# Florissant Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/florissant-formation Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Colorado Plateau / Rockies Regional setting: Central Colorado volcanic highlands Age: Late Eocene Age detail: About 34 million years ago Depositional setting: Freshwater lake, streams, and a volcanically active valley Rock types: Shale, Mudstone, Tuff, Volcaniclastic conglomerate States in recognized usage: Colorado ## Summary The Florissant Formation records a high-elevation Eocene lake periodically disturbed by volcanic eruptions, with fine lake beds preserving insects and leaves while volcanic mudflows buried enormous redwood relatives. ## Distribution The formation is centered on the Florissant basin west of Colorado Springs. Its celebrated exposures are protected within Florissant Fossil Beds National Monument, and the geographically limited unit is far smaller than continental-scale formations such as the Morrison. Lake shales, volcanic ash, stream deposits, and coarse volcanic debris occur in a compact basin. Their arrangement records a landscape repeatedly changed by eruptions from the nearby Thirtynine Mile volcanic field. ## Geological history A freshwater lake occupied the valley while streams supplied sediment and nearby volcanoes supplied ash and debris. Fine mud settled through quiet water in thin layers. At other times, lahars—fast volcanic mudflows—swept into the valley and buried trees and older sediment. Volcanic ash could trigger algal blooms and changes in lake chemistry that helped preserve delicate organisms. The formation therefore links ordinary lake sedimentation with episodic volcanic disturbance rather than representing a single catastrophic burial. ## Fossil record Florissant is famous for detailed insects, spiders, leaves, seeds, pollen, fish, birds, and mammals. The lake beds preserve delicate wings and leaf venation, while upright petrified stumps record huge trees related to modern redwoods. The assemblage provides an unusually broad view of a late Eocene ecosystem. Fossils document both aquatic and terrestrial organisms, allowing scientists to reconstruct vegetation, climate, food webs, and environmental change. ## Minerals and geological resources Volcanic ash and tuff contain fine quartz and feldspar, with clay minerals produced as ash altered after deposition. Silica played a major role in petrifying wood, and iron minerals contribute color to weathered beds. The formation is scientifically important for fossils rather than as a collecting district. Mineral grains are commonly microscopic or locked within soft tuff and shale, and all natural materials are protected inside the national monument. ## Field context - Fossil and mineral collecting is prohibited within Florissant Fossil Beds National Monument. - Soft shale can crumble quickly after exposure; scientific excavation and conservation are specialized work. - Do not confuse the Eocene formation with much younger Ice Age deposits that also occur in the area. ## Connected field guides - Shale (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/shale - Conglomerate (rock; Named rock type) — https://lithomap.app/resources/rock-identification/conglomerate - Mudstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/mudstone - Feldspar (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/feldspar - Quartz (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/quartz Mapped location and formation context support an identification but cannot establish one. Compare several physical properties and use laboratory analysis when the material remains ambiguous. ## Connected state geology guides - Colorado geology — https://lithomap.app/resources/geology/united-states/colorado ## Authoritative sources - Florissant geologic trail — National Park Service: https://www.nps.gov/flfo/learn/geologic-trail.htm - Florissant Fossil Beds frequently asked questions — National Park Service: https://www.nps.gov/flfo/faqs.htm ## Formation photograph - Image: https://lithomap.app/_delivery/formations/florissant-formation.webp - Description: Paper shale exposure of the Florissant Formation at a fossil quarry in Colorado - Creator: Kent G. Budge - License: CC0 1.0 — https://creativecommons.org/publicdomain/zero/1.0/ - Source record: https://commons.wikimedia.org/wiki/File:Florissant_Formation.jpg Formation-level information does not prove that a fossil or mineral occurs at a specific coordinate. It also does not establish ownership, access, collecting permission, present conditions, or specimen identity.