Outline of Fossil Lake laid over map of today overlapping borders of Idaho, Utah, and Wyoming. Fossil Butte National Monument is outlined and the remaining sediments of the lake are labeled.
The extent of ancient Fossil Lake shown overlying a modern map for reference. The yellow sections show the remaining sediments from Fossil Lake where there may be fossils.

NPS image

The layered rock of the Green River Formation is a historical record of three former lakes: Fossil Lake, Lake Gosiute, and Lake Uinta. These lakes once covered large parts of present-day Wyoming, Utah, and Colorado. Fossil Lake was the smallest. Oriented on a north south trend, its length was about twice its width. Never more than 100 feet deep, it covered around 1500 square miles at its maximum. The depth, size, and shape would have varied considerably during the lake’s existence.

Today, Fossil Butte National Monument protects nearly 13 square miles of land in southwestern Wyoming. Here the memory of Fossil Lake exists only in cliff-forming carbonates that cap the more gently sloping shoulders of the flat-topped ridges. These top-most rock layers were once mud at the lake’s bottom. Their calcium and magnesium-rich chemistry attests to the alkalinity of the lake water.

Stratigraphic column showing the 3 members and the beds going top to bottom sandwich, 18-Inch Layer, Prisky Layer, K-spar tuff, minifish, and gastropod beds.
Stratigraphic column of the Green River Formation highlighting the many layers within the Fossil Butte Member.

NPS image

The Green River Formation within the Fossil Lake basin is sub-divided into three members. They are from bottom (oldest) to top (youngest), the Road Hollow, Fossil Butte, and Angelo members. The Road Hollow Member is the early formative stage in the lake’s existence when sedimentation varied between river-delivered and lake-formed. During the Fossil Butte Member time, the lake was at its deepest and most expansive. Lake-precipitated calcite-rich sediment predominated. This is also the most productive fossil-bearing unit. Finally, the Angelo Member finds the lake’s fortunes diminished. It was shrinking, shallowing and becoming saltier as evaporation outpaced water supply. Its deposits are rich in dolomite, chert, halite, and trona.

The ledge-forming middle third of these picturesque escarpments is the Fossil Butte Member. Composed mostly of laminated calcite-rich mudstone, this unit is world renowned for the preservation, abundance, and diversity of its fossils. Within this member, many of the most productive fossil-bearing layers have informal names like sandwich bed horizon, 18-inch layer, minifish bed, and gastropod bed that are more descriptive than scientific. The sandwich bed is composed of two closely spaced volcanic ashes (slices of bread) between which you find alternating limestone (turkey) and kerogen (roast beef) laminations. The average thickness of the 18-inch layer is, you guessed it, 18 inches. It produces the largest and best-preserved fossils. In the minifish bed, the fossils are more abundant, albeit much smaller on average. Snails are very common in the gastropod bed.

A hill topped with white sediments with ledges of the Fossil Butte Member visible on the side. Red and purple hills of the Wasatch Formation are in the foreground.
From this vantage you can see the tan ledges of the Fossil Butte Member and the reds and purples of the Wasatch Formation.

NPS photo

The more gently sloping base of the flat-topped ridges is the Main Body of the Wasatch Formation. Its red, tan, and gray sedimentary rock, deposited as stream gravel, sand, silt, and clay, also has fossils. This includes teeth and bone fragments from at least 46 different kinds of land mammals. Among these are the hippo-like Coryphodon and the diminutive primate Cantius. As has occasionally happened with the early horse Protorohippus venticolum and lemur-like Apatemys chardini, discovery of more complete specimens within the Green River Formation is possible given that Wasatch Formation deposition continued at the same time. Although much less common, these remains only needed to wash into the lake shortly after death to fossilize intact.

Compressional forces that pushed up the modern Rocky Mountains warped the crust to the west downward in places. This created shallow basins that captured rainfall and river flow to form lakes. One such basin, known as Fossil Basin, became home to Fossil Lake. After millions of years, the lakes vanished as these forces waned and the basins filled with sediment. Later, these same areas experienced slow regional uplift. This allowed meandering rivers to transform a high, featureless plain into a landscape of broad valleys and flat-topped ridges. Within Fossil Basin today, the elevation of the valleys and ridges average 6600 and 7600 feet respectively.

Fossil of a palm frond from the Green River Formation
Palm frond catalog number FOBU490.

NPS photo

The landscape wasn’t the only thing that was changing. The flora and fauna of Fossil Lake included palm trees and crocodile relatives. The area had a warm temperate climate much like the Gulf Coast of the United States today. This was during the height of the Early Eocene Climatic Optimum around 52 million years ago, despite its position in the interior of a continent at mid-latitude. Since that time the Earth has experienced a long, slow cooling and drying trend that culminated in the Pleistocene glaciations. Over this time span, the area went from a forested lake to a sagebrush grassland ecosystem where the iconic animal is the pronghorn (Antilocarpa americana).

Last updated: March 11, 2024

Park footer

Contact Info

Mailing Address:

P.O. Box 592
Kemmerer, WY 83101


307 877-4455

Contact Us