Fort Gratiot County Park: Rockhounding in Saint Clair County, Michigan

Quick details

Access

Check locally

County

Saint Clair County

State

Michigan

Postcode

48059

Coordinates

43.08204, -82.47258

Geology at Fort Gratiot County Park

This spot sits on Antrim Shale, mapped as black shale that formed roughly 385 to 360 million years ago. Sedimentary rock like this erodes unevenly, so the most productive collecting is typically along fresh cuts, badland slopes, and active drainages.

Mapped unit

Antrim Shale

Age

385–360 Ma (Late Devonian)

Rock types

black shale

Elevation

610 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 Fort Gratiot County Park

Best months: April to November. Typical highs run 81°F in midsummer and 31°F in midwinter, with winter lows near 20°F.

Climate normals: NOAA 1991-2020 monthlies, PORT HURON station (8 mi away).

Land & collecting status

Confirm rules before collecting

Land status here is not confirmed. Verify with the managing agency before collecting — federal, state, and tribal rules differ.

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.

State guidance last verified: April 1, 2026

• Michigan DNR — Rockhounding rules
• Michigan Geological Survey
• Isle Royale National Park — Rules
• Community submissions reviewed against published agency rules