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.