The Florissant Fossil Beds National Monument is a national monument located in Teller County, Colorado. The location is famous for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales of the Florissant Formation. Based on argon radiometric dating, the formation is Eocene (approximately 34 million years old ) in age and has been interpreted as a lake environment. The fossils have been preserved because of the interaction of the volcanic ash from the nearby Thirtynine Mile volcanic field with diatoms in the lake, causing an algal bloom. As the algae fell to the bottom of the lake, any plants or animals that had recently died were preserved by the algal mats. Fine layers of clays and muds interspersed with layers of ash form "paper shales" holding beautifully-preserved fossils.