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Where is the Eau Claire Formation?
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.
How did it form?
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.
Compare observable anatomy in the fossil identification guides, then review the U.S. fossil collecting-law guide before disturbing any find. A formation-level record does not prove a fossil is present, exposed, common, or legal to collect at a particular place.
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.
Identify rocks and minerals named in this guide
Use physical properties to investigate materials discussed above. A formation name or mapped location supports context, but it cannot identify a hand sample by itself.
What to know in the field
- 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.
Formation, fossil, and mineral information is educational. It does not establish land ownership, legal access, collecting permission, specimen identity, or present-day field conditions.
Authoritative sources
This guide synthesizes the following sources in original language. Consult the linked records for local stratigraphic detail and the underlying scientific literature.
- Geolex Eau Claire references U.S. Geological Survey
- Northern Midwest aquifer hydrogeology U.S. Geological Survey
Editorial review: September 16, 2026.