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!
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.
Sure would be fun for some of the old Geology Club to do a field trip to Harper’s Ferry sometime. Hi Callan!
Happy to – come late August, I’ll have some free time.
I really like how the ‘anthropogenic’ rock wall was founded on the outcrop, with the base course of stones being placed to fit the foliation and kink banding.
Anyway for you to know how old the wall is? Without being able to see more of it, I would be interested to know how steep the facing of the wall is – considering I have seen photos of ancient, vertical walls still standing, despite the fact that many builders today fail to build stable walls with a batter.
Didn’t get to this Friday, but certainly makes for a great start to my Monday 🙂