14 June 2019

Friday fold: kinked cleavage at Harpers Ferry

Posted by Callan Bentley

Last weekend was the annual meeting of the eastern section of the National Association of Geoscience Teachers. On Friday afternoon, we visited Harpers Ferry, West Virginia, and my colleague Beth Doyle led a great field trip to examine the rocks exposed there. This was my favorite outcrop we saw:

Here is a close up of this outcrop, which is framed by (anthropogenic) rock wall:

Dipping shallowly from upper left to lower right is a tectonic foliation: that is to say, slaty cleavage. This foliation has been folded into a well-defined kink band by late Paleozoic Alleghanian deformation. The rock itself was a mudrock (~shale) when it was first deposited: the Cambrian-aged Harpers Formation. Bedding is also visible in places at Harpers Ferry, but I can’t see any in this outcrop.

Here is my sketch, from my field notebook:

…And lastly, here’s an annotated copy of the second photo:

…Happy Friday to you!