A year or so ago, I picked up this nice sample of limestone in the Shenandoah Valley of Virginia (easternmost valley of the Valley and Ridge physiographic province). It was a cobble in a stream, not in situ, but it can't have come very far (by natural means anyhow) since it's quite angular. I liked it because of the alternating colors of its layers. I was not totally sure why they are different colors, but I strongly suspected it had something to do with different reactions to weathering (perhaps different calcite / aragonite ratios, or an increased silica / clay content in some layers?). I also liked the patterns of sedimentary layering, thinking back to undergraduate discussions of Flaser bedding and the like, but not remembering the details clear enough to interpret this one. Perhaps one of the sedimentary geologists can help clue me in? Still, I suspected it had something to teach me, so I brought it back to my lab at NOVA. Side view:

There, I sawed it in half. Top view:

To my chagrin, but not my surprise, the interior showed the layering less clearly. In the sawn section, I could clearly see where the weathering "front" had penetrated a short distance into the rock along the lighter-colored layers. While they were yellow-tan on the face of the sample, they were merely light gray in the interior. I decided to try and create a little weathering of my own, and reached for one of the students' acid bottles. I dropped about ten drops of acid on the sawn face, let it fizz for a bit, rinsed it off, and repeated the acid application. Almost instantly, the different layers jumped out into high contrast. The light-colored one was much more reactive than the dark colored one. Here's a view from the scanner which offers a comparison between the un-acid-treated sample (left) and the one I gave the brief acid bath to (right):

Not only does the layering jump out at you, you can see some micro-faulting too. Here's another view, from the camera, of the two samples, one stacked atop the other. I'm astonished at how 30 seconds of acid produced such a remarkable difference in their appearances:

As soon as I had documented the efficacy of the technique, I treated the second sample the same way as the first. One is now in the NOVA teaching collection, and the other is a proud new member of the CB office deskcrop collection.

Labels: limestone, nova, teaching, valley and ridge