OS Ref (GB): SN108345 / Sheet: 145
Latitude: 51° 58' 34.38" N
Longitude: 4° 45' 18.18" W
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The stones only project above the surface for 40 cms or so, and they vary a lot in shape and dimensions -- and rock type too. Some are made of dolerite, some are volcanic ashes, rhyolites and what appear to be gabbros. I did not think it was a good idea to knock chunks off so as to examine the lithologies a bit more carefully. The stones have almost certainly just been picked up in the immediate vicinity and built into the monument. They are typical glacial erratics made of rocks from the Fishguard Volcanic Series -- and not one of them has travelled very far from its place of origin. The facets and broken surfaces seem to be of several different ages, but none of the edges is particularly sharp.
Where are the capstones? There are plenty of flattish stones embedded in the turf in the vicinity. Some may actually be embedded in the turf in the "passage" itself.
As we can see in the photos, many of the stones lean inwards towards the axis of the monument. The passage is not aligned towards anything significant.......
There are a number of boulders with rounded-off edges in the vicinity which seem to be made of a very similar bluish rhyolite as that which is exposed at Rhosyfelin. The rock outcrops there are less than 2 km away, towards the NNE. Without proper geological analysis, the origins of these boulders cannot be reliably guessed at -- and there are of course many other outcrops of bluish foliated rhyolite in the Fishguard Volcanic Series to the N and NW.