# Pierre Shale Canonical page: https://lithomap.app/resources/geologic-formations/united-states/pierre-shale 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 and central Great Plains Age: Late Cretaceous Age detail: Mostly Campanian to Maastrichtian; member ages vary across the Great Plains Depositional setting: Offshore Western Interior Seaway and its shifting margins Rock types: Shale, Marl, Siltstone, Sandstone, Bentonite States in recognized usage: Colorado, Kansas, Montana, Nebraska, North Dakota, South Dakota, Wyoming ## Summary The Pierre Shale is a vast Late Cretaceous marine succession of the Great Plains, known for gray shale, bentonite, fossil-bearing concretions, and pronounced engineering problems on clay-rich slopes. ## Distribution Pierre strata extend from the Dakotas and Montana through Wyoming, Nebraska, Colorado, and Kansas, both at the surface and beneath younger deposits. The formation can be hundreds to thousands of feet thick depending on basin position. Members differ across the region, and western equivalents become sandier or pass into other named units near ancient sediment sources. Central South Dakota terminology therefore should not be transferred unchanged to every basin. ## Geological history Marine mud accumulated far from shore while periodic currents supplied silt and sand. Volcanic ash from western eruptions settled into the seaway and altered into bentonite, forming distinctive beds used for correlation. Sea-level change moved shoreline facies across the basin. Limestone and iron-rich concretions grew within sediment during early burial, sometimes enclosing fossils before compaction flattened the surrounding mud. ## Fossil record Ammonites, baculites, inoceramid bivalves, oysters, fish, sharks, marine reptiles, and microscopic organisms occur in the Pierre. Fossil zones defined by rapidly changing cephalopods help date and correlate members. Concretions can preserve shells or bones in three dimensions, but many are unfossiliferous. Removing a concretion without recording its exact bed discards much of its scientific value. ## Minerals and geological resources Clay minerals make up much of the shale, accompanied by quartz, plagioclase, calcite, dolomite, and organic matter. Bentonite differs mineralogically from ordinary shale and commonly contains abundant smectite-group clay. Pyrite, siderite, gypsum, jarosite, zeolites, and iron-manganese oxides occur locally. Weathering sulfides can create acidic, salt-crusted surfaces, while swelling clay contributes to landslides and infrastructure damage. ## Field context - Pierre slopes are prone to slumping and can fail after prolonged rain or river erosion. - Dry bentonite may look firm but becomes exceptionally slick and adhesive when wet. - Fossil collecting rules depend on land ownership and fossil type; roadcuts are not automatically public. ## Connected field guides - Shale (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/shale - 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 - Dolomite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/dolomite - Gypsum (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/gypsum 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 - 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 - Composition and properties of the Pierre Shale — U.S. Geological Survey: https://www.usgs.gov/publications/composition-and-properties-pierre-shale-and-equivalent-rocks-northern-great-plains - Geolex Pierre references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/PierreRefs_9624.html ## Formation photograph - Image: https://lithomap.app/_delivery/formations/pierre-shale.webp - Description: Broken fossil-bearing concretion weathered from the Pierre Shale near Ekalaka, Montana - Creator: Wilson44691 - License: Public domain — https://creativecommons.org/publicdomain/mark/1.0/ - Source record: https://commons.wikimedia.org/wiki/File:BrokenConcretion22.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.