The cupmark is on a low earthfast boulder a little to the north of the standing stone. I found it accidentally while looking for the arrow stones, having had no idea it existed.
Gwynedd Archaeological Trust description:
A flat-topped, ground-fast boulder with a single large cup marked on top. Cup mark 8cm dia and 3cm deep, max rather large and flat bottomed compared to most and poss therefore not of same origin and date as most cup marks. The stone is very close to the cross-roads of tracks at the Bwlch and may be associated with the tracks. Boulder c.1.2m long and wide and 0.4m high. (Hopewell and Smith, 2010)