Green River Formation (Historic Quarry)
Three ancient great lakes existed in the region of Wyoming, Utah, and Colorado 50 million years ago: Lake Gosiute, 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.
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 the 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 park's two fossil trails, or elsewhere, leave them undisturbed. And while you're here take time to enjoy the life and landscape of the park.
