Above is an extract from the map showing the Rhosyfelin - Pont Saeson area. The site of the famous "bluestone quarry" is slap in the centre of the photo, where the minor road has a distinct Z-bend. The pinky-purple colour shows the extent of Fishguard Volcanics, which also sweep from here towards the NW, with a number of outcrops on the northern flanks of Carningli and Dinas Mountain.
The green colours represent sedimentary rocks -- for the most part Ordovician shales in this area.
The Fishguard Volcanics in this area are rhyolite lavas and ashes with considerable internal variation, which leads Rob Ixer and Richard Bevins to think they can tie down some of the Stonehenge fragments to specific locations at Craig Rhosyfelin with great accuracy. We have already had some discussions as to whether you can tie down particular fragments to a location with an accuracy of +/- 2m!!
We await with interest their paper on this matter....