25 June 2021

Friday fold: The Blue Ridge Tunnel

Posted by Callan Bentley

On Wednesday of this week, I went for the first time to the newly-opened-to-the-public Blue Ridge Tunnel, a county park in Nelson and Augusta counties, Virginia.

Built in the 1850s to serve as a railway tunnel, it provides a unique perspective on the geology of the Blue Ridge.  The western side is largely bricked-over, but in the middle, about at the Nelson/Augusta County Line, there are a series of extraordinary exposures that Chuck Bailey has dubbed “The Hall of Boudins:

The rocks here are all Catoctin Formation, a Neoproterozoic series of rift-related lava flows and intercalated sedimentary rocks that were metamorphosed during late Paleozoic mountain-building. In the photo above, you can see buff-colored Catoctin greenstone (metamorphosed basalt) with a pronounced east-dipping foliation. Within it are lozenges of green-colored metasandstone. Those pod-shaped blobs are green because they host a lot of epidote in them (a metamorphic mineral that is frequently found in areas of low-grade metamorphism under wet conditions). They are not in their original depositional orientation or shape, however; they have been boudinaged (stretched into asymmetric taffylike segments) as a result of tectonic stresses that shoved Blue Ridge rocks up from the east toward the west.

The kinematics of those same Alleghanian forces can be inferred from not only the asymmetric boudinage, but also asymmetric folds within the same rocks. These folds can be mapped out with careful measurements of bedding and foliation orientations, but sometimes they are more plain, as with this hinge:

There’s also a more subtle Z-fold on the left, above my fingers. The photo also shows well the difference in color between fresh Catoctin (dark) and weathered Catoctin (buff tan).

This is a really cool location to have as a local geologic resource; I look forward to my next visit.

If you visit, first download a geologic guide to the Blue Ridge Tunnel (PDF) by Katie Lang and Chuck Bailey. It calls attention to several subtle things that I missed on my visit this week.