Tag Archives: Stanley Formation

Geo-pic of the week: Sigmoidal Veins

Sigmoidal vein in sandstoneedited

The picture above shows a boulder of Hot Springs Sandstone with well-developed sigmoidal veins.  Sigmoidal veins – sometimes called tension gashes – form in rock by shear stress.  That’s stress that causes adjacent parts of a rock to slide past one another.  In the above picture the yellow arrows indicate the approximate orientation of the stresses that were applied to this boulder to create the sigmoidal veins.

Sigmoidal veins, at their inception, are shaped like parallel lines that bulge toward the center and taper at the ends.  They originate due to tension created between the two opposing forces acting on the rock.  Essentially the rock tears to alleviate this tension.  If the shearing continues long enough, these openings in the rock begin to rotate.  The eventual shape, seen above, is like the letter S.  The ends of each S point opposite of the direction of the force that created them.  Therefore, sigmoidal veins can indicate the forces at work on bedrock when it was buried underground.

The veins pictured here are at the edge of a parking lot next to the Arlington Hotel in Hot Springs Arkansas.  After they developed the veins were in filled with quartz.  The Hot Springs Sandstone is a member of the Mississippian Stanley Formation.

Geo-pic of the week: Freshly exposed anticline

Mcloed St. Doctored

This is an anticline exposed on Mc Leod Street, southwest of Hot Springs, Garland County, Arkansas.   It’s not unique as, anticlines are common in the Ouachita’s and other mountain ranges throughout the world.  Most often though, these structures are large scale and cover expanses of land that can’t be viewed from a human vantage point.  When they do form on a scale that’s small enough for human observation, we typically don’t have the benefit of a freshly blasted exposure like this one. 

In fact, many times geologists must infer that folds like this exist in places deep underground that no one has or will ever see.  That’s why, if you see a geologist on the side of the road, taking something like this in, as in the picture above, just let him have his little moment.  The exposure is of deep marine sedimentary deposits of the Stanley Formation.