As the Paleocene gave way to the Eocene, 60 million years ago, a trio of lakes were forming in the intermountain west. Fossil Lake, the smallest but deepest of the three, covered the extreme southwestern edge of Wyoming; Lake Gosiute covered much of the rest of southwest Wyoming while Lake Uinta spread across northeastern Utah and northwestern Colorado. All three of these lakes formed within basins that downwarped as the Rocky and Uinta Mountains rose. Over the course of their history, some 20 million years for the larger two lakes, these bodies expanded and contracted with the climate; during wet periods they were interconnected while, during extended droughts, they were separated by vast salt flats and desert.
Through most of their history, these lakes occupied a subtropical zone and lush vegetation grew along their margins. Crocodiles, turtles, tropical birds, amphibians, lizards and primitive mammals inhabited these bordering wetlands and a wide variety of fish lived within the lakes. Fossil evidence also reveals that riparian woodlands, including groves of sycamore, covered parts of the basin.
Today, these Great Lakes of the Eocene, known as the Green River Lake System, are represented by the sediments that collected within them. Fossil Lake filled in with sediments of the Wasatch Formation and early Green River Formation by the middle of the Eocene; today, Fossil Butte National Monument sits within its former basin, protecting remnant towers and cliffs of these sediments and the numerous fish fossils that they harbor. Lake Gosiute and Lake Uinta have become the vast High Plateau region that characterizes the tristate of Colorado, Utah and Wyoming. The upper strata of these tablelands are composed of the Green River Formation, famous for its layers of oil shale. Draining and molding the plateaus are numerous tributaries of the Green River itself, which heads in the Wind River Range of Wyoming and flows south to join the Colorado in Canyonlands National Park.