Thomsonite Beach: Chlorastrolite & Lintonite Rockhounding in Cook County, Minnesota

Plan your visit

Generally open to casual rockhounding

County

Cook County

State

Minnesota

Nearest road

MN 61

Coordinates

47.72778, -90.45836

Coordinates are approximate. Confirm the exact ground in the RockHoundR app, where land overlays show which agency manages the area.

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.

Best time to visit Thomsonite Beach

Comfortable months (45 to 90°F highs)

Best months: May to October. Typical highs run 69°F in midsummer and 23°F in midwinter, with winter lows near 6°F.

Climate normals: NOAA 1991-2020 monthlies, GRAND MARAIS station (5 mi away), adjusted for elevation.

Geology at Thomsonite Beach

This spot sits on Basalt and basaltic andesite, mapped as basalt and basaltic andesite that formed roughly 1200 to 1000 million years ago. Volcanic ground like this weathers into rocky slopes and float, and fresh exposures in cuts and drainages are usually the most productive places to look.

Mapped unit

Basalt and basaltic andesite

Age

1200–1000 Ma (Mesoproterozoic)

Rock types

basalt and basaltic andesite

Elevation

910 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.

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.

• Minnesota Geological Survey
• Minnesota DNR state land rules
• Superior National Forest
• Community submissions reviewed against published agency rules