Grand Marais: Lintonite & Thomsonite Rockhounding in Cook County, Minnesota
Updated July 2026
The Cook County shore around Grand Marais holds Minnesota's best thomsonite, a zeolite that filled gas cavities in the billion-year-old North Shore basalt flows and weathers out as rounded nodules with concentric green, pink, and white eyes. Good Harbor Bay is the most productive public thomsonite beach in the state, and the same lava flows yield lintonite, a green massive variety, alongside Lake Superior agate. Collecting is best after storms, when wave action turns fresh gravel onto the beach. Reported finds include lintonite, thomsonite. Below: coordinates, access notes, nearby spots, and trip-planning links.
Grand Marais photos
Public image records connected to this spot or its reported material.
Map showing Grand Marais in Cook County, Minnesota
Quick details
- Access
- Public area
- County
- Cook County
- State
- Minnesota
- Nearest road
- Harbor View Trail
- Coordinates
- 47.74462, -90.35314
Geology at Grand Marais
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
- 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 Grand Marais
Best months: April to October. Typical highs run 70°F in midsummer and 24°F in midwinter, with winter lows near 7°F.
Climate normals: NOAA 1991-2020 monthlies, GRAND MARAIS station (0 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 Grand Marais
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 Grand Marais.
- Thomsonite BeachCook County, Minnesota · 5 mi awayChlorastrolite, Lintonite, Thomsonite Nodules
- Arrowhead ParkCook County, Minnesota · 15 mi awayLake Superior Agate
- Little Marais RiverLake County, Minnesota · 42 mi awayThomsonite, Zeolite
- Pigeon PointCook County, Minnesota · 43 mi awayChalcocite, Chalcopyrite, Pyrite
- Beaver BayTop pick in MinnesotaLake County, Minnesota · 55 mi awayLake Superior Agate, Thomsonite
- Gull RockLake County, Minnesota · 59 mi awayLake Superior Agate
- WintonSaint Louis County, Minnesota · 68 mi awayLake Superior Agate, Evergreen Jasper
- ElyTop pick in MinnesotaSaint Louis County, Minnesota · 73 mi awayJasper, Quartz, Chalcedony
Across the state line from Grand Marais
Grand Marais is close enough to the Minnesota border that the next-closest rockhounding spots are in a neighboring state. Worth knowing if you are already on the road.
- Carnelian beachKeweenaw County, Michigan · 64 mi awayAgate Nodules, Carnelian, Quartz
- Big Bay Town ParkTop pick in WisconsinAshland County, Wisconsin · 68 mi awayLake Superior Agate, Jasper, Thomsonite
- Union Bay CampgroundOntonagon County, Michigan · 72 mi away
- Gull PointOntonagon County, Michigan · 73 mi awayKeweenaw Agate
- Siskiwit Lake (South Side)Keweenaw County, Michigan · 74 mi awayChlorastrolite
- Green ParkOntonagon County, Michigan · 76 mi awayCharlevoix Stone, Honeycomb Coral, Rhyolite
Grand Marais FAQ
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More rockhounding in Minnesota
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