Prim boulder is a name given to curious, round, often nearly spherical sandstone boulders that are common in the area around the town of Prim in Cleburne county, Arkansas. Though the town of Prim boasts a noteworthy abundance of these unusual specimens, they actually have been found within a 100 mile area from Newton to White county. The one pictured here is still attached to the outcrop of sandstone: Many are found laying around at the surface, the rock from which they came having long ago weathered away.
Geologists believe these formed by precipitation of calcite and iron minerals from ground water. The calcite and iron minerals precipitate in concentric bands and make that part of the sandstone more resistant to weathering so that the boulders remain after the rest of the sandstone has eroded away.
If you are interested in seeing Prim boulders for yourself there are many on display around the community of Prim within easy sight from highway 263.