Geo-pic of the week: Sigmoidal Veins

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: Folded Rock In 3 Dimensions

 

                                                         

Above are several images of the same rock sample: a highly deformed quartzose siltstone collected from the Womble Formation, Ouachita Mountains, Arkansas.  The uppermost image shows a cut and polished surface.  The green line that’s been added to the picture defines a fracture that split the sample after it was cut.  Ordinarily, that would be a bitter turn of events but, in this case, it was a fortunate accident.  The fracture provides a rare, multi-dimensional view inside a tightly folded rock (lower photo).  Luckily, the fracture propagated across the bedding rather than breaking along a bed, which makes the beds of the fold appear to fan out like a deck of cards showing a lot of the detail of the structure.