16 May 2012

Brecciation & percussion in Antietam Formation

Posted by Callan Bentley

Further upstream from the Skolithos and the snake and the diabase rip-rap… The field review team wandered down onto a creekside outcrop of Antietam Formation. The Antietam is a quartz sandstone, with variable levels of deformation, depending on where you look. In some places, it has been gently strained with the little Skolithos tubes taking on elliptical cross sections, or the individual sand grains undergoing a moderate amount of pressure solution, producing a through-going fabric in the rock. In other places, though, it has been shattered into a distinctive breccia. This breccia may mark the trace of the thrust fault at the Blue Ridge / Valley & Ridge boundary. It was natural enough to highlight some instances of brecciation on the new Elkton East quadrangle field review at the beginning of last week.

Here’s an example of the brecciation:

Another thing that caught my eye at this outcrop was a lovely instance of percussion marks – the little cone-shaped fractures that form when bouncing bedload (saltating cobbles) slam into stationary outcrops of resistant rock like quartzite.  Take a look:

Can tell me which way is upstream? 🙂