There is another stone, which also has the same odd markings, also situated about 3 yards from the stream at SH63336774, commanding excellent views down to Bethesda and across to Anglesey.
I have searched for this stone quite extensively but cannot find it in this location. The only one I could find was a large boulder with another resting on top of it. The markings on these stones all looked natural to me. Apart from a quite deep circular hole, possible cup?
I have taken pics of this.
This stone is listed by the Gwynedd Archaeological Trust as of "unknown period"
"Some Arrow Stones and Other Incised Stones in North Carnarvonshire and North Denbighshire" by W. Bezant Lowe in Archaeologia Cambrensis 1924 pp. 340 - 362 gives the following description:
"An immense boulder of fine-grained rock, situated on the SE slope of Moel Faban, about 3 yards to the NW of a stream that flows down the valley between Gyrn and Drosgl, and about 12 yards to the NW of the path from Tanyfoel in the direction of Gyrn. The stone roughly measures 12 ft. by 12 ft. and 10 ft. high, but 3 large fragments have evidently been split off by frost. The markings occur in different places on the main block of stone.
(a) The most interesting group are those on the highest point of the stone; they are in a cup-shaped oval cavity which measures 6½ in. by 9 in. The markings, 23 in number, radiate from the centre of the cavity, and seem to be in 4 groups, each group having one deep and broad mark, about 6 in. long; the remainder are from 2½ in. to 3 in. long.
(b) On the NW face of the stone are two groups. In one group are 10 marks having lengths from 1½ to 6 in. They are all well marked and the deepest is 3/16 in. deep. To the right of this group are 5 fine marks measuring from 3½ to 5 in. About 15 in. above the last set is a narrow flat surface and on this are 11 marks, with one cutting across them: the middle one is ¼ in. deep, and the length of these marks is about 3 in."