3 September 2013

A new outcrop of Catoctin meta- volcanic breccia

Posted by Callan Bentley

Lily and I took our son to the Virginia Scottish Games on Sunday. This is an annual competition of events in traditionally Scottish feats – throwing heavy things, mainly. There is also fried food, whisky, and some terrific music. The event is held at the Great Meadow, south of The Plains, Virginia. I had never been there before – it’s a site often used for horsey sorts of events, and though my wife is into that, I’m not. So I was delighted to find that there were terrific outcrops there of highly foliated, green-colored, matrix-supported metaconglomerate, rich in vesicular/amygdular greenstone clasts. I interpreted it to be a volcanic breccia unit within the Catoctin Formation, such as are seen in many places along Skyline Drive in Shenandoah National Park, Virginia.


Great Meadow is on the east limb of the Blue Ridge anticlinorium, and Shenandoah National Park on the west limb, and so in no way should it be surprising to see this rock unit cropping out far east of where I am used to seeing it.

It also occured to me that it might be the basal layer of the Fauquier Formation, a rock unit that “feels” more local to me. That’s a Neoproterozoic metasedimentary rock unit that some interpret as recording Rodinian rifting, and others interpret as the sedimentary signature of Snowball Earth glaciation. I’m used to seeing it up at Groveton Farm, near Aldie, but this sure looked a bit like it, too. But greener, and without the coarse, foliated clasts of the Marshall meta-granite that dominate in the Fauquier Formation. So it was probably Catoctin, right? The two are in direct contact in this area, so I looked up the most recent geologic map of the area on the USGS’s user-friendly new geologic map server, and found that Gary Espenshade (1986) interpreted the Great Meadow site as dominated by Catoctin meta-volcanic-breccia, of both high- (peach colored on the map) and low- titanium flavors (purple on the map):


Zooming in on a particularly distinct clast:


Lots of nice boulders of this stuff lying around, too – I think I’m going to ask the Great Meadow management if I can have one for the blooming NOVA rock garden…