Fossils & Geology

A butte is an isolated flat-topped hill with steep sides. When viewed from the road or behind the visitor center, Fossil Butte appears to be a butte, but it is actually a ridge. The top tan layer of this ridge and the surrounding ridges was the bottom of a freshwater lake 52-million years ago. It is in these rocks that the fossils are found. The majority are from the middle layers known as the Fossil Butte Member (FBM) of the Green River Formation (GRF).

Fossils from the FBM are world famous for their abundance, diversity, and preservation. The most common vertebrate fossil in the world, Knightia eocaena, is found here. The oldest complete bat fossils come from the FBM as well as the only complete freshwater stingrays. Preserved wing venation of damselflies allows the identification of different species. In one species (Tynskysagrion brookeae), preserved color patterns in the wings differentiated males and females. The diversity and abundance of well-preserved fossils paints a more complete picture of the ancient ecosystem than most fossil locations. With each new fossil found, more is learned about life in and around ancient Fossil Lake.