# Green River Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/green-river-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: Green River, Uinta, and Piceance basins Age: Eocene Age detail: Principally about 54–43 million years ago, varying by basin and member Depositional setting: Long-lived freshwater to saline lakes and their margins Rock types: Limestone, Dolostone, Mudstone, Sandstone, Organic-rich shale States in recognized usage: Colorado, Utah, Wyoming ## Summary The Green River Formation is the layered archive of large Eocene lake systems in Wyoming, Utah, and Colorado, preserving exceptional fossils, organic-rich mudrocks, and an unusual suite of minerals formed in changing lake water. ## Distribution Green River strata occupy several separate intermontane basins: the Greater Green River Basin in Wyoming, the Uinta Basin in Utah, and the Piceance Basin in Colorado. These basins contained related but distinct lakes, so rock character and member names differ among them. Some of the most accessible exposures rim the basins, while thick successions also continue underground. Fossil-bearing laminated limestone near Fossil Basin is only one expression of a formation that also includes deltaic sandstone, shoreline mudstone, carbonate beds, and deep-basin oil shale. ## Geological history Sediment accumulated as lake levels, salinity, sediment supply, and climate changed through time. Nearshore areas received sand and silt from rivers; quieter water preserved fine carbonate and organic material in thin layers. Evaporation in closed or restricted lake phases concentrated dissolved chemicals and produced distinctive authigenic minerals. Because the lakes evolved independently, a statement that is true for one basin or member may not apply to another. The formation is best understood as a family of lake deposits rather than a single identical layer spread across three states. ## Fossil record Fine-grained beds preserve fish, plants, insects, turtles, crocodilians, birds, bats, and other organisms. At Fossil Butte, rapid burial and quiet lake-bottom conditions produced especially detailed fossils, including complete fish and delicate leaves and insects. The fossil record varies sharply with depositional setting. Some central-lake layers are rich in articulated fossils, whereas saline or strongly organic intervals may record mainly microorganisms, pollen, or chemically stressful conditions. ## Minerals and geological resources Quartz, dolomite, and feldspar are widespread. USGS core studies also document calcite, clay minerals, analcime, albite, pyrite, marcasite, and basin-centered evaporite or authigenic minerals such as nahcolite and dawsonite. Mineral distribution is stratigraphically and geographically controlled. Nahcolite, for example, is an industrial soda-ash resource in parts of the Piceance Basin, not a blanket feature of every Green River outcrop. Organic-rich beds are also a major oil-shale resource, though oil shale is a rock resource rather than a mineral species. ## Field context - Thinly laminated fossil beds can split easily; collecting is prohibited at Fossil Butte National Monument. - Oil-shale slopes may be steep, loose, and hot, and old resource workings can be hazardous. - Check the basin and member name before comparing ages, fossils, or minerals between localities. ## Connected field guides - Shale (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/shale - Dolostone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/dolostone - Limestone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/limestone - Mudstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/mudstone - Sandstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/sandstone - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite 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 - Utah geology — https://lithomap.app/resources/geology/united-states/utah - Colorado geology — https://lithomap.app/resources/geology/united-states/colorado - Wyoming geology — https://lithomap.app/resources/geology/united-states/wyoming ## Authoritative sources - Green River Formation mineral occurrence data — U.S. Geological Survey: https://www.usgs.gov/data/mineral-occurrence-data-eocene-green-river-formation-piceance-and-uinta-basins - Distribution of mineral phases in the Green River Formation — U.S. Geological Survey: https://pubs.usgs.gov/publication/70210516 - Geolex Green River references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/GreenRiverRefs_8483.html ## Formation photograph - Image: https://lithomap.app/_delivery/formations/green-river-formation.webp - Description: Layered Green River Formation rising behind the historic Haddenham Cabin at Fossil Butte National Monument - Creator: Magicpiano - License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ - Source record: https://commons.wikimedia.org/wiki/File:FossilButteNM_HaddenhamCabin_(cropped).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.