24 February 2011
Another serendipitous sighting on last weekend’s field trip to Thoroughfare Gap was this boulder of Weverton Formation, lying in the no-man’s-land between the railroad tracks and Broad Run:
As you look at this boulder, I hope you will notice what caught my eye and prompted me to take the photo. There’s a big flat surface (white) at left, but at right, in the gray portion of the rock, you can see a series of sub-parallel joint surfaces, dipping steeply towards the camera. They look like Ping-Pong paddles! Here’s a closer look at them, with my Swiss Army knife for scale:
I think what we might be looking at here is a single big joint face, that started in the bottom and then propagated to the top. It met slightly tougher gray quartzite at right, and at that mechanical discontinuity, the joint surface broke up and flared into an array of twist hackles. Typically, these features are relatively small (in my experience), so this seemed like a nice large example. Each of the hackle faces bears its own petite plumose structure.
Some other links to elucidate what the hell I’m talking about here:
I didn’t notice it when I took the photo, but the arc-shaped feature (highlighted by oxide/mud staining) at the bottom of the main joint face may be a concentric rib. Thoughts?