11 February 2011

Friday fold: Difficult Run fun

Posted by Callan Bentley

Last Sunday morning, Lily and I met up with some students of mine (past and present) for a hike down Difficult Run in Virginia. This defile leads down from Georgetown Pike near McLean, Virginia towards a confluence with the Potomac across from the south end of the Billy Goat Trail, and just upstream from Black Pond (on the property of Madeira School). The trail was icier than we had anticipated, and Difficult Run was “up,” making it dangerous to get too close to the water, so we didn’t. But we had an enjoyable hike regardless, and spotted some new folds in the migmatitic schist and gneiss exposed there. Check them out:

Here’s a bonus for you — schist chips in migmatite:

Migmatite is a rock that falls on the border between “metamorphic” and “igneous” — basically, it’s a metamorphic rock that got heated up so much it began to melt, but didn’t finish the job. The minerals with the lowest melting points (quartz, potassium feldspar, muscovite) melted out as “granite” flavored magma. After the rock was heated up enough to express out this liquid felsic distillation, squeezed-dry sheets of unmelted mica-rich schist were wafting in the magmatic breeze, and boudinaged. Torn asunder, these liberated schist chips could begin to act as mechanically independent particles. They could get tumbled along in the magmatic currents, and even imbricated against their neighbors. Later, when the granitic magma around them crystallized, they were locked in these illicit positions for the next half a billion years…

Happy Friday; enjoy your weekend. Here’s hoping spring comes sooner rather than later. I could use more hikes like this, and less time sheltering inside with only the blog for company…