Fossil Butte is a 50 million year old lakebed and one of the richest fossil resources in the world. It is part of the Green River Formation, a layer of rock composed of laminated limestone, mudstone, and volcanic ash. Complete paleo-ecosystems are preserved in the formation, which is the geologic remnant of the Green River Lake System of the Eocene era. Designated on October 23, 1972, Fossil Butte National Monument encompasses a part of land that was once under Fossil Lake.
Fossil Lake was the smallest and shortest-lived of the Green River System lakes. At its largest size, the lake covered 932 square miles and existed for approximately three million years. One phenomenon of the Fossil Lake fossils is their density. There are as many as several hundred fossilized fish per square meter in some portions of Fossil Butte. At least thirteen genera of fish are found in the sediments of fossil lake, as well as a wide variety of species, from large predators like gar and bowfin to the now extinct Knightia, Diplomystus, and Priscacara. Plant and invertebrate records are equally diverse. Evidence of lily pads, horsetails, ferns, palms, and poplar and elm trees exist along Fossil Lake's floodplain and lower elevations. The remains of dragonflies, mosquito larvae, and water striders bear testament to the vibrancy of this freshwater ecosystem. In addition, evidence of freshwater mollusks, snails, crayfish, and shrimp has been found.
The paleontologist who worked at Fossil Butte have deposited specimens at the Field Museum in Chicago, IL, the American Museum of Natural History, in New York City, and Smithsonian Institution's National Museum of Natural History in Washington D.C.