To come back to this, I just chanced upon this paragraph:
"At a late meeting of the Archaeological institute, the Hon. W. O. Stanley gave an account of a remarkable block of stone in one of the mountain passes of Caernarvonshire, which is known as the 'CArreg-y-Saelhan,' or 'the stone of the arrows.' It is a flat stone, nearly circular in form and about six feet over, with a number of straight scorings upon it from a quarter to half-an-inch deep; and these, according to tradition, were produced by the arrows and other weapons of the chieftains, who, on the commencement of war, were accustomed to sharpen them upon this rock."
(from On a Discovery of Flint Arrowheads and Other Stone Implements at Little Salisbury Hill, near Bath, John Evans, Transactions of the Ethnological Society of London, Vol. 4. (1866), pp. 240-243.)

- And it struck me, am I right in thinking that most of these arrow stones are in 'mountain passes'? Are most of them in valleys? (Whereas (please correct me for I haven't a clue) lots of rock art is in more prominent spots?) Could this be significant??