# Fox Hills Sandstone

Canonical page: https://lithomap.app/resources/geologic-formations/united-states/fox-hills-sandstone
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 and adjacent basins
Age: Latest Cretaceous
Age detail: Late Campanian to Maastrichtian, depending on basin and mapped interval
Depositional setting: Retreating-sea shorelines, barrier islands, tidal inlets, and shallow marine shelves
Rock types: Sandstone, Siltstone, Shale, Coquina
States in recognized usage: Colorado, Montana, North Dakota, South Dakota, Wyoming

## Summary

The Fox Hills Sandstone records the final retreat of the Western Interior Seaway across the northern Great Plains in shoreface, barrier-island, and tidal deposits rich in marine traces and shells.

## Distribution

Fox Hills strata extend through parts of the Dakotas, Montana, Wyoming, and Colorado, commonly between offshore Pierre Shale and younger continental deposits. They form sandy ridges and ledges where exposed and continue beneath broad areas of younger cover.

The transition into adjacent formations is commonly intertonguing rather than a single sharp surface. Thickness, member names, and even the precise time represented change between the Williston, Powder River, Denver, and other basins.

## Geological history

As the Western Interior Seaway withdrew, sand accumulated on beaches, shorefaces, barrier islands, tidal deltas, and inlets. Mud remained in quieter offshore and lagoonal settings, producing repeated sandstone-to-shale transitions.

Wave and tidal structures, coarsening-upward beds, burrows, and shell concentrations reveal moving shorelines. Later groundwater made calcite- or iron-cemented concretions and influenced the unit’s aquifer properties.

## Fossil record

Oysters and other marine mollusks can form dense shell beds, while ammonites, shark teeth, and varied trace fossils occur in suitable facies. Burrows such as Ophiomorpha are especially characteristic of energetic sandy shore environments.

Fossil content differs by shoreline position. A clean upper shoreface sandstone may hold mainly traces, whereas tidal-channel or offshore beds can concentrate shells and other remains.

## Minerals and geological resources

Quartz dominates the sandstone, accompanied by feldspar, clay minerals, mica, and heavy-mineral grains. Calcite and iron oxides locally cement beds and concretions.

The formation can be an aquifer and has petroleum relevance in some basins, but water quality and reservoir behavior vary with depth, cementation, and interbedded shale.

## Field context

- Intertonguing contacts require a measured section or detailed map to locate confidently.
- Shell beds and concretions can weather out below unstable sandstone ledges.
- Much of the outcrop belt crosses private ranchland; obtain permission before entry.

## Connected field guides

- Sandstone (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/sandstone
- Shale (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/shale
- 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

- 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

## Authoritative sources

- Geolex Fox Hills references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/FoxHillsRefs_8187.html
- Fox Hills stratigraphy and depositional environments — U.S. Geological Survey: https://pubs.usgs.gov/pp/1532/report.pdf
- Fox Hills hydrogeologic characteristics — U.S. Geological Survey: https://www.usgs.gov/publications/geologic-and-hydrogeologic-characteristics-white-river-formation-lance-formation-and

## Formation photograph

- Image: https://lithomap.app/_delivery/formations/fox-hills-sandstone.webp
- Description: Sandstone-capped ridge of the Fox Hills Formation in eastern Colorado
- Creator: IveGoneAway
- License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/
- Source record: https://commons.wikimedia.org/wiki/File:Fox_Hills_Formation_01.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.