Thompson's Harbor State Park: Granite & Gneiss Rockhounding in Presque Isle County, Michigan

Updated August 2026

Thompson's Harbor State Park is a mapped rockhounding spot in Presque Isle County, Michigan. Reported finds include granite, gneiss, quartz, stromatolite. Below: coordinates, access notes, nearby spots, and trip-planning links.

Quick details

Access: Paid / fee dig
County: Presque Isle County
State: Michigan
Coordinates: 45.35195, -83.61985

Geology at Thompson's Harbor State Park

This spot sits on Dundee Limestone, mapped as limestone that formed roughly 395 to 385 million years ago. Carbonate rock like this dissolves and weathers readily, so fresh road cuts, quarries, and creek exposures are typically the most productive ground.

Mapped unit: Dundee Limestone
Age: 395–385 Ma (Middle Devonian)
Rock types: limestone
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 Thompson's Harbor State Park

Best months: April to October. Typical highs run 80°F in midsummer and 28°F in midwinter, with winter lows near 14°F.

Climate normals: NOAA 1991-2020 monthlies, ROGERS CITY station (11 mi away).

Land & collecting status

Pay-to-dig site

This is a fee-dig location. Hours, prices, and what you can keep vary by operator. Call ahead before driving out.

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

Found at Thompson's Harbor State Park

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 Thompson's Harbor State Park.

Thompson's Harbor State Park FAQ

Where is Thompson's Harbor State Park?+

Thompson's Harbor State Park is in Presque Isle County, Michigan, at 45.35195, -83.61985.

What rocks and minerals can you find at Thompson's Harbor State Park?+

Thompson's Harbor State Park is reported to produce Granite, Gneiss, Quartz, Stromatolite.

Is collecting allowed at Thompson's Harbor State Park?+

Pay-to-dig site. This is a fee-dig location. Hours, prices, and what you can keep vary by operator. Call ahead before driving out. Always confirm current rules with the managing agency before a trip.

What tools should I bring to Thompson's Harbor State Park?+

A rock hammer, a small pry bar, gloves, and safety glasses cover most crystal ground. A screen helps in loose pocket soil, and newspaper protects points on the way out.

When is the best time to visit Thompson's Harbor State Park?+

The most comfortable months at Thompson's Harbor State Park are April to October.

How do I get to Thompson's Harbor State Park?+

Open the directions link to navigate to 45.35195, -83.61985 in Google Maps. Some spots are remote, so check road conditions before driving out.

More rockhounding in Michigan

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Open Thompson's Harbor State Park on the map

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