Thanks to Alun Roach for posting these pics on Facebook. Carreg Samson is a rather splendid cromlech near the north Pembrokeshire coast -- in the most spectacular of settings. It is supposed to have been created in the Neolithic, about 5,500 years ago. There are six uprights and a capstone. As we can see, the stones used in the construction are varied -- all erratics, and all used (as usual) more or less where found. The burial chamber was constructed in a pit -- thought by the archaeologists to be the excavation pit from which the capstone was lifted. So this cromlech was built exactly where a potential capstone was found; the uprights might then have been imported from a short distance away.
That's a rather utilitarian conclusion relating to the cromlech's location -- unlike other suggestions which are a good deal more romantic and imaginative.....
Here is a photo from Alan Hughes, showing the capstone in close-up.
The geology has not been well described -- but at least three of the stones are made of Ordovician dolerite, and the capstone, like one of the uprights, is made of what appears to be a rough volcanic agglomerate or ignimbrite, probably derived from one of the local igneous outcrops. This might be the rock referred to by BGS as a pyroclastic "crystal tuff" belonging to the Llanrian Volcanic Formation.
Grid ref: SM 848335