Tag Archives: diagenesis

Geopic of the week: Cone in cone structure

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Pictured above is a good example of cone in cone structure.  It’s a common feature in limestone, although it can form in other rock types as well.  This sample was collected from the DeQueen Limestone of the Gulf Coastal Plain of southern Arkansas. 

From the picture, you can see that cone in cone structure results in numerous nested cones, which resemble Bugles Corn Snacks (product placement).  The craters, visible above, are places where cones have worked themselves out of the rock.  In other places you can see the round bottom of a cone: those cones have yet to work free from the rock.

Thus far, no one is certain how cone in cone structure forms.  Several theories have been offered, but scientists have yet to arrive at a consensus.  However it forms, it’s fascinating to look at and wonder about!

Geopic of the week: Sandstone pipe

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This is a “sandstone pipe” in the Witt Springs Formation of north central Arkansas.  At first glance, it seems that someone has managed to insert a pipe into the outcrop (either that, or someone had fairly sophisticated plumbing 320 million years ago).  Actually, these naturally occurring features result from iron minerals precipitating out of ground water as it moves through rock.  When minerals precipitate from a solution, they do so in concentric bands known as liesegang bands.  They were named for Raphael Liesegang: the chemist that first produced them in the laboratory. Once the band of minerals has formed, it makes that part of the rock harder, and, as the rock erodes, the iron-fortified band stands out in relief.  Typically liesegang bands form in organic shapes like the ones that surround the pipe above.  When they form a cylindrical band, however, they look almost identical to iron pipes.

Photo taken by Richard Hutto

For more views of sandstone pipes click here

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.