17 May 2012
Yesterday, I introduced percussion marks to this blog space. Here are some other shots of this distinctive “shatter” structure, but in a vein of hydrothermal (milky) quartz exposed on Bear Island, between the C&O Canal and the Potomac River (about right here, just west of the trail):
Sometimes percussion marks are accompanied by radial fracture sets like this one…
I interpret these radial fractures (sometimes with small cone-fractures at the center) as signs of a really big impact. Volcanic complexes sometimes bear radial dikes and ring dikes in a similar orientation around the main vent.
Have you learned to see percussion marks? Where do you find them in your area?