6 June 2013
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: