7 May 2021

Friday fold: Lynchburg Group

Posted by Callan Bentley

Last weekend, I went to get my second vaccination, and because of the ridiculous quirks of the way the vaccination campaign is (dis)organized, I had to travel to Lynchburg, Virginia, to get the shot. An extra two hours on the road (roundtrip) may sound like a pain, but it was ameliorated by getting to see some cool outcrop along the way.

This is in Riveredge Park in Madison Heights, Virginia, directly across the James River from downtown Lynchburg:

The rock here is Neoproterozoic Lynchburg Group metasediments that were folded and faulted during Appalachian mountain-building. The layering you see is primary sedimentary bedding, and it serves as strain markers for westward-verging asymmetric folds (and tectonic cleavage in the muddier layers).

In the upper right (east), the outcrop features sandy layers that show off more open folding:

Look for the variation in dip across this field of view: essentially vertical in the upper left, moderately left-dipping in the center, and approaching horizontal at the lower right edge:

Small-scale parasitic folds and cleavage are particularly well expressed at this spot:

(I had teased that site on Twitter earlier this week.) Another example of the little crenulations:

Rock fall aficionados will also appreciate this outcrop for its instability. There’s a skin of soil and vegetation stretching across a rock-free chasm at the top of the cliff, and a pile of debris at the bottom. Access to the cliff is fenced off with a “no trespassing” sign posted; a reasonable precaution it seems to me. Still, the debris run-out reached to the edge of the parking area and had broken through one of the fence cross-beams, so maybe they should go further.

It’s also interesting to see the various processes of weathering playing out here. The outcrop was much prettier just two years ago, as this Google Maps Streetview capture shows. Since then, it’s rotted and painted itself with iron oxides and what appears to be travertine:

There are even some little “stalactites” building downward off some of the overhangs:

I collected one sample there; which I intend to turn into a 3D model – Maybe in time for next week’s Friday fold…?

Happy Friday, and Happy End-of-the-Spring-Semester to those who observe!