|I was intrigued by this, having seen it apparently mentioned in the Mabinogion: "And by force of strength, and fierceness, and by the magic and charms of Gwydion, Pryderi was slain. And at Maen Tyriawc, above Melenryd, was he buried, and there is his grave."* Felenrhyd is just downstream, and the stone that marks his grave stands in the churchyard of Maentwrog.
'Maen Twrog' however implies 'Twrog's stone' - Twrog being the celtic St Twrog. The stone stands beside the church in Maentwrog. A website about the church suggests the stone marks St Twrog's grave.
It would be interesting to know how old the church is (the current one seems Victorian?). The wikipedia doesn't say where it gets its version of the legend from, but suggests Twrog was trying to destroy a pagan alter with reckless stone throwing from a mountain, and that explains why Maen Twrog and the church are where they are.
It also mentions the belief that "if one rubs this boulder one is fated to return to the village in the future."
Moss's kind researches from 'Welsh Saints' by Breverton turned up the information that:
At Maentwrog, a huge stone different to local rocks (possibly a glacial boulder) is attached to the angle of the church, and is known as Maen Twrog. It was supposed to have been thrown by Twrog from the top of the mountain of Moelwyn.
*(from Math, son of Mathonwy, online at the sacred texts archive
Posted by Rhiannon
21st June 2006ce
Edited 21st June 2006ce