# Eau Claire Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/eau-claire-formation Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Midwest / Great Lakes Regional setting: Upper Mississippi Valley and northern interior basins Age: Late Cambrian Age detail: Late Cambrian; correlations and rank vary across the northern Midwest Depositional setting: Shallow marine shelf during a regional transgression Rock types: Fine sandstone, Siltstone, Shale, Dolostone States in recognized usage: Illinois, Iowa, Minnesota, Wisconsin ## Summary The Eau Claire Formation is a fine-grained Cambrian marine interval above the Mount Simon Sandstone, notable for trilobite-bearing beds and for acting as a regional groundwater confining unit. ## Distribution The Eau Claire is exposed near its Wisconsin type area and in a limited number of valleys, but most of its extent is known from wells across Wisconsin, Minnesota, Iowa, and Illinois. It changes laterally into more sandy or carbonate-rich equivalents. Because it is commonly buried, drill logs and regional hydrogeologic sections are central to mapping it. Formation, shale, and sandstone ranks have all appeared in historical usage. ## Geological history Fine sand, silt, clay, and carbonate accumulated as a shallow sea spread across the continent above coarser Mount Simon sand. Alternating energy and sediment supply produced thin beds, ripple structures, and shaly intervals. Its finer texture gives it lower permeability than adjacent sandstone aquifers over much of the region. Southward increases in dolomite and facies changes complicate correlations with neighboring basins. ## Fossil record Trilobites, brachiopods, and trace fossils occur in selected beds, and classic trilobite zones helped establish the unit’s Late Cambrian age. Fossiliferous horizons are separated by intervals with few visible remains. Body fossils are most likely in fine sandstone and shale deposited under quieter marine conditions; they should not be expected uniformly through the formation. ## Minerals and geological resources Quartz dominates sandy beds, with clay minerals, feldspar, calcite, and dolomite in finer or more calcareous intervals. Glauconite and iron-bearing minerals may occur locally. The formation’s principal regional resource significance is hydrogeologic: it restricts vertical groundwater movement between major sandstone aquifers where shale and siltstone are thick. ## Field context - Most mapped extent is subsurface, so surface appearance alone understates its regional importance. - Thin-bedded sandstone and shale can break into unstable plates along roadcuts. - Confirm local rank and equivalent-unit terminology before comparing well logs between states. ## 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 and mineral context) — https://lithomap.app/resources/rock-identification/siltstone - Dolostone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/dolostone - 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 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 - No directly overlapping state guide is currently published. ## Authoritative sources - Geolex Eau Claire references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/EauClaireRefs_1490.html - Northern Midwest aquifer hydrogeology — U.S. Geological Survey: https://www.usgs.gov/publications/hydrogeology-cambrian-ordovician-aquifer-system-northern-midwest-united-states-a ## Formation photograph - Image: https://lithomap.app/_delivery/formations/eau-claire-formation.webp - Description: Shale-rich Eau Claire Formation displayed in a deep Warren County drill core from Ohio - Creator: James St. John - License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ - Source record: https://commons.wikimedia.org/wiki/File:Eau_Claire_Formation_(Middle_to_Upper_Cambrian%3B_Warren_County_core%2C_Ohio%2C_USA)_1.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.