Tag Archives: gulf coastal plain

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: Salt casts

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Salt casts are sedimentary structures that indicate a very saline environment of deposition.  This example was collected from the DeQueen Limestone in Howard County, Arkansas (peppermint for scale). About 146 million years ago, much of Arkansas south of the Ouachita Mountains was under near-shore marine conditions.

The cubes that stand in relief on the underside of this rock bed were once crystals of the mineral Halite – or common table salt.  These crystals grew in the mud of a tidal flat that was periodically flooded with very salty sea water.  The salt crystals were later dissolved, leaving cube-shaped molds that filled with mud, preserving the casts of the Halite crystals.

Ho Ho Ho, Merry Christmas! from the Arkansas Geological Survey