Geo-pic of the week: Veins

Any rockhound worth their salt knows that the best place to hunt for interesting minerals is in the void spaces in rock.  Void spaces come in two types; vugs and veins.  Vugs are usually found in igneous rock and result from trapped gas bubbles.  Veins, on the other hand, can be found in any type of bedrock. 

Veins are fractures, that have been plugged with minerals, typically by precipitation from circulating water.  The above picture was taken in the Ron Coleman quartz mine, near Hot Springs, Arkansas.   The near-parallel white streaks that riddle the sandstone are quartz-filled veins.  The fractures resulted from the intense deformation of the Ouachita Mountains, by plate tectonic forces, around 300 million years ago.  That deformation opened up space for quartz to grow in, and the tremendous heat and pressure from the mountain-building generated the mineral-rich fluid that deposited the crystals.      

Geo-pic of the week: Herringbone Cross-Bedding

 

Pictured above is sandstone displaying classic herringbone cross-beds.  Cross-bedding results from either sediment transport by flowing water, such as in this example, or by wind flow, as in the case of dunes.

Cross-beds form by the migration of sediment, and tilt in the direction of flow.  As sediment grains are carried by the current, they migrate up the gentle ramp of previously deposited cross-beds.  When they reach the end, they tumble down the steeper face there and are deposited to become part of the next cross-bed.  In this way the sediment migrates in the downstream direction.

Each group of similarly tilted cross-beds is known as a set.  In herringbone cross-bedding, the sets are oriented contrarily, which gives the outcrop a fishbone appearance.  These differently oriented cross-bed sets indicate changing flow directions.