Located where the States of Utah, Wyoming and Colorado converge, The Green River Formation is one of the world's most important and renowned paleontological sites due to the extremely rare fossils that have been found in the region. Fossils of prehistoric plants, fish, reptiles and mammals have been discovered in the Green River Formation, where a series of lakes existed during the Eocene Epoch, approximately 33 to 55 million years ago.
The first documented Green River fish fossil was discovered in 1856 by Dr. John Evans. Over the next two decades visitors to the region began discovering more fossils. In 1871, Edward Drinker Cope published a book about the findings.
The Green River Formation was comprised of three prehistoric lakes: Fossil Lake, Lake Ulinta and Lake Gosiute. Fossil Lake was the smallest and deepest, located in lower regions of western Wyoming; Lake Gosiute was likely the shallowest and covered a large portion of southwestern Wyoming, while Lake Ulinta was the largest and was located in northeastern Utah.
According to The Smithsonian National Museum of Natural History in Washington, D.C., approximately 35,000 fossiliferous Green River rocks are housed in the institution. Many of the rocks in the collection contain fossils of reptiles, insects and flowers.
The Fossil Butte National Monument, located in Kemmerer, Wyoming, is also home to a large Green River fossil collection, including fossilized fish, reptiles and mammals.
