Fossil nerds unite! Get sedimental at Mazon Creek

The Mazon Creek fossil beds, situated across Grundy, Will, Kankakee, and Livingston counties in northeastern Illinois, are renowned worldwide for their exceptionally well-preserved plant and animal fossils encased in ironstone (siderite) concretions within the Francis Creek Shale. These concretions are formed from hard, round rocks that often contain fossils inside ancient clay layers. Most fossils are recovered from natural outcrops, abandoned strip‑mine spoil piles, and stream exposures in this region.

The site is prized globally as a Lagerstätte, where both soft-bodied and complex‑part organisms from approximately 300 million years ago remain remarkably preserved. Over 400 plant species from at least 130 groups of species and more than 320 animal species, including soft-bodied creatures, have been identified.

Amateur and guided fossil hunts usually take place at old mining sites where fossil-filled rocks have been exposed by weather. Hunters search through piles and creek banks for concretions, many of which can be cracked open to reveal internal fossils. A Chicago Tribune story describes trips organized by the Field Museum, where participants have the opportunity to uncover ancient specimens firsthand.

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The plant fossils from Mazon Creek, dominated by ferns, pteridosperms (extinct seed plants resembling ferns), and sphenopsids (ancient plants related to modern horsetails), provide a vivid window intoPennsylvanian ecosystems. The flora included giant scale trees (lycopsids), horsetail relatives (sphenopsids), seed ferns, and cordaites (tall, tree-like plants with long, strap-shaped leaves), which contributed to the formation of Illinois’ coal beds.

Animal fossils include diverse invertebrates, fish, amphibians, and the famous “Tully Monster” (Tullimonstrum gregarium), the only known soft-bodied vertebrate of its kind and Illinois’ state fossil, found exclusively at Mazon Creek.

How to Participate

Plan through the Field Museum – While the Field Museum doesn’t facilitate trips directly, its scientists and staff often collaborate with ECSOX, researchers, and volunteers in community science efforts. Stay tuned to its Community Science pages or newsletters, future collaborations or outreach events supporting fossil collecting may be announced.

The Earth Science Club of Northern Illinois (ESCONI) hosts the only confirmed fossil‑collectingfield trips. Contact ESCONI directly to ask if they plan to add fall or late‑summer field outings for 2025

Keep an eye on the I&M Canal National Heritage Area events page,which will have future New Collectors Day fossil tours if and when they resume.

Collecting is allowed primarily from spoil-pile areas in the Francis Creek Shale, provided a free day-use permit is obtained from the Illinois Department of Natural Resources. Permits are required, and collecting must follow regulations outlined by the Illinois State Museum and the Illinois Department of Natural Resources. The permit allows individuals to gather up to one five-gallon bucket of fossil-bearing rocks per day from the surface; digging is not allowed. Collecting is permitted only between March 1 and Sept. 30, and hobbyists must complete a short form documenting what they’ve taken. The process is simple, open to the public, and ensures that this remarkable site remains protected for future generations.

1. What to Bring: Bring tools for prying open concretions (freeze-thaw or soaking methods help), gloves, sturdy boots, and a permit. Come prepared for uneven, potentially muddy terrain.

2. Learn from Experts:  The Field Museum’s collections include materials collected over decades by researchers such as George Langford and Dr. Adolf Carl Noë, as well as earlier paleobotanists. Noé and Langford’s efforts in assembling extensive plant fossil collections helped make the museum a center of Mazon Creek research.

Whether you're new to fossil hunting or already captivated by Earth’s deep history, Mazon Creek offers a hands-on, scientifically rich adventure straight from the Carboniferous past.