The Green River Formation is an Eocene epoch (55.8 – 33.9 million years ago) geologic formation that records the sedimentation in a series of intermountain lakes. The sedimentary layers were formed in a large area of interconnecting lakes, named for the present-day Green River, a tributary of the Colorado River. Fossil Butte National Monument in Lincoln County, Wyoming is located in a part of the formation known as Fossil Lake because of the abundance of exceptionally well preserved fish fossils found in the area.
Within the Green River Formation of southwest Wyoming in the area known as Fossil Lake, two distinct zones of very fine-grained lime muds are particularly noted for preserving a variety of complete and detailed fossils. These layers are an Eocene Lagerstätte, a rare place where conditions were right for a rich accumulation of undisturbed fossils. The most productive zone—called the 18 inch layer— consists of a series of laminated lime muds containing abundant fish and other fossils. These are easily split along the layers to reveal the fossils. This thin zone represents some 4000 years of deposition.