Gars, paddlefish, bowfins, and stingrays survive today, as do herring, perch, and mooneyes. The shore bustled with crocodiles; primates and dog-sized horses inhabited the land; birds and bats took to the air.
This scene is gone now because of climate changes and the lake's disappearance. But the site of ancient Fossil Lake and its fossils will be protected in perpetuity at Fossil Butte National Monument, where paleontologists and park visitors can discover the past.
Fossils of Ancient Fossil Lake
For number, variety, and detail of fossil fish, few places can equal ancient Fossil Lake. Its fossils enable us to take a close look at what life was like at Fossil Lake 50 million years ago. Fish are the most common fossils found here. Millions of herring that swam in schools are preserved as images-in-stone. Specimens of bigger predatory fish, like five-foot-long gars and four-foot-long bowfins, are rarer. Over 20 species of freshwater fish have been identified in Fossil Lake. Many are recognizable as ancestors or are related to some of today's species. Besides fossil fish, hundreds of other life forms are captured in stone. The bones of fossil bats, the oldest complete bats in the world, and remarkably complete fossil snakes are preserved here. Snail shells, insect impressions, crocodiles, freshwater turtles, bird skeletons, feather impressions, and plant remainsleaves, seeds, stems, and flowers blown or washed into the lakethese, and more, are part of the buried treasure of fossils unearthed here.
Ideal Conditions for Making Fossils No one knows for sure what events led to the preservation of Fossil Lake's animals and plants. But scientists have developed theories to explain the process. One essential ingredient for preservation was burial in calcium carbonate. which precipitated out of the water and fell like a constant gentle rain to the bottom of Fossil Lake. This protective blanket covered whatever sank to the bottomdead fish, fallen leaves. This took place, year after year, for hundreds of thousands of years. Some of the most perfectly preserved fossils come from the deep-water sediment layers of whitish to gray limestone alternating with brown organic layers commonly called the 18-inch layer.
These fossils are generally adult fish. An equally important fossil-bearing layer comes from nearer the lake shallows and is composed of a tan limestone with faint lamination that splits easily due to its lack of organic material. This layerthe split-fish layeraverages 6½ feet thick. Here are younger fish and species like crayfish and stingrays that lived in the near-shore shallows.
Unsolved Mystery While many of Fossil Lake's animals and plants probably died natural deaths, occasionally huge numbers of fish died suddenly. These die-offs are recorded on slabs of the Green River Formation called mass mortality layers. What killed these fish? A super-bloom of blue-green algae that used all the oxygen? A sudden change in water temperature or salinity? All of these? Ongoing research may solve the mystery.
The Age of Mammals
Life in the Cenozoic Era—the Last 65 Million Years
From simple beginnings great numbers and varieties of life forms have evolved and populated the Earth. For 140 million years before the Cenozoic Era, dinosaurs held dominion over the land. Mammals also existed, but they were small and not abundant. As the dinosaurs perished the mammals took center stage. Even as mammals increased in numbers and diversity, so did birds, reptiles, fish, insects, trees, grasses, and other life forms. The fossil record gives us a fascinating glimpse into the Cenozoic Era. Without fossils we would have little way of knowing that ancient animals and plants were different from today's. With fossils we discover that an extraordinary procession of organisms lived in North America and around the world. Species changed as the epochs of the Cenozoic Era passed. Those that could tolerate the changes in the environment survived. Other species migrated or became extinct. The fossil record tells these stories, but the study of fossil remains, paleontology, also raises many questions: What types of environments did these plants and animals live in? How did they adapt to climatic changes? How did different groups of plants and animals interrelate? How have they changed through time?
Fossils are studied in the context in which they were found and as one element in a community of organisms. Every fossil can serve as a key to unlock knowledge, so the National Park Service is especially concerned with the protection of these keys as the questions unfold. The Cenozoic: Era continues today and scientists estimate that as many as 30 million species of animals and plants now inhabit the Earth. This is a mere fraction of all life forms that have ever existed. Scientists now think that about 100 species will become extinct every day, a rate accelerated by human actions. Pollution of the air and water; destruction of forests, grasslands, and other ecosystems; and other adverse changes to Earth's environment challenge life's very ability to survive. "Looking back on the long panorama of Cenozoic life," Finnish scientist Björn Kurten has said, "I think we ought to sense the richness and beauty of life that is possible on this Earth of ours." It is no longer enough to plan for the next generation or two, Kurten suggests. We should plan "for the geological time that is ahead .... It may stretch as far into the future as time behind us extends into the past."
