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Mandan: Agate & Chalcedony Rockhounding in Morton County, North Dakota

Updated July 2026

Mandan lies in the Cannonball Formation belt that produces Teredo-bored petrified wood, North Dakota's state fossil: ancient driftwood riddled by marine shipworms and later silicified. Morton County gravels and badlands exposures also yield agate, chalcedony, and chert. Petrified wood and other plant fossils may be collected for personal use, but vertebrate fossils on public land are protected by state law. Reported finds include agate, chalcedony, chert, teredo petrified wood. Below: coordinates, access notes, nearby spots, and trip-planning links.

Mandan photos

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

1 photo

Reconstructed Mandan earthlodges at On-A-Slant Village, Fort Abraham Lincoln State Park near Mandan, Morton County, North Dakota

Quick details

Access: Public area
County: Morton County
State: North Dakota
Nearest road: Sunny Place South
Postcode: 58554
Coordinates: 46.81736, -100.93526

Geology at Mandan

This spot sits on Cannonball Formation, mapped as shale and sandstone that formed roughly 66 to 56 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: Cannonball Formation
Age: 66–56 Ma (Paleocene)
Rock types: shale and sandstone
Elevation: 1,660 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 Mandan

Best months: April to October. Typical highs run 83°F in midsummer and 23°F in midwinter, with winter lows near 3°F.

Climate normals: NOAA 1991-2020 monthlies, MANDAN EXP STN station (1 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 Mandan

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 Mandan.

Across the state line from Mandan

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

Little EagleCorson County, South Dakota · 79 mi awayPetrified Wood, Opal

Mandan FAQ

Where is Mandan?+

Mandan is in Morton County, North Dakota, at 46.81736, -100.93526. Nearest road: Sunny Place South.

What rocks and minerals can you find at Mandan?+

Mandan is reported to produce Agate, Chalcedony, Chert, Teredo Petrified Wood.

Is collecting allowed at Mandan?+

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 Mandan?+

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 Mandan?+

The most comfortable months at Mandan are April to October.

How do I get to Mandan?+

Open the directions link to navigate to 46.81736, -100.93526 in Google Maps. The nearest road is Sunny Place South.

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