Quartz veins in the Rhosyfelin rhyolites

I was a bit mystified when I first saw clean white boulders and stones at Rhosyfelin, made of quartz.   They were incorporated in the till and other deposits.  Then I realized that they are entirely local, having come from a number of quartz veins within the rhyolite sequence.  In the top two photos we see what appears to be a bedrock quartz exposure, and a detached boulder which has within it "inclusions" of bluish rhyolite. This is very close to the position of the famous big stone about which the archaeologists get very excited.

The other three photos are from the NE tip or end of the spur -- and we can see here that there is an obvious vein of secondary quartz up to 20 cms thick.  It does not seem to follow one of the primary fracture planes, and I suppose all we can say about it is that it is younger than the rhyolite by an unknown amount........

We can see in two of these photos that chunks of bluish rhyolite have been incorporated into the quartz veins.

It will be interesting to get further geological insights on this quartz from Richard and Rob.