Fossil Hunting in Wisconsin

Wisconsin’s Paleozoic bedrock records a long history of marine environments from the Cambrian through the Silurian. The state’s quarries, road cuts, and lakeshores offer accessible collecting for a wide range of marine invertebrate fossils, including some classic Cambrian trilobite localities.

Why Wisconsin?

Wisconsin’s bedrock tells a story of repeated marine transgressions across a stable continental shelf. Cambrian sandstones record the earliest flooding of the continent, Ordovician limestones and dolomites preserve a thriving marine ecosystem, and Silurian reef complexes document a tropical shallow sea teeming with corals and crinoids.

The Driftless Area of southwestern Wisconsin is especially important. This region was never glaciated, so bedrock is exposed at the surface in ways that are unusual for the upper Midwest. Road cuts and creek beds in the Driftless Area cut cleanly through Ordovician and Cambrian strata.

What You’ll Find

Cambrian fossils (~500-520 million years old):

• Trilobites — Dikelocephalus and Crepicephalus from the Tunnel City Group and related formations
• Brachiopods — Early inarticulate forms
• Trace fossils — Burrows and trackways in Cambrian sandstones

Ordovician marine fossils (~440-470 million years old):

• Brachiopods — Diverse assemblages in the Platteville and Galena formations
• Cephalopods — Straight-shelled nautiloids, sometimes large
• Bryozoans — Colonial forms in Galena Dolomite
• Trilobite fragments — Less common than in Ohio but present in the right beds
• Receptaculitids — “Sunflower coral” fossils, actually an enigmatic organism

Silurian fossils (~420-440 million years old):

• Tabulate corals — Halysites (chain coral) and Favosites (honeycomb coral) are iconic Wisconsin Silurian fossils
• Rugose corals — Both solitary and colonial forms
• Crinoid stems — Often large and well-preserved in Niagara Dolomite
• Trilobites — Calymene (the classic “rolled-up trilobite”) from Silurian quarries
• Brachiopods — Pentamerus and other Silurian genera

Best Time to Visit

• Spring (April-June): Snowmelt and spring rains expose fresh fossils on road cuts and shorelines
• Late summer/early fall: Lake Winnebago water levels may be low, exposing more shoreline
• After storms on Lake Winnebago: Wave action washes fossils out of dolomite ledges onto beaches

Tips for Wisconsin Collecting

1. Walk the Lake Winnebago shoreline — This is the easiest and most productive public collecting in the state; Silurian corals and brachiopods litter the beach after storms
2. Ask permission at quarries — Milwaukee-area Silurian quarries are legendary for Calymene trilobites, but access must be arranged through the operators
3. Explore the Driftless Area — Road cuts in Grant, Iowa, and Crawford Counties expose fossil-rich Ordovician beds that are easy to access
4. Look for receptaculitids — These distinctive “sunflower” fossils are a unique Wisconsin find that most visitors overlook
5. Bring a chisel for dolomite — Silurian dolomite is hard but splits along bedding planes to reveal well-preserved fossils

Collecting Sites

Legal Rules for Wisconsin

Key Geological Formations

• Galena Dolomite (Upper Ordovician)
• Platteville Formation (Middle Ordovician)
• Niagara Dolomite (Silurian)
• Tunnel City Group (Upper Cambrian)

Local Fossil Clubs & Museums

• Milwaukee Gem and Mineral Society
• Badger Lapidary and Geological Society
• University of Wisconsin Geology Museum
• Weis Earth Science Museum