(Park names following captions indicate where fossils were found.)
The Park Today
Fossil Butte National Monument is a semi-arid landscape of flat-topped buttes and ridges dominated by sagebrush, other desert shrubs, and grasses. Can you imagine that once this was a sub-tropical climate with a lake teeming with life? Look for evidence in the rocks around you.
Green River Formation
Planning Your Visit
Visitor Center Here you'll find information, over 200 fossil exhibits, and a bookstore. It is open daily except winter holidays; hours vary by season. Visit www.nps.gov/fobu.
Picnic Area A shaded picnic area has tables, fire grills, restrooms, and drinking water.
Historic Quarry Trail Strenuous: 2.5-mile loop. Climb Fossil Butte to a Green River Formation quarry worked in the 1960s. Exhibits describe the geology and history of collecting.
Chicken Creek Trail Moderate: 1.5-mile loop. This trail winds through sagebrush and an aspen grove. Exhibits tell the area's natural history.
Today's Natural World Watch for mule deer, pronghorn, elk, coyotes, and eagles in this land of sagebrush, aspen, willows, and pines. Wildflowers bloom seasonally.
Climate Temperatures range from 90s°F in summer to subfreezing in winter. High winds are common.
Accessibility Contact the park about accessible facilities. Service animals are welcome.
Food, Lodging, Services Find food, lodging, campgrounds, and services in Kemmerer. Bridger-Teton National Forest has camping.
Safety, Regulations Be alert. Remember, your safety is your responsibility. • Wear sturdy shoes and carry plenty of water when hiking. Be aware of elevation gains and steep downhill sections. • Protect yourself from the sun and insects. • Pets must be leashed and attended. • Build fires only in picnic grills. • Use caution on unpaved roads; they may be impassable if wet. • Do not feed or approach wildlife. • For firearms regulations see the park website. Emergencies: call 911.
Collecting FossilsPast and Present
In 1856 geologist Dr. John Evans collected some of the first fossil fish from the Green River Formation. Paleontologist Dr. Joseph Leidy identified the fossils and gave them scientific names.
Not all early collectors were scientists. In the 1860s Union Pacific Railroad workers discovered the Petrified Fish Cut near the Green River station. In 1881 the Oregon Short Line Railroad was routed near Fossil Butte, improving access to the area.
Men, women, even families came here for what became a life's work. Robert Lee Craig spent 40 years (1897-1937) quarrying and preparing fossils for museums and private collections throughout the world. David C. Haddenham worked here into the 1960s.
Many of these fossils are exhibited today in museums across the United States, including Chicago's Field Museum, New York's American Museum of Natural History, Laramie's University of Wyoming Geological Museum, and Washington, D.C.'s Smithsonian Institution.
Collecting continues outside the park on private land and on state land leased by permit only. Rare finds on state land are given to the Wyoming Geological Survey for study.
Collecting within Fossil Butte National Monument is limited to special permit research projects that advance scientific understanding of Fossil Lake.
Fossil Butte National Monument was established to protect the fossils and geologic features within its boundary. Do not disturb or damage any fossil; If you find a fossil, leave it in place and report it to a ranger. All natural and historic features are protected by federal law.
Warning Non-permit removal of fossils violates the National Park Service Organic Act (16 U.S.C.) and 36CFR2.1 (a) (1) (iii). Violators will be prosecuted.
