Florissant Fossil Beds

The Florissant Fossil Beds, formally the Florissant Formation, is a Late Eocene sedimentary formation around Florissant in Teller County, Colorado, dated by argon radiometric methods to roughly 34 million years old, within a range of about 37.2 to 33.9 million years. It consists mainly of shale and mudstone about 74 meters thick, including fine, paper-like shale layers of interbedded clay, mud and volcanic ash that formed in an ancient lake; the exceptional preservation of its fossils is attributed to volcanic ash from the nearby Thirtynine Mile volcanic field triggering diatom blooms in the lake. The formation preserves over 1,500 species of insects and spiders, numerous fossil plants, and roughly 30 petrified redwood stumps of Sequoia affinis, and much of the site is protected as Florissant Fossil Beds National Monument, established in 1969.