Tag Archives: Mississipian

Geopic of the week: Septarian concretions



The “fossilized turtle shell” pictured above is actually one of the more commonly misidentified pseudo-fossils in the rock record, a septarian concretion.  Concretions aren’t the remains of animals or their shells at all:  They are inorganic nodules that grow in rocks by precipitation from groundwater.  This kind of concretion is called septarian – a Latin word for partition – because of the cracks that divide it into polygons.

Though these are not uncommon, how they form isn’t well understood.  Explanations for the cracks range from dehydration and shrinkage (similar to mud cracks) to fracturing by either crushing or violent shaking (such as in an earthquake).  Once fractured, circulating ground water deposits more minerals into the cracks.

This septarian concretion was collected from the Fayetteville Shale near Leslie, Arkansas in the Ozarks.


To see more views of septarian concretions, click here

GeoPic of the Week: Crinoid Stem In Limestone

Crinoid stem in limestone

Crinoid stem in limestone

Crinoids are a group of marine invertebrates that were abundant in the Mississippian Period (359-323 million years ago). It is unusual to find a complete crinoid fossil; however, pieces of the stalk (commonly called stem or column) are abundantly preserved in the rocks in northern Arkansas.  The center portion of the stem in this picture has dissolved away and ooliths were deposited in it.  Ooliths are small round bodies, usually 0.5-1 mm in size, and are commonly formed of calcium carbonate (calcite).  They are formed as successive concentric layers precipitate around a nucleus such as a shell fragment, algal pellet, or a quartz-sand grain.  The layers form in wave-agitated water such as a shoal environment along a shoreline.  The rock type is called an oolitic limestone.  Fossiliferous and oolitic limestone is present in the Pitkin Limestone in northern Arkansas.