Source: NPS Brochure (2010)
Brochures ◆ Site Bulletins ◆ Trading Cards
Annotated Checklist of Vascular Flora, Fossil Butte National Monument NPS Natural Resource Technical Report NPS/NCPN/NRTR-2009-205 (Walter Fertig and Clay Kyte, May 2009)
Archeological Assessment, Fossil Butte National Monument (George M. Zeimens, 1976)
Dam Removal and Channel Restoration at Fossil Butte National Monument (Clayton R. Kyte, Vincent L. Santucci and Richard R. Ingles, Jr., extract from Wildland Hydrology, June/July 1999)
Geologic Resources Inventory Report, Fossil Butte National Monument NPS Natural Resource Report NPS/NRPC/GRD/NRR-2012/587 (J. Graham, October 2012)
Invasive Exotic Plant Monitoring at Fossil Butte National Monument
Invasive Exotic Plant Monitoring at Fossil Butte National Monument: 2008 Pilot Field Season NPS Natural Resource Technical Report NPS/NCPN/NRTR-2009/234 (Dustin W. Perkins and Rebecca Weissinger, July 2009)
Invasive Exotic Plant Monitoring at Fossil Butte National Monument: Annual Report 2009 NPS Natural Resource Technical Report NPS/NCPN/NRTR-2010/322 (Dustin W. Perkins, May 2010)
Invasive Exotic Plant Monitoring at Fossil Butte National Monument: Field Seasons 2008-2012 NPS Natural Resource Technical Report NPS/NCPN/NRTR-2013/751 (Dustin Perkins, May 2013)
Invasive Exotic Plant Monitoring at Fossil Butte National Monument: 2014 Field Season NPS Natural Resource Report NPS/NCPN/NRR-2015/988 (Dustin W. Perkins, July 2015)
Invasive Exotic Plant Monitoring at Fossil Butte National Monument: 2016 Field Season NPS Natural Resource Report NPS/NCPN/NRR-2017/1555 (Dustin W. Perkins, December 2017)
Invasive Exotic Plant Monitoring at Fossil Butte National Monument: 2018 Field Season NPS Natural Resource Report NPS/NCPN/NRR-2019/1920 (Amy Washuta and Dustin W. Perkins, May 2019)
Museum Management Plan (Arvid Aase, Kent Bush, Linda Clement, Ann Elder, Lynn Marie Mitchell and Matthew Wilson, 2002)
National Register of Historic Places Nomination Form
Haddenham Cabin (Christine E. Maylath, Benjamin Brower and Kathy McKoy, February 23, 1995, revised May 10, 2000)
Natural Resource Condition Assessment, Fossil Butte National Monument NPS Natural Resource Report NPS/FOBU/NRR-2017/1394 (Kevin M. Benck, Kathy Allen, Andy J. Nandeau, Lonnie Meinke, Anna M. Davis, Sarah Gardner, Matt Randerson and Andy Robertson, February 2017)
Observations of Elk Movement Patterns on Fossil Butte National Monument (Edward M. Olexa, Suzanna C. Soileau and Leslie A. Allen, Wyoming Landscape Conservation Initiative, October 2014)
Preliminary Mammal Survey of Fossil Butte National Monument, Wyoming (Tim W. Clark, extract from Great Basin Naturalist, Vol. 37 No. 1, March 1977)
Paleogeography and Paleoenvironments of the Lower Unit, Fossil Butte Member, Eocene Green River Formation, Southwestern Wyoming (©Roberto E. Biaggi, Master's Thesis Loma Linda University, June 1989)
Paleontology of the Green River Formation, with a Review of the Fish Fauna The Geological Survey of Wyoming Bulletin 63 (Lance Grande, 1984, 2nd ed.)
Proposed Fossil Butte National Monument (August 1964)
Railroads, Tourism and Fossils Park Paleontology Vol. 6 No. 3 (Vincent L. Santucci, Fall-Winter 2002)
The Geologic History of Fossil Butte National Monument and Fossil Basin NPS Occasional Paper No. 3 (Paul O. McGrew and Michael Casilliano, 1975)
Vascular Plant Species Checklist and Rare Plants of Fossil Butte National Monument (Walter Fertig, October 9, 2000)
Vascular Plant Species Discoveries in the Northern Colorado Plateau Network: Update for 2008-2011 NPS Natural Resource Technical Report NPS/NCPN/NRTR-2012/582 (Walter Fertig, Sarah Topp, Mary Moran, Terri Hildebrand, Jeff Ott and Derrick Zobell, May 2012)
Vegetation Classification and Mapping Project Report, Fossil Butte National Monument NPS Natural Resource Technical Report NPS/NCPN/NRTR-2010/319 (Beverly Friesen, Steve Blauer, Keith Landgraf, Jim Von Loh, Janet Coles, Keith Schulz, Amy Tendick, Aneth Wright, Gery Wakefield and Angela Evenden, May 2010)
Handbooks ◆ Books
Last Updated: 18-Apr-2022