Yeah, theres a huge 'mother rock' (sh490440) a few miles outside Criccieth on the Afon Dwyfor in the valley not far from 'Ystumcegid'. I stumbled across it when when i took a day out from the family, on holiday last year. It's mostly white quartzite granite and covered in green lichen , its probably some sort of glacial erratic, but it is apsolutely huge (20 ft long by about 15 ft high by about 10 ft wide), and shaped like a hump back whale. The shear presence of the thing was overwhelming and i spent a good hour just chillin', reclining along it's 'back' having my dinner on this scorching summers day. If we go back to wales again this year, i'm definately going back, as it's setting, in a tree lined hollow next to a stream and a quiet farm track is pure bliss.
There was also another 'portal dolmen' nearby which Julian does'nt mention in TMA, but is well worth a visit. The name is 'Beudy r Gromlech' (sh 489404) and its situated about 2 miles as the crow flies from Ystum'. It's looks like a mini 'Pentre Afan' from certain angles, and is in a tree lined field, not far from the B4411, just before the village / hamlet of Rhoslan. (The pathfinder for the Lleyn Peninsula East No:13 is excellant for getting about, though i did find that there were a few more high fences than i expected.) If you've got a car theres plenty more 'stones 'n'stuff' nearby, but i had to rely on the cambrian coaster train service to get about.
When i get a scanner sorted out, i'l find somwhere to post the photo's.