Mobridge: Opalized Wood & Agatized Wood Rockhounding in Walworth County, South Dakota

Public accessBest spot2 mineralsWalworth County, South DakotaUpdated June 2026

Minerals reported: 2
Elevation: 1,610 ft
Best months: March to November

The Missouri River banks and gravels around Mobridge are one of the few South Dakota localities that produce opalized wood, a blend of petrified wood and silica or precious opal, along with agatized wood. It rounds out the list with a distinctive fossil-wood material from the central Missouri River corridor. Reported finds include opalized wood, agatized wood. Below: coordinates, access notes, nearby spots, and trip-planning links.

Plan your visit

Generally open to casual rockhounding

County: Walworth County
State: South Dakota
Postcode: 57601
Coordinates: 45.52016, -100.41246

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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.

Access note: Collecting here focuses on the public gravels and bars of the Missouri River (Lake Oahe) in Walworth County. Surface collecting of petrified and opalized wood on a navigable riverbed is generally allowed, but collectors should check rules for Army Corps of Engineers shoreline lands at the reservoir. The Standing Rock and Cheyenne River reservations lie across the river to the west and south, so stay on the Walworth County (east) side public ground unless tribal permission is obtained.

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 Mobridge

27°J

32°F

45°M

59°A

71°M

80°J

88°J

86°A

77°S

61°O

45°N

32°D

Comfortable months (45 to 90°F highs)

Best months: March to November. Typical highs run 88°F in midsummer and 27°F in midwinter, with winter lows near 7°F.

Climate normals: NOAA 1991-2020 monthlies, MOBRIDGE MUNI AP station (2 mi away).

Geology at Mobridge

This spot sits on Pierre Shale, mapped as shale and bentonite that formed roughly 100 to 66 million years ago. Sedimentary beds like these are the typical host for petrified and silicified wood, which weathers out as durable float on slopes and in washes.

Mapped unit: Pierre Shale
Age: 100–66 Ma (Late Cretaceous)
Rock types: shale and bentonite
Elevation: 1,610 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.

Today's weather

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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.

BLM Montana-Dakotas rockhounding rules
Buffalo Gap National Grassland
South Dakota Geological Survey
Community submissions reviewed against published agency rules

Found at Mobridge

Each name 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 Mobridge.

Mound CityCampbell County, South Dakota · 16 mi awayPetrified Wood, Opal
Little EagleCorson County, South Dakota · 21 mi awayPetrified Wood, Opal
Rattlesnake ButteZiebach County, South Dakota · 81 mi awayCalcite
Fox RidgeMeade County, South Dakota · 109 mi awayMoss Agate

Across the state line from Mobridge

Mobridge is close enough to the South Dakota border that the next-closest rockhounding spots are in a neighboring state. Worth knowing if you are already on the road.

Cannonball RiverSioux County, North Dakota · 63 mi awayAgate, Chalcedony, Silicified Wood

Mobridge FAQ

Where is Mobridge?+

Mobridge is in Walworth County, South Dakota, at 45.52016, -100.41246.

What rocks and minerals can you find at Mobridge?+

Mobridge is reported to produce Opalized Wood, Agatized Wood.

Is collecting allowed at Mobridge?+

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. Always confirm current rules with the managing agency before a trip.

What tools should I bring to Mobridge?+

A rock hammer, sturdy gloves, and a padded bag cover petrified wood hunting. Most pieces are surface float, so walking slopes and washes beats digging.

When is the best time to visit Mobridge?+

The most comfortable months at Mobridge are March to November.

How do I get to Mobridge?+

Open the directions link to navigate to 45.52016, -100.41246 in Google Maps. Some spots are remote, so check road conditions before driving out.

More rockhounding in South Dakota

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