Mazon River: Fossil Hunting in Grundy County, Illinois

Quick details

Access

Public area

County

Grundy County

State

Illinois

Coordinates

41.29336, -88.36670

Geology at Mazon River

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

540 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 Mazon River

Best months: March to November. Typical highs run 85°F in midsummer and 31°F in midwinter, with winter lows near 16°F.

Climate normals: NOAA 1991-2020 monthlies, MORRIS 1NW station (6 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 Mazon River

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

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