I've already introduced you to two of my Honors students' field projects. Now for the last of the three -- Jason's project on the strained metaconglomerate of Klingle Road. Klingle Road is a "road" in D.C. that was damaged by a storm some years back, and never repaired. Some people have started using it as a park, while others clamor for the road to be fixed. Geologically, it's interesting because it exposes a rock unlike any other nearby: a distinctly foliated metaconglomerate. Because I am so clever, I call it the Klingle Road Metaconglomerate. It's part of the "Laurel Formation," which is one of many flavors of metagraywacke / accretionary wedge complex that make up the bulk of the Piedmont in this area. Here's some of the squished clasts that Jason is interested in:

We know these rocks got heated up a fair bit. How do we know this? Well, they flowed out into elongated shapes all oriented in the same direction for one (see the additional photos here). The outcrop is peppered with clusters of little plus-shaped protuberances: they are clusters of sericite (cryptocrystalline muscovite) in the shape of staurolite porphyroblasts. Staurolite is a reasonably high grade metamorphic mineral, and when we see the three-dimensional shape of staurolite, but it's been turned into relatively-low-grade sericite, it's an indication of "retrograde metamorphism." Basically, after hitting the peak of its particular metamorphic conditions (high temperature and pressure, growing staurolite), the rock is readjusting to lower temperatures and pressures, and those staurolite crystals are reacting to a mineral that's more stable at those lower temperatures and pressures: sericite.

But anyhow -- back to the metaconglomerate. It's made of clasts, and those clasts have been stretched. The question is: how much have they been stretched. Sometimes when strain estimates are made, we assume an initial sphere shape, and then measure the lengths of the various axes of the resulting ellipsoidal shape (the "strain ellipsoid"). But is the assumption of initial sphericity valid? Jason is testing this issue by measuring the axes of cobbles and pebbles from the metaconglomerate as well as loose cobbles and pebbles found in nearby Rock Creek. We want to get a sense of how ellipsoidal cobbles are before they experience orogenic shortening/stretching. Here's a shot of Jason, Spencer, and Victoria measuring cobble axis lengths on a gravel bar near the National Zoo:

And a shot of the crew close-up:

And, just for fun, here's one more shot from Victoria's field area on Broad Branch. We hiked up to the contact with the Kensington Tonalite (a ~464 Ma felsic intrusive rock -- essentially a granite) and found a series of small waterfalls over this resistant rock unit. In the sequence of cascades were a series of deep pools. I submerged myself in one of them:

Labels: appalachians, dc, field trips, nova, piedmont, sediment, structure, teaching