Riverbed Joints

March 13, 2013

Riverbedjoints0087 (2)

Photographer: Johan Spee; Johan's Web site
Summary Author: Johan Spee

As I was hiking a picturesque valley trail in the French Pyrenees Mountains in the spring of 2011, my eye was caught not only by a wild river and waterfalls but also by the exposed rock in the riverbed itself. In many places, lines ran crisscross in the rock as if snow scooters had left scratch marks during the winter. Closer inspection revealed that the lines weren't scratched into the bedrock (sunken relief) but rather raised above it. These lines are in fact joints -- fractures in the rock.

Joints generally occur as sets, with each set consisting of joints sub-parallel to each other. In this case, however, the lines run in all directions indicating that the rock has been under pressure many times and from many different directions. In the course of time and with temperatures in excess of approximately 250 F or 121 C degrees, the fractures became filled with quartz. When the rock eventually reached the surface, natural processes such as wind and water flow eroded both the rock and the quartz. The harder quartz is more resistant to these forces than the surrounding rock causing “selective erosion” that leaves the quartz standing out in relief -- a very common phenomenon but rarely displayed as clearly as here. Photo taken during a hike from la Rallière to Pont d'Espangne, France on May 28, 2011.

Photo details: Camera Model: Canon PowerShot A700; Focal Length: 11.5mm; Aperture: f/3.5; Exposure Time: 0.010 s (1/100).