# Hell Creek Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/hell-creek-formation Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Great Plains Regional setting: Northern Great Plains Age: Latest Cretaceous Age detail: Late Maastrichtian, immediately before the end-Cretaceous extinction Depositional setting: Rivers, channels, floodplains, ponds, and coastal lowlands Rock types: Sandstone, Siltstone, Claystone, Mudstone, Lignite States in recognized usage: Montana, North Dakota, South Dakota, Wyoming ## Summary Hell Creek records the last slice of dinosaur-dominated time in the northern Great Plains, when rivers crossed a low coastal plain east of the rising western mountains and near the retreating Western Interior Seaway. ## Distribution The formation is best exposed in eastern Montana and the western Dakotas, with recognized use extending into Wyoming. Badlands reveal stacked channels and floodplain deposits, while the unit continues beneath younger cover over a broader area. Its upper boundary lies near the Cretaceous–Paleogene transition, but local exposures can be incomplete. Correctly locating that boundary requires stratigraphic evidence; a color change or isolated fossil alone is not sufficient. ## Geological history Sandstone bodies mark former river channels and bars. Finer claystone and siltstone settled on floodplains and in abandoned channels, while thin lignite beds formed from plant material in wet, poorly drained settings. Bentonite records altered volcanic ash, and siderite concretions or nodules occur locally. The landscape shifted repeatedly as channels migrated. That patchwork explains why fossils can be dense at one horizon and scarce a short distance away: preservation depended on the local environment and the timing of burial. ## Fossil record Hell Creek is renowned for Tyrannosaurus, Triceratops, Edmontosaurus, and other dinosaurs. Its broader ecosystem also included fish, turtles, crocodilians, mammals, lizards, amphibians, mollusks, and an extensive plant record preserved as leaves, wood, pollen, and spores. Fossil assemblages extend close to the end-Cretaceous boundary, but the uppermost fine-grained beds can be poor in fossils where channel deposits are absent. Scientific surveys therefore distinguish ecological change from gaps caused by preservation and exposure. ## Minerals and geological resources Quartz and feldspar dominate many sandstones; clay minerals make up much of the finer rock. Calcite and iron-carbonate minerals can cement concretions, and siderite nodules are locally common. Altered ash contributes bentonitic clay. Thin lignite seams occur in some areas. Lignite is an organic sedimentary rock, not a mineral, but it is geologically important because it records wet, vegetation-rich environments and may influence groundwater chemistry and slope stability. ## Field context - Badlands erode quickly, so maps and exposures can change after storms. - Dinosaur and other vertebrate fossils on federal land may not be collected without the appropriate research permit. - Private ranchland is extensive throughout the region; visible outcrop does not imply public access. ## Connected field guides - Mudstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/mudstone - Sandstone (rock; Named rock type) — 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 - 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 - Wyoming geology — https://lithomap.app/resources/geology/united-states/wyoming - Montana geology — https://lithomap.app/resources/geology/united-states/montana ## Authoritative sources - Geolex Hell Creek unit record — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/Units/HellCreek_8598.html - Vertebrate biostratigraphy of the Hell Creek Formation — U.S. Geological Survey: https://www.usgs.gov/publications/vertebrate-biostratigraphy-hell-creek-formation-southwestern-north-dakota-and - Hell Creek depositional ages study — U.S. Geological Survey: https://www.usgs.gov/publications/a-case-study-maximum-depositional-ages-terrestrial-sandstones-near-cretaceous ## Formation photograph - Image: https://lithomap.app/_delivery/formations/hell-creek-formation.webp - Description: Eroded Hell Creek Formation badlands near Fort Peck Reservoir, Montana - Creator: Anky-man - License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ - Source record: https://commons.wikimedia.org/wiki/File:Hell_Creek.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.