Visited 15th April 2003: This is the strangest natural feature I've seen in a while. It's a long narow curving band of scattered rocks, stretching from the Carn Menyn Chambered Cairn down into the valley. From a distance it looks river like (and a bit alien) winding it's way down from the Preseli ridge. I pinched the name 'Stone River' from the book Prehistoric Preseli by N.P. Figgis.
I'm usually a bit sceptical about the connection between 'natural' sites and artificial prehistoric sites, but this one left me relatively certain that it was of significance to the people who erected the Preseli sites. The fact that the Carn Menyn Cairn was built directly at the head of this feature makes this likely. From Bedd Arthur the river can be seen stretching away from majesty of Carn Meini, down towards the flat fertile land where Gors Fawr is sited. The river is also very easy to see from below (e.g. from the Rhos Fach Standing Stones), like an arrow pointing to Carn Meini.
For some reason the stones are showing up as pink in this shot, which is kind of handy. Carn Meini is evident as a mass of pink to the right of the centre of the shot. The Stone River wins its way downhill from the south west corner of this mass of stones.