Portsmouth Mine: Agate & Chalcedony Rockhounding in Crow Wing County, Minnesota

Plan your visit

Generally open to casual rockhounding

County

Crow Wing County

State

Minnesota

Nearest road

Portsmouth Campground Trail

Postcode

56441

Coordinates

46.49079, -93.97144

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 Portsmouth Mine

Comfortable months (45 to 90°F highs)

Best months: April to October. Typical highs run 81°F in midsummer and 21°F in midwinter, with winter lows near 0°F.

Climate normals: NOAA 1991-2020 monthlies, BRAINERD CROW WING CO AP station (10 mi away).

Geology at Portsmouth Mine

This spot sits on Iron-formation in the Cuyuna North and South Ranges and Mille Lacs Group, mapped as banded iron formation that formed roughly 2500 to 1600 million years ago. Tough silica like agate and jasper typically survives weathering and collects as float in drainages and gravel.

Mapped unit

Iron-formation in the Cuyuna North and South Ranges and Mille Lacs Group

Age

2500–1600 Ma (Paleoproterozoic)

Rock types

banded iron formation

Elevation

1,230 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