Florissant Fossil Beds National Monument, Colorado
The Florissant Fossil Beds in eastern Colorado are a six thousand acre prairie and forest preserve overlaying multiple deposits of late Eocene-era fossils. Over fifty thousand high quality samples have been uncovered here from seventeen hundred different species. Largely a temperate valley 34 million years ago, the land was flooded suddenly by lahars – mudflows from a nearby volcanic eruption – burying massive redwood trees as well as the smaller plants and trapping mammals. As eruptions continued into the Miocene era, later lahars dammed a stream forming Lake Florissant and depositied ash, pumice, and clay into the lake bed; as this fine grained sediment accumulated and solidified into shale it preserved smaller marine mammals and aquatic plants. A fossil of a tsetse fly – the modern equivalent of which is only found in tropical Africa – was even found in the shale layers here. The tsetse as well as many of the other more delicate molds have been excavated and removed to permanent archaeological collections but numerous petrified redwood stumps remain in situ on the prairie surface.