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Mazonia-Braidwood State Fish & Wildlife Area (Pit 11): Fossil Hunting in Grundy County, Illinois

Updated July 2026

The Mazonia unit preserves the reclaimed spoil of the old Pit 11 coal strip mine, the source of the world-famous Mazon Creek fossil biota and its soft-bodied preservation inside siderite (ironstone) concretions. This is the type ground of the Tully Monster, Tullimonstrum gregarium, named the Illinois state fossil in 1989. It is one of the very few public sites in Illinois where collecting is expressly allowed, under a free permit from the Illinois Department of Natural Resources. Reported finds include concretions with fossils, siderite, tully monster, fern fossils. Below: coordinates, access notes, nearby spots, and trip-planning links.

Mazonia-Braidwood State Fish & Wildlife Area (Pit 11) photos

Public image records connected to this spot or its reported material.

1 photo

Fossiliferous ironstone (siderite) concretion holding a fern fossil, of the type collected from the Mazon Creek beds at the Pit 11 / Mazonia-Braidwood area, Grundy County, Illinois

Quick details

Access: Public area
County: Grundy County
State: Illinois
Nearest road: Illinois Route 53
Postcode: 60408
Coordinates: 41.19750, -88.27080

Geology at Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)

This spot sits on Carbondale Formation, mapped as shale, sandstone, and coal that formed roughly 315 to 305 million years ago. Sedimentary layers like these are where fossils are typically preserved, and fresh exposures after rain or in active cuts tend to produce best.

Mapped unit: Carbondale Formation
Age: 315–305 Ma (Pennsylvanian)
Rock types: shale, sandstone, and coal
Elevation: 590 ft

Geologic mapping via Macrostrat and the underlying state geological survey maps (CC BY 4.0). Bedrock at the surface can vary within a mapped unit.

Best time to visit Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)

Best months: March to November. Typical highs run 84°F in midsummer and 31°F in midwinter, with winter lows near 14°F.

Climate normals: NOAA 1991-2020 monthlies, DWIGHT WWTP station (10 mi away).

Land & collecting status

Generally open to casual rockhounding

Most public-tagged spots sit on BLM, U.S. Forest Service, or other federal land where reasonable hand collecting of common rocks and minerals is allowed. Confirm posted rules and active mining claims before you dig.

Public-land rules vary by agency, season, and field office. The RockHoundR app pulls live BLM, USFS, NPS, and tribal overlays so you can see exactly which agency manages the ground at this spot.

Sources & verification

Spot details combine the public RockHoundR location dataset, normalized mineral labels, agency land-status checks in the app, and community submissions. Coordinates are approximate until verified in the field.

Sources: RockHoundR public spot dataset, app land overlays, and local agency review before each trip.

Found at Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)

Each chip opens all spots that produce that material; the encyclopedia link opens the full ID and field guide.

Nearby rockhounding spots

Other rockhounding spots within driving distance of Mazonia-Braidwood State Fish & Wildlife Area (Pit 11).

Across the state line from Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)

Mazonia-Braidwood State Fish & Wildlife Area (Pit 11) is close enough to the Illinois border that the next-closest rockhounding spots are in a neighboring state. Worth knowing if you are already on the road.

Mazonia-Braidwood State Fish & Wildlife Area (Pit 11) FAQ

Where is Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)?+

Mazonia-Braidwood State Fish & Wildlife Area (Pit 11) is in Grundy County, Illinois, at 41.19750, -88.27080. Nearest road: Illinois Route 53.

What rocks and minerals can you find at Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)?+

Mazonia-Braidwood State Fish & Wildlife Area (Pit 11) is reported to produce Concretions With Fossils, Siderite, Tully Monster, Fern Fossils.

Is collecting allowed at Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)?+

Generally open to casual rockhounding. Most public-tagged spots sit on BLM, U.S. Forest Service, or other federal land where reasonable hand collecting of common rocks and minerals is allowed. Confirm posted rules and active mining claims before you dig. Always confirm current rules with the managing agency before a trip.

What tools should I bring to Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)?+

A rock hammer, a cold chisel, safety glasses, and newspaper or foam for wrapping finds cover most fossil ground. A trowel and a sieve help in soft exposures.

When is the best time to visit Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)?+

The most comfortable months at Mazonia-Braidwood State Fish & Wildlife Area (Pit 11) are March to November.

How do I get to Mazonia-Braidwood State Fish & Wildlife Area (Pit 11)?+

Open the directions link to navigate to 41.19750, -88.27080 in Google Maps. The nearest road is Illinois Route 53.

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