Geopic of the week: Prim boulders

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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.  

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One thought on “Geopic of the week: Prim boulders

  1. R. Dugan King

    I live on the south shore of Greers Ferry Lake in north central Arkansas … Van Buren County.
    Along the shoreline it is easy to find the most bizarre and beautiful specimens of what appear to be Ironstone Concretions. They make excellent ashtrays. I have found one that appears to have a trilobite stuck to it. I haven’t seen rocks like these anywhere else. As the lake rises up and down these things are getting washed out of the clay. The beach is covered with shale, sandstone and these ironstone concretions.

    Reply

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