You are browsing the archive for percussion marks Archives - Mountain Beltway.
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 …
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.