21 September 2018

Friday fold: quartz veins in metagraywacke of the Mather Gorge Formation

Posted by Callan Bentley

It’s Friday!

Here’s a lovely sight, contributed by reader Fred Atwood:

Those are quartz veins in one of my favorite local rock units, the Mather Gorge Formation. Fred reports,

This is at Madeira School in Great Falls between Black Pond and the Potomac.

The rocks around Great Falls, particularly those on the Billy Goat Trail’s “A” Loop, are exemplary in many regards. That’s why I am taking my Physical Geology students there next month. Of relevance to understanding Fred’s video is that there are two generations of quartz veins in these rocks: (1) a pre-late-Ordovician set, which were ductilely deformed during the Taconian Orogeny (local metamorphic ages ~464 Ma), and (2) a post-Taconian set, which are undeformed, cutting across their crumpled predecessors. The latter set of veins, whose traces appear as straight white lines on the rock outcrop surface, are gold-bearing, but that’s not the case for these “folded oldies.”

I guess we can console ourselves that they have something even more valuable than gold to share: beautiful deformation! sublime wiggles! classic contortions! uplifting undulations!

Thanks for sharing, Fred; Great outcrops!