Today, in glorious sunshine with hardly a breath of wind, my wife and I enjoyed a walk on Brynberian Moor (Waun Brwynant). We had our picnic lunch at Bedd yr Afanc, which I have described before.
Now I am quite convinced that the bedrock on the slight ridge (on which the grave of the fearsome monster is located) is rhyolite, which looks remarkably similar to that of Rhosyfelin. The geology map confirms this, showing Fishguard Volcanics for some distance south of Brynberian -- rhyolitic tuffs (ashes) and lavas. Rhyolite fragments are littered about everywhere, and there is one large slab projecting though the turf which I think is a bedrock outcrop: