Tag Archives: gypsum

Geo-pic of the week: Satin Spar

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Satin spar is a variety of the mineral gypsum and, aside from it’s attractive fibrous appearance, it’s used for many practical purposes, including for making plaster, chalk, and drywall.  Some ideally suited varieties are carved by sculptors.  This piece was collected from a gypsum mine in Howard County, Arkansas, near the town of Nashville, where it’s mined and processed to make drywall.

Around 100 million years ago, the water of the Gulf of Mexico reached all the way to southern Arkansas, forming a huge marine bay.  Because that water was somewhat isolated from the ocean’s circulation, evaporation concentrated dissolved minerals there, to the point that the water became oversaturated and minerals, such as gypsum, began to crystallize out of it.  It’s the same process by which most of the world’s salt deposits formed.  In fact, gypsum is often found associated with salt.

Geopic of the week: Dinosaur tracks in Arkansas

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Pictured above are tracks of an Acrocanthosauras Atokensis: a bi-pedal predatory dinosaur.  The tracks were discovered in 2011 in a gypsum mine north of Nashville Arkansas by workers at the mine.  It is one of two such “dinosaur trackways” – as they are called – that have been discovered in this mine;  The first one was unearthed in 1983.  Dinosaur tracks are not common in Arkansas as most of the rocks here, which are very old, were deposited long before the dinosaurs existed. 

The rocks where the tracks are preserved were deposited in the early Cretaceous Period sometime between 145 and 100 million years ago.  At that time, the area south of the Ouachita Mountains was a broad coastal plain and the Gulf of Mexico waters reached all the way to southern Arkansas.  A variety of dinosaur species tracks, both herbivore and carnivore, have been discovered in these trackways, indicating that the coastal area at that time was quite the dinosaur stomping ground.