The formation is noted for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales.

Based on argon radiometric dating, the formation is Eocene (approximately 34 million years old [4]) in age and has been interpreted as a lake environment.

[7] The formation was first formally named as the Florissant Lake Beds by Charles Whitman Cross in 1894.

[8] In 1969, the Florissant Fossil Bed National Monument was established after a long legal battle between local land owners and the federal government.

Today, the park receives approximately 60,000 visitors a year, and is the site of ongoing paleontological investigations.

[10] In the late Eocene to early Oligocene, approximately 34 million years ago, the area was a lake environment with redwood trees.

[11] The massive unconformity is due to erosion that occurred during the uplift of the modern Rocky Mountains, the Laramide Orogeny.

The Florissant Formation itself is composed of alternating units of shale, mudstone, conglomerate, and volcanic deposits.

[6] The Laramide Orogeny, which created the modern Rocky Mountains, had been uplifting the area to the west since the end of the Cretaceous,[13] although the exact timing of the orogeny is debated [12] In the late Eocene to the Early Oligocene, volcanic episodes began to occur to the southwest of the Florissant area.

The fossil bearing paper shales are intercalated with larger deposits of volcanic material.

[15][16] Most of the remaining units are composed of clasts of weathered Pikes Peak Granite, volcanics, and mud that were transported by streams that flowed through the area.

The ash that settled created the tuff, and the lahars formed the mudstones and the conglomerates of the Florissant formation.

[16] The volcanic material that caused so much destruction led to the preservation of the fossils within the Florissant Formation's shales and mudstones.

The lahars then covered the base of the redwoods that were living at the time, and the trunks of the trees became harder and fossilized.

As the population of the diatoms massively increased, the stress from the volcanic episodes at the same time caused large die-offs of the local biota.

This process was repeated often, possibly yearly, as the runoff from rain collected in the lake, causing cyclical diatom blooms and die-offs.

Each microlayer of ash and clay was compacted by overlaying sediments to create "paper shales" (usually between 0.1 and 1.0 mm thick).

Based on this information, it has been estimated that the lake could have lasted 2,500 to 5,000 years, if the diatom couplets represent annual cycles.

[citation needed] The majority of the stumps have been identified as belonging to Sequoia affinis, a close relative of the modern coast redwood (S.

There are also specimens of fruits, seeds, cones, and flowers, which are all preserved in the paper shales of the formation.

The great preservation of these animals gives insight into the environment that they survived in, as well as some aspects of their behavior.

The invertebrate fossils of the Florissant are arthropods, such as spiders, millipedes, insects, and ostracods; and mollusks such as clams and snails.

Mayflies, dragonflies, damselflies, grasshoppers, crickets, katydids, cockroaches, termites, earwigs, web-spinners, cicadas, snake flies, lacewings, beetles, flies, mosquitoes, butterflies, moths, wasps, bees, ants, and other insects have all been found in the Florissant.

These fossils are aquatic and terrestrial insects, giving insight into the ecology and biology of the area.

Other birds were described in early literature from the 19th century, but the exact identification of these samples cannot be done due to lack of information.

The largest mammal that has been discovered so far is a brontothere, an elephant-sized animal with a pair of thick horns on the top of its head.

The toxicity of the water due to volcanic activity could be to blame, but there are documented cases of aquatic animals in the lake.

[6] Fossil plants, and in particular their leaves, have been the most useful sources of information of paleoclimate during the time of deposition of the Florissant Formation.

[24] Comparing fossil plants and leaves to modern analogs enables inferences about the climate to be made based on physiological and morphological similarities.

[16] Based on the small size and the features of the teeth, the precipitation during the late Eocene to early Oligocene has been estimated to be around 50-80 centimeters per year, with a distinct dry season.