6 June 2013
Percussion marks on quartzite cobbles in DC
Posted by Callan Bentley
Hey, that quartzite boulder has measles! Or is it bubonic plague? Or maybe it’s just acne?
Look closer and see if you can deduce what these things are:
These conical fractures are percussion marks. They form when a cobble smacks into another cobble underwater, propelled forward by a powerful current. As the two rocks knock together, the strength of the collision can overwhelm the strength of the bonds holding the rock together, and a series of cone-shaped fractures grow into the interior of the cobble.
Multiple overlapping percussion marks in the same area can cause an interesting weathering pattern: