# Ogallala Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/ogallala-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: High Plains east of the Rocky Mountains Age: Miocene to Pliocene Age detail: Principally late Miocene to early Pliocene, varying across the High Plains Depositional setting: Braided rivers, alluvial plains, soils, and local lakes Rock types: Sand, Gravel, Silt, Clay, Sandstone, Caliche States in recognized usage: Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, Wyoming ## Summary The Ogallala Formation is a widespread apron of river-borne sediment shed from the Rocky Mountains, forming much of the High Plains and containing the principal geologic material of the High Plains aquifer. ## Distribution Ogallala deposits underlie or crop out across a broad belt from South Dakota and Wyoming to Texas and New Mexico. They cap tablelands, fill older valleys, and form the resistant rim of escarpments where caliche or cemented sandstone protects softer sediment. The formation is highly variable because streams occupied different routes through time. A nearby exposure may shift from loose sand and gravel to cemented sandstone, silt, volcanic ash, or carbonate-rich soil horizons. ## Geological history East-flowing river systems carried sediment from the uplifting Rocky Mountains and spread it across preexisting valleys and plains. Repeated channel migration built overlapping bodies of gravel, sand, silt, and clay. Soil formation and groundwater evaporation precipitated calcium carbonate as caliche, especially near the top. Some sediment became cemented into resistant rock, while much remained only weakly consolidated and now serves as an important aquifer. ## Fossil record Fossil mammals, tortoises, plants, seeds, and freshwater organisms occur in selected intervals. Horse, camel, rhinoceros, proboscidean, and other vertebrate remains help document changing Great Plains environments. Fossil seeds have been especially useful for regional correlation, but fossil-bearing horizons are discontinuous. Reworked older shells or rock fragments in stream gravel should not be mistaken for Ogallala-age organisms. ## Minerals and geological resources Quartz and feldspar dominate much sand and gravel, with calcite concentrated in cement and caliche. Clay minerals occur in fine sediment, while volcanic ash contributes glass and altered material in some beds. Silica can form opaline cement or hard quartz-rich lenses. Gravel also contains many recycled rock types from the mountains, so an individual pebble may be far older than the formation carrying it. ## Field context - Loose sand and gravel can collapse in steep cuts or excavations. - The aquifer is a water-bearing system, not an underground lake, and saturated thickness varies greatly. - Caprock blocks can detach above softer slopes, especially along escarpments and reservoir shorelines. ## Connected field guides - 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 - 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 - Wyoming geology — https://lithomap.app/resources/geology/united-states/wyoming - Kansas geology — https://lithomap.app/resources/geology/united-states/kansas ## Authoritative sources - High Plains aquifer and Ogallala Formation — U.S. Geological Survey: https://pubs.usgs.gov/ha/ha730/ch_d/D-text2.html - Geolex Ogallala references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/OgallalaRefs_9656.html - Ogallala geology and hydrogeology in Wyoming — U.S. Geological Survey: https://www.usgs.gov/publications/geologic-and-hydrogeologic-characteristics-ogallala-formation-and-white-river-group ## Formation photograph - Image: https://lithomap.app/_delivery/formations/ogallala-formation.webp - Description: Ogallala Group rocks exposed at Ash Hollow State Historical Park, Nebraska - Creator: Chris Light - License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ - Source record: https://commons.wikimedia.org/wiki/File:Ash_Hollow_Nebraska.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.