Monthly Archives: November 2014

Geopic of the week: La Petite Roche (Little Rock)

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Pictured above is about all that remains exposed of the rock outcrop that is the namesake of the capitol of Arkansas, La Petite Roche or The Little Rock.  The outcrop was given its name by early french explorers to the area who first arrived in 1722.  Though unimpressive in stature, it is notable because it is the first exposure of solid rock that one sees when navigating up the Mississippi and Arkansas rivers, starting at the Gulf of Mexico.

The geologic explanation for this is simple;  The land between New Orleans and Little Rock Arkansas along that route is in the Mississippi River Valley, where the bedrock is buried beneath river-lain and wind-blown sediment.  The city of Little Rock straddles the boundary between the lowlands of the Mississippi River Valley and uplands of the Ouachita Mountains, which stretch west well into Oklahoma.  Therefore this little sandstone and siltstone outcrop marks that physiographic boundary. 

If you boated all the way from New Orleans to Little Rock without seeing a single rock you would probably be impressed by it too.

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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.