15 March 2013
Yesterday, I spent a pleasant day in the field with John Singleton, the new structural geology professor at George Mason University. I was showing John a couple of sites I’ve used as field trip locations for the GMU structural geology class, and John was showing a couple of new sites to me – places he visited on last fall’s Virginia Geological Field Conference. I missed VGFC last fall, as I had something else going on that weekend, so I was pleased to finally get to see these new sites. Today’s “Friday fold” is one that the conference participants kept talking about long after the conference was over: a recumbent anticline in a quarry on the northeast shore of the South Fork of the Shenandoah River. The quarry unfortunately has a lot of trees in it these days, but the fold is still plenty clear.
Two GigaPans of the site:
The axial surface of these folds (there’s a more obscure recumbent syncline at the bottom of the quarry wall) is more or less horizontal. This is unusual in the Appalachian Mountains’ Valley & Ridge province. Usually the folds are upright. So why is this one different? Steve Whitmeyer of James Madison University and his students interpret it as formerly having been upright, but then it got rotated over to the west due to a cryptic west-directed thrust fault (with these rocks being in the footwall of that putative fault).