Beautifully preserved fish . . . with delicate fins, tail rays and scales, all virtually undisturbed . . . are entombed here in thinly layered sediments recording the abundant life and ecology of an ancient subtropical lake. Wilmot Bradley, US Geological Survey
Three ancient great lakes existed in the region of Wyoming, Utah, and Colorado 50 million years ago: Lake 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 rock layer known as the Green River Formation, made up of laminated limestone, mudstone, and volcanic ash. The fossils are among the most nearly perfectly preserved remains of ancient plant and animal life in the world. Some of the most extraordinary of these fossils came from Fossil Lake, represented today by a flat-topped remnant of rock that stands where the center of Fossil Lake once was. Fossil Butte National Monument preserves the butte and its invaluable, fascinating record of the past.
The fossils of Fossil Lake are remarkable for their numbers and the broad spectrum of species found here: more than 20 kinds of fish, 100 varieties of insects, and an as yet uncounted number of plants. Paleontologists, the scientists who study fossils, and private collectors have unearthed thousands of specimens during the past 100 years. Many billions more lie buried in the butte and surrounding ridges protected and preserved for future paleontologists to study. The fossils are remarkable for their detail. Many of the fish, for example, retain not only their entire skeletons, but their teeth, delicate scales, and skin as well. And perhaps most remarkable of all is the story the fossils tell of an ancient life and landscape.
The scene 50 million years ago, during the Eocene Epoch of the Cenozoic Era, was quite different from that today. Fossil Lake, 50 miles long and 20 miles wide at its maximum, nestled among mountains in a lush green forest of palms, figs, cypress, and other subtropical trees and shrubs. Willows, beeches, oaks, maples, and ferns grew on the lower slopes, and on the cool mountain sides was a spruce and fir forest. In and around the warm waters of the lake, animal life was diverse and abundant. A broad range of fish inhabited the tributaries, shallows, and deep water of Fossil Lake during its unusually long life of more than 2 million years. Gars, paddlefish, bowfins, and stingrays, though they may appear primitive to some, still survive today, as do herring, perch, and mooneyes. The lakeshore was alive with crocodiles and turtles; insects, dog-sized horses, and early primates inhabited the land; birds and bats mastered the air.
This vibrant scene is gone now because of profound climate changes and the disappearance of the lakes. But the site of ancient Fossil lake and many of the fossils that tell its story will be protected in perpetuity at Fossil Butte National Monument. Here, paleontologists and park visitors alike 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 by far. Millions of herrings that swam in schools are preserved as images-in-stone. Specimens of bigger predatory fish, such as 5-foot-long gars and a 4-foot-long bowfin, are rarer. Altogether more than 20 species of freshwater fish have been identified at Fossil Lake; many are recognizable as ancestors or close cousins of some of today's species. Besides the fossil fish, there are hundreds of other forms of life captured in stone. The delicate bones of a fossil bat, the oldest known in North American, and a remarkably complete fossil snake were preserved here. Snail shells, insect impressions, crocodiles, freshwater turtles, bird skeletons, feather impressions, and plant remains (leaves, seeds, stems, and flowers blown or washed out into the lake) - these, and more, are part of the buried treasure of fossils unearthed at Fossil Lake.
Ideal Conditions for Fossil-Making. What events led to the preservation of so much of Fossil Lake's life as fossils? No one knows for sure, but after careful study scientists have developed theories to explain the process. One essential ingredient for preservation, they believe, was rapid burial in calcium carbonate, which precipitated out of the water and fell like a constant gentle rain to the bottom of Fossil Lake. Whatever sank to the bottom - dead fish, fallen leaves - was covered by this protective blanket. Year after year for hundreds of thousands of years, this reoccurred. Some of the most perfectly preserved fossils come from the deep-water sediment layers of whitefish to buff-colored calcite limestone alternating with brown oil shale commonly called the 18-inch layer. The fossils are generally adult fish. An equally important fossil-bearing layer comes from nearer the lake shallows and is composed of a lighter-colored limestone with faint lamination that splits easily due to the lack of organic material. Thus, its name: the split-fish layer, which averages 6½-feet-thick. Here one finds younger fish and species that would have survived better in the near shore shallows - crayfish and stingrays, for example.
