Fossil Butte National Monument
P.O. Box 592
Kemmerer, WY 83101
(307) 877-4455
(307) 877 4457 Fax
Three ancient great lakes existed in the region of Wyoming, Utah, and Colorado 50 million years
ago -- Lake Goslute, 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, 00 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 lake shore 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 America, 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 whitish 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 6%-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 superbloom of blue-green
algae that emitted poisons into the water? A sudden change in water temperature or in salinity? All
of these? Ongoing research may solve the mystery.
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 this 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 parks two fossil trails, or elsewhere, leave them undisturbed. And while you're here
take time to enjoy the life and landscape of the park.
The Visitor Center
The visitor center is a good place to start your exploration of the park. The center is open every
day except for winter holidays. A museum has displays of fossils and an artist's recreation of what
this area may have looked like in the time of ancient Fossil Lake, as well as other exhibits. Books
and brochures are available, and park rangers can help plan your visit. A schedule of events,
including guided walks, is posted.
Trails - Two park trails offer an opportunity to experience Fossil Butte and its fossil story
first hand. The Quarry Trail, a 2.5 mile-long loop trail, climbs up Fossil Butte to the
site of a fossil quarry in the Green River Formation that was worked from the late 1800s to the
1970s. Trailside exhibits recount some of the history of fossil collecting. Along the trail is a cabin
once used by fossil hunters. The 1.5-mile Fossil Lake Trail provides a close-up view of
the flora and fauna in the park. It winds through an aspen grove, high desert landscape, and near a
beaver pond. The record of the past can be seen in rock exposures of the Green River Formation;
today's life and landscape are all around you. Trailside exhibits interpret the areas natural history,
past and present.
Crosscountry hiking is permitted.
Wildlife and Vegetation - Besides its fossil resources, the park protects other natural
features. Wildlife watchers may see mule deer, moose, elk, pronghorn, coyotes, golden and bald
eagles, and a variety of other birds in this land of sagebrush, grass, and scattered aspen groves,
willow thickets, and pine-fir stands. Wildflowers bloom seasonally.
Weather - In whatever season you visit, come prepared. Temperatures range from the 90s
F in the summer to sub-freezing in the winter. Precipitation averages only 9 inches per year, and
more than half falls as snow. Strong winds are common.
Picnic Area - A picnic area, located in an aspen grove, has tables, fire grills, restrooms, and
drinking water.
Nearby Visitor Services - The town of Kemmerer, located 10 miles east of the park, has
lodging, a campground, restaurants, grocery stores, and gasoline stations. Camping is not permitted
in the Monument. Primitive camping is permitted on surrounding Bureau of Land Management
land where caution should be used with campfires. No potable water is available. Commercial
campgrounds are located in Kemmerer; public camping in Bridger-Teton National Forest -- 50
miles northeast, at Fontenelle Reservoir -- 50 miles east, and Cokeville town park -- 30 miles
northwest. More information is available through the Chamber of Commerce, Kemmerer, WY
83101, phone: 307-877-9761.
Mileage from: Cheyenne, WY: 352; Denver, CO: 450; Rock Springs, WY: 95; Evanston,
WY: 65; Salt Lake City, UT: 150; and Vernal, UT: 150.
Books & Maps
Browse through the Adventurous Traveler Bookstore's database to find an extensive collection of books and maps on Wyoming.
Map of Fossil Butte National Monument.
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