5 November 2010

Friday fold: a gummy worm

Posted by Callan Bentley

A week ago today, I went out on the Billy Goat Trail (near Potomac, Maryland) with a group of students: five from George Mason University’s GeoClub, and two that are current Physical Geology Honors students with me at Northern Virginia Community College. One of my students, Robin, observed this lovely fold, and called my attention to its gummy-worm-like form. When we saw it, we all knew it was destined to be featured on Mountain Beltway as a “Friday Fold:”

These rocks are meta-graywackes of the Mather Gorge Formation, where beds of mudstone between the original graywacke layers have been metamorphosed to a muscovite schist. These layers were then folded — not once, but twice — to produce the fold we see here. An annotated copy of the photo would look like this:

The yellow and orange lines are traces of the compositional layering in the rocks, and the blue line shows the trace of the axial olane from the first episode of folding (“F1”), with the black lines labeled “F2” showing the more recent folds’ axial plane traces. Note that they overprint the first fold, and deform its (formerly planar) axial plane.

If this doesn’t make any sense, then watch this demonstration of how to make a similar fold exposure using paper:


Weird how the audio and video seem to be mismatched by a fraction of a second… very disconcerting. Oh well… Happy Friday!