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Mound City: Petrified Wood & Opal Rockhounding in Campbell County, South Dakota

Updated July 2026

The country around Mound City, west toward the Missouri River, is a documented source of petrified and agatized wood, with occasional opal, weathering out of the banks and gravels of the river and its tributaries. It is one of the better petrified-wood opportunities in the north-central part of the state. Reported finds include petrified wood, opal. Below: coordinates, access notes, nearby spots, and trip-planning links.

Mound City photos

Public image records connected to this spot or its reported material.

1 photo

Petrified wood logs weathering out of badlands strata in the northern Great Plains, the kind of fossil wood found in the Missouri River gravels near Mound City

Quick details

Access: Permission required
County: Campbell County
State: South Dakota
Coordinates: 45.72064, -100.23723

Geology at Mound City

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: 1,760 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 Mound City

Best months: April to October. Typical highs run 85°F in midsummer and 24°F in midwinter, with winter lows near 3°F.

Climate normals: NOAA 1991-2020 monthlies, POLLOCK station (13 mi away).

Land & collecting status

Permission required before collecting

Requires landowner or agency permission before any collecting. Do not enter without explicit authorization.

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 Mound City

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 Mound City.

Across the state line from Mound City

Mound City 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.

Mound City FAQ

Where is Mound City?+

Mound City is in Campbell County, South Dakota, at 45.72064, -100.23723.

What rocks and minerals can you find at Mound City?+

Mound City is reported to produce Petrified Wood, Opal.

Is collecting allowed at Mound City?+

Permission required before collecting. Requires landowner or agency permission before any collecting. Do not enter without explicit authorization. Always confirm current rules with the managing agency before a trip.

What tools should I bring to Mound City?+

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 Mound City?+

The most comfortable months at Mound City are April to October.

How do I get to Mound City?+

Open the directions link to navigate to 45.72064, -100.23723 in Google Maps. Some spots are remote, so check road conditions before driving out.

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