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6 June 2013

Percussion marks on quartzite cobbles in DC

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 …


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