Geo-pic of the week: Pinnacle Mountain

This is a picture of Pinnacle Mountain – one of several steep-sided hills up to about 1000 feet tall, located at Pinnacle Mountain State Park, northwest of Little Rock, Arkansas.  Though its appearance may be misleading and its origin is debatable, Pinnacle Mountain is not a volcano!

What’s intriguing about Pinnacle and the smaller nearby hills is that they’re sandstone lenses surrounded by shale.  That, from a geologic perspective, is difficult to explain since sedimentary rocks are suppose to form in layers, not lenses or blocks.  This has led geologists to a variety of interpretations for their origin ranging from giant undersea landslides, to sand that got trapped in the empty gouges left by large undersea landslides, to beds of sandstone caught up and scrambled with shale beds along huge thrust faults.

Whatever their origin, because they are sedimentary rock and contrary to some satirical publications I’ve seen circulating online, they are not volcanoes and they are not going to blow up!

Geo-pic of the week: Satin Spar

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.