Green River Formation
another area, in Fossil Butte National Monument, Lincoln County, Wyoming is Fossil Lake. It has exceptionally well preserved fish fossils. The most productive zone — the split fish layer — is a series of laminated or varved lime muds about 6 ft (1.8 m) thick. They contains many fish and other fossils. The rock is easily split along the layers to reveal the fossils. This thin zone took about 4000 years to deposit.