Fossil Hunting in Iowa

Iowa may be known for its cornfields, but beneath those fields lies a remarkable seafloor. During the Devonian and Mississippian periods, Iowa sat near the equator beneath warm, shallow seas brimming with corals, crinoids, and brachiopods. Today, those ancient ocean floors are exposed in quarries, river bluffs, and one of the most extraordinary flood-scoured fossil pavements in the country.

Why Iowa?

Iowa occupies a special place in paleontology for two reasons. First, the Devonian Fossil Gorge at Coralville Lake is one of the most visually stunning fossil sites in the Midwest — a vast bedrock surface carpeted with corals, brachiopods, and crinoids exposed by catastrophic flooding. Second, the Burlington Limestone of southeastern Iowa is the type locality for one of the most crinoid-rich formations on Earth. The town of Burlington has been synonymous with crinoid paleontology since the mid-1800s.

For beginners, Iowa is an ideal introduction to Paleozoic marine fossils. The specimens are abundant, easy to identify, and well exposed. You do not need to be an expert to come home with brachiopods, crinoid stems, and horn corals.

The state also offers excellent Ordovician exposures in the northeast (the Maquoketa Formation), and the Keokuk and Warsaw formations of the southeast produce beautiful quartz-lined geodes alongside marine fossils.

What You’ll Find

Devonian marine fossils (~375-385 million years old):

• Colonial corals — Hexagonaria (“Petoskey stone” type), Favosites (honeycomb coral), chain corals
• Horn corals (Rugosa) — Solitary corals, abundant and often well preserved
• Brachiopods — Atrypa, Strophodonta, and many other genera
• Crinoid stems — Disc-shaped columnals scattered everywhere in limestone
• Bryozoans — Lacy and branching forms
• Gastropods — Snail shells in limestone matrix
• Trilobites — Phacops and other genera (less common but present)

Mississippian marine fossils (~325-345 million years old, Burlington/Keokuk formations):

• Crinoid stems and calyxes — Burlington Limestone is one of the densest crinoid accumulations known; some beds are essentially solid crinoid hash
• Blastoids — Star-shaped echinoderms related to crinoids
• Brachiopods — Productid and spiriferid brachiopods
• Geodes — Hollow Keokuk Formation concretions lined with quartz crystals

Ordovician fossils (northeastern Iowa, ~445 million years old):

• Brachiopods — Various genera in the Maquoketa Formation
• Bryozoans — Abundant in shale exposures
• Cephalopods — Straight-shelled nautiloids

Best Time to Visit

• Spring (April-May): Snowmelt and spring rain expose fresh material in creek beds and road cuts
• Summer (June-August): Ideal for wading in streams; Devonian Fossil Gorge is most accessible
• Fall (September-October): Low water levels in streams expose gravel bars; pleasant field conditions
• After flooding: Major flood events, like those in 1993 and 2008, can dramatically expose new fossil surfaces (though these are rare)

Tips for Iowa Collecting

1. Visit Devonian Fossil Gorge first — the exposed fossil pavement below Coralville Dam is an unforgettable classroom; interpretive signs help you identify what you are seeing
2. Learn to spot crinoid stems — these disc-shaped segments with a hole in the center are the most common Iowa fossil and your gateway to recognizing other specimens
3. Check creek beds — Iowa’s many streams cut through Paleozoic limestone, tumbling fossils into gravel where they are easy to pick up
4. Split limestone slabs — brachiopods and corals are often revealed when you hammer along bedding planes
5. Visit Burlington for crinoids — Starrs Cave Park and stream exposures around the city are the type locality for world-class crinoid material
6. Collect geodes too — southeastern Iowa’s Keokuk Formation geodes are found in the same creeks as marine fossils; look for round, rough-surfaced rocks
7. Bring a bucket and newspaper — corals and crinoid calyxes can be fragile; wrap carefully for transport
8. Stop at the University of Iowa Museum of Natural History — Iowa City’s museum provides excellent context for the state’s Paleozoic marine life

Collecting Sites

Legal Rules for Iowa

Key Geological Formations

• Cedar Valley Group (Middle Devonian)
• Burlington Limestone (Lower Mississippian)
• Maquoketa Formation (Upper Ordovician)
• Lime Creek Formation (Upper Devonian)

Local Fossil Clubs & Museums

• Iowa Geological Survey
• University of Iowa Museum of Natural History
• Cedar Valley Rocks and Minerals Society
• Blackhawk Gem and Mineral Society
• Burlington Geological Society