By Doug Nichols
The previous “Our Natural World” column covered the origin of the 34-million-year-old lake deposits at Florissant Fossil Beds National Monument and the legislative struggle to have the site preserved for the public as part of the National Park system. This column provides some details about the fossils present in this unique area.
Most prominent are enormous petrified trunks of prehistoric sequoia trees, relatives of the giant redwoods of California. They were buried in mudstone at the bottom of ancient Lake Florissant, which developed after a lava flow from a nearby volcano dammed a stream flowing through the valley now occupied by the fossil beds.
Layers of fine mud and volcanic ash accumulated in the lake forming paper-thin shale. The shale preserves twigs of these great trees with characteristic scale-like leaves and small spheroidal cones that testify to their botanical affinity. A number of the buried stumps have been partially excavated by the National Park Service and can be seen along trails on monument grounds. A favorite is the Big Stump, which is 11.6 meters (38 ft) in circumference. Embedded in the top of the Big Stump are two rusting saw blades, left there by early visitors to the site (before it became a National Monument) who tried unsuccessfully to cut apart the solid stony mass of this gigantic fossil. Many other stumps can be seen along the trails, and numerous others are still buried, their presence marked by fragments of petrified wood at the surface.
Not exposed along the trails, but on display at the visitor’s center, are beautifully preserved leaves of a wide variety of plants that grew in the forest along with the sequoias. Hundreds of species have been identified. Studies of the fossil flora began in the late 1800s as paleobotanists came to area to dig into the lakebed deposits. Many of these fossils now reside in collections of major museums around the country. Digging and collecting are no longer allowed within the monument, but a small commercial quarry is in operation in the town of Florissant just north of the monument. For a fee, visitors can take chunks of shale and split them apart to reveal not only fossil leaves, but also delicately preserved insects.
The fossil insects of Florissant are world-famous for their quality and variety. They include a paper wasp (the symbol of Florissant Fossil Beds National Monument), fossil lacewings, mayflies, damselflies, grasshoppers, termites, cicadas, beetles, and even a tsetse fly (a relative of the living tsetse flies of Africa). Florissant is especially well known for its fossil butterflies, some of which still show faded color patterns on their wings. These insects flew or crawled about in the forest that grew on the slopes above Lake Florissant or along its shores. Insects and leaves that fell into the lake were preserved by volcanic ash that fell intermittently over the area in latest Eocene time.
To visit the National Monument, take I-25 south to Colorado Springs, then U.S. 24 west through Woodland Park to Florissant.
Doug Nichols is a Scientist Emeritus with the U.S. Geological Survey and a Research Associate with the Denver Museum of Nature & Science. He is a resident of Berthoud.
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<p>Big Stump (fossil sequoia)</p>