Unsolved Mysteries. While many of Fossil Lake's animals and plants probably died natural deaths, on several occasions huge numbers of fish were killed suddenly. These die-offs are recorded on great slabs of the Green River Formation called mass mortality layers.
- What killed these fish?
- A super-bloom of blue-green algae that emitted poisons into the water?
- A sudden change in water temperature or in salinity?
- All of these?
The Park Today
Fossil Butte National Monument today is a semi-arid landscape of flat-topped buttes and ridges dominated by sagebrush, other desert shrubs, and grasses. It might be hard to imagine that 50 million years ago a lake teeming with life existed here in the midst of a subtropical climate. But the evidence is in the rocks around you.
In the park, the Green River Formation, which consists of the lake sediments of Fossil Lake, appears as layers of tan-buff-colored sedimentary rock near the top of Fossil Butte and surrounding ridges. It is in the 200- to 300-foot-thick formation that millions of fish and other fossils are found. Other fossil-bearing rock formations, laid down at different times or at different sites, also exist here. The most prominent is the red, pink, and purple Wasatch Formation - a stream deposited layer underlying, interfingering, and overlaying the Green River Formation - that has yielded the fossil remains of primitive horses, a rhino-sized mammal known as Coryphodon, early primates, crocodiles, turtles, lizards, and plants. Fossils are best discovered at the visitor center museum. If you happen to see fossils along the park's two fossil trails, or elsewhere, leave them undisturbed. And while you're here take time to enjoy the life and landscape of the park.Fossil Collecting, Past and Present
Some of the first fossil fish from the Green River Formation were collected by geologist Dr. John Evans in 1856 and described scientifically by Joseph Leidy. Not all early collectors were scientists; Union Pacific Railroad workers discovered the "Petrified Fish Cut" near Rock Springs station in the 1860s. A few years later the railroad was routed near Fossil Butte. Other collectors were men and women - sometimes whole families - fascinated by the fossils they found. Robert Lee Craig, who quarried fish from Fossil Butte and Fossil Ridge to the south, spent 40 years, from 1897 to 1937, quarrying and preparing fish for museums and private collections throughout the world. David C. Haddenham worked the Fossil Butte sites for more than 50 years in this century.
Several families have come to the basin for what has amounted to a life's work for many of them.
Many of the countless fossils they dug out and carefully prepared are studied and exhibited today in museums across the United States, including the Field Museum in Chicago, the American Museum of Natural History in New York, the University of Wyoming Geological Museum at Laramie, and the Smithsonian Institution's National Museum of Natural History in Washington, DC The extensive collections reflect the toils of private and scientific collectors of the past 100 years.
Today collecting continues outside the park on private lands and on Wyoming state lands leased by permit only. Rare finds on state lands are turned over to the University of Wyoming for scientific study. Collecting within Fossil Butte National Monument is now limited to special permit research projects that will further scientific understanding of Fossil Lake. Other collecting is disallowed by the Antiquities Act of 1906 that makes it illegal to remove or injure "objects of antiquity" found on federal lands.
The General park map handed out at the visitor center is available on the park's map webpage.
View the park's map to create your own personal maps and images right here.For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.
A photo album for this park can be found here.For information on other photo collections featuring National Park geology, please see the Image Sources page.
Fossil Butte National Monument: Along the Shores of Time.
Ambrose, Peter D.
Dinosaur Nature Association.
paperback, 28 pages, black and white illustrations.
Includes illustrations and descriptions of fossils found at the Fossil Butte National Monument.
Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
9" x 10.75", paperback, 550 pages, full color throughout
The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!
Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.
For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.
The NPS maintains a searchable data base of research needs that have been identified by parks.
A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.
NPS Geology and Soils PartnersAssociation of American State Geologists
Geological Society of America
Natural Resource Conservation Service - Soils
U.S. Geological Survey
Currently, we do not have a listing for any park-specific geology education programs or activities.
General information about the park's education and intrepretive programs is available on the park's education webpage.For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.