1 March 2011

News from the Billy Goat Trail (3): grab bag

Posted by Callan Bentley

Okay, a final post (for now, anyhow) sharing some images from last Friday’s field trip to the Billy Goat Trail, in Potomac, Maryland.

Yesterday we looked at lamprophyre dikes, but there are other dikes on the Billy Goat Trail, too. Like this granite pegmatite:

What’s interesting to me about this is that the joint set seems to have contradictory cross-cutting relations with the dike. Did the joints come first? Or the crystallization of the magma? If the latter, are the joints not really joints, but small faults? Blerg.

A look upstream at Mather Gorge, with the Rocky Islands in the distance. Students working at upper right:

A class portrait adjacent to Mather Gorge, looking upstream, from the “Pothole Alley” stretch of trail:

Okay, what are these?

I have some ideas, but before I pollute your mind with them, let’s hear what you have to say…

The full Flickr photoset from the trip (including really big versions of these images) can be found here.