Florissant Fossil Beds National Monument is a national monument located in Colorado. Famous for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales of the Florissant Formation, the formation is Eocene (approximately 34 million years old) in age and has been interpreted as a lake environment. Fossils are preserved because of the interaction of the volcanic ash from the nearby Thirtynine Mile volcanic field with diatoms in the lake.
