# Morrison Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/morrison-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: Western Interior and Colorado Plateau Age: Late Jurassic Age detail: About 157–150 million years ago Depositional setting: River channels, floodplains, lakes, and wetlands Rock types: Mudstone, Sandstone, Siltstone, Limestone, Bentonitic clay States in recognized usage: Arizona, Colorado, Kansas, Montana, New Mexico, Oklahoma, South Dakota, Utah, Wyoming ## Summary A broad sheet of continental sediment famous for Late Jurassic dinosaurs, the Morrison records shifting rivers and floodplains across much of the western interior before the modern Rocky Mountains existed. ## Distribution The Morrison is exposed across a wide belt of the western United States. Its best-known outcrops lie on the Colorado Plateau and along the flanks of Rocky Mountain uplifts, but equivalent named Morrison rocks extend much farther across the interior. Its appearance changes from place to place. Resistant sandstone ledges commonly mark former river channels, while softer, colorful mudstones represent floodplain soils, ponds, and low-energy basins. Member names also vary regionally, so a local geologic map is essential for precise identification. ## Geological history Morrison sediment accumulated in a continental landscape crossed by rivers. Sand and gravel settled in channels; silt and clay spread across floodplains; and carbonate mud formed in some lakes and wetlands. Volcanic ash altered into swelling clay in portions of the unit, especially the Brushy Basin Member. The formation is not one uniform bed. Its members intertongue, pinch out, and change composition across hundreds of miles. Those changes let geologists reconstruct drainage patterns, volcanic influence, soil formation, and climate during the closing part of the Jurassic. ## Fossil record The Morrison preserves one of North America’s most celebrated dinosaur records. Documented animals include large sauropods, stegosaurs, meat-eating theropods, and smaller plant-eating dinosaurs, alongside turtles, crocodile relatives, fish, amphibians, mammals, freshwater mollusks, and insects. Bones are not evenly distributed. Channel sandstones and rapidly buried deposits may concentrate and protect remains, while broad mudstone intervals can be comparatively sparse. A formation-level identification therefore predicts geological context, not the presence of a fossil at a particular spot. ## Minerals and geological resources Common rock-forming minerals include quartz and feldspar in sandstone, calcite in limestone and cement, and clay minerals in mudstone. Bentonite-rich intervals contain abundant altered volcanic material, commonly dominated by smectite-group clay. Parts of the Morrison on the Colorado Plateau host localized uranium–vanadium mineralization, historically important in several mining districts. That regional association does not mean every exposure is mineralized or safe to enter; old workings can contain unstable ground, radiation hazards, and private claims. ## Field context - Swelling clay can become exceptionally slick when wet and can form deeply rutted roads. - Fossils on federal land are protected under different rules depending on type; vertebrate fossils generally require a research permit. - Use a detailed local map because member names, contacts, and colors change across the formation’s large range. ## Connected field guides - Limestone (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/limestone - Mudstone (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/mudstone - Sandstone (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/sandstone - Siltstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/siltstone - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Feldspar (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/feldspar 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 - Arizona geology — https://lithomap.app/resources/geology/united-states/arizona - Wyoming geology — https://lithomap.app/resources/geology/united-states/wyoming - Montana geology — https://lithomap.app/resources/geology/united-states/montana - Kansas geology — https://lithomap.app/resources/geology/united-states/kansas ## Authoritative sources - The Morrison Formation — National Park Service: https://www.nps.gov/subjects/fossils/the-morrison-formation.htm - Morrison Formation at Dinosaur National Monument — National Park Service: https://home.nps.gov/dino/learn/nature/morrison-formation.htm - Geolex Morrison references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/MorrisonRefs_9394.html ## Formation photograph - Image: https://lithomap.app/_delivery/formations/morrison-formation.webp - Description: Distinctively banded Morrison Formation outcrop in Dinosaur National Monument, Utah - Creator: Michael Overton - License: CC BY-SA 2.5 — https://creativecommons.org/licenses/by-sa/2.5/ - Source record: https://commons.wikimedia.org/wiki/File:Green_River_UT_2005-10-14_2104.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.