I took this image in 2005, when I was working up a geologic history of the C&O Canal National Historical Park. It’s a vein of quartz, gracefully folded within the Catoctin Formation. The exposure is along the railroad tracks at Point of Rocks, Maryland, easternmost extent of the Blue Ridge province on the north shore of the Potomac River. The Culpeper Basin begins about 100 meters to the east of this site.
Note that the Catoctin (a former basalt) has been metamorphosed to greenschist here, with a pronounced foliation. Here’s a Photoshop-highlighted and annotated view of the same fold:
It is typical when folding and foliation form at the same time due to the same consistent tectonic differential stress that the foliation should be parallel to the axial plane of the folds. We saw the same parallelism in completely different rocks in the Canadian Rockies a few weeks ago. You know what they call that? Uniformitarianism!
Yes, there exists a previous folded cleavage, probably syngenetic with the quartz vein
So the basalt was injected along fault with hot siliceous groundwater, possibly near the surface, which precipitated the quartz. Then later, the entire unit was folded, which is when the basalt morphed into green schist?
And we know this green schist was a basalt how? From it’s chemical composition?
Also, I can make out another parallel fold just above, in what looks like another thinner vein.