Yellowstone River - Black Bridge Boat Launch: Montana Moss Agate & Petrified Wood Rockhounding in Dawson County, Montana

Plan your visit

Generally open to casual rockhounding

County

Dawson County

State

Montana

Nearest road

Mary Avenue

Postcode

59330

Coordinates

47.09995, -104.72662

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 Yellowstone River - Black Bridge Boat Launch

Comfortable months (45 to 90°F highs)

Best months: March to October. Typical highs run 88°F in midsummer and 29°F in midwinter, with winter lows near 9°F.

Climate normals: NOAA 1991-2020 monthlies, GLENDIVE station (1 mi away).

Geology at Yellowstone River - Black Bridge Boat Launch

This spot sits on Alluvium, mapped as gravel, sand, and silt that formed within roughly the last 2 million years. Petrified wood typically travels as tough float, and worked gravels like these are exactly where weathered pieces tend to collect.

Mapped unit

Alluvium

Age

2 Ma to present (Pleistocene to Holocene)

Rock types

gravel, sand, and silt

Elevation

2,030 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.

State guidance last verified: April 1, 2026

• BLM Montana, Recreational mineral collecting
• Montana Bureau of Mines and Geology
• Gem Mountain Sapphire Mine (fee-dig operator)
• Community submissions reviewed against published agency rules