INTRODUCTION

Fifty million years ago the southwestern corner of what is now Wyoming was part of a system of three freshwater lakes that covered that area plus adjacent parts of Colorado and Utah.

These lakes began with Lake Flagstaff and later became Lake Gosiute, Lake Uinta, and Fossil Lake (Fig. 1). These lakes developed in intermontane basins that were created as a result of the geologic events that formed the Rocky Mountains. The three lakes, collectively known as the Green River Lake System, had a long, complex history of expansion and contraction. Varying climatic and geologic conditions were responsible for the changes in lake size and distribution.

The shores surrounding the lakes were blanketed by a lush, green canopy of palm, cinnamon, maple, oak, and other familiar trees. Hazel and lilac covered the forest floor. Rushes and other aquatic plants lined the lake shore.

The air was humid and warm. Streams flowing down from the hills and mountains built up flood plains and fed sediment into the lake, where it was deposited in shallow water near the shore. Deposits formed by chemical processes settled to the bottom further from shore in deep, quiet water. From these processes were formed the rocks from which the past history of Green River Lake System is read.

In the forest and undergrowth lived the ancestors of modern mammal groups. Ancestral rodents and tiny insectivores lived a furtive existence in the brush and mold that carpeted the forest floor, or else they sought a livelihood among the branches of trees. Large, bizarre animals with strange names lumbered through the reed-covered streamsides eating soft, succulent plants. The earliest members of the horse family browsed on the soft vegetation. The trees overhead were alive with the chattering and antics of early primates. Carnivorous mammals preyed on their plant-eating neighbors and so maintained a balanced community. Crocodiles and turtles basked in the sun on the beach. Flamingos concentrated in large nesting grounds. Snakes and lizards crawled about in the undergrowth. Insects, many exceedingly similar to modern types, flew about in the warm air or crept about on the plants.

The waters of the lakes teemed with many types of fish. Relatives of the perch, herring, and sting ray swam in the warm lake water. Occasionally, large-scale mortalities of the fish occurred. As the fish died, they sank to the bottom of the lake, and were preserved in the lake sediments.

Today, the lakes are gone and where once there were lush tropical forests there is now a semi-desert covered with sagebrush and greasewood. The history of these lakes, forests, and animal life can be read in the thick sediments deposited so long ago.

This report concerns itself with one of these lakes, the smallest of the three, Fossil Lake. Famous for its fossil fish beds, part of this ancient lake is now Fossil Butte National Monument. It is the purpose of this report to relate the history of Fossil Lake and its now fossilized inhabitants, thereby, hopefully presenting the order of events in a landscape obscured by 50 million years of time.

LOCATION

Fossil Butte National Monument is near the geographical center of Fossil Basin. The basin itself is located in the extreme southwestern part of Wyoming in Uinta and Lincoln counties, near the Utah-Idaho border (Fig. 2).

The monument is situated about 10 miles west of Kemmerer, Lincoln County, Wyoming (Fig. 3). U.S. Highway 30N runs just south of the monument as does the Union Pacific Railroad line to Oregon. The abandoned town of Fossil is also immediately south of the monument.

The Kemmerer and Sage quadrangles of the United States Geological Survey cover the entire monument and the surrounding area.

HISTORICAL BACKGROUND OF RESEARCH

The principal rocks involved at Fossil Butte are the Green River and Wasatch formations. The first published notice of these two rock units was by Hayden (1869). Although short, Hayden’s descriptions were the basis for later refinement of the stratigraphy of the Green River and Fossil basins. Hayden also mentioned the quantities of fossil fish from the Green River Formation.

The work done by Hayden was conducted under the auspices of the U.S. Department of the Interior, Geological and Geographical Survey of the Territories. The survey lasted from 1867 to 1878, during which time Hayden and his associates published annual reports of their findings.

In the 1870 report, Hayden (1871) mentioned the discovery of the Petrified Fish Cut. This cut is located on the main line of the Union Pacific Railroad about 2 miles west of Green River, Wyoming. A. W. Hilliard and L. E. Rickseeker, employees of the Union Pacific Railroad, discovered the cut and obtained many fossil fish which they turned over to Hayden.

Previous to Hayden, rocks now known as the Green River and Wasatch formations were occasionally mentioned in the various diaries, journals, and reports of early missionaries like those of S. A. Parker or those of explorers like Fremont (Knight 1955).

Fig. 1. Paleocene-Eocene lake system (after Schaeffer and Mangun 1965).

Fig. 2. Geographic features of southwest