English: Maen Llia Standing Stone A short distance from the minor road leading from Ystradfellte to Heol Senni, the massive monolith of Maen Llia stands on the moorland in a magnificent and isolated spot in the Brecon Beacon National Park. It is a large diamond-shaped slab of conglomerate, 3.7m (12ft) high and 2.8m (9ft) wide and only 0.6m (2ft) thick. It is likely that a quarter to a third of the stone is below ground, so it has managed to stand up to thousands of years of wild Welsh weather.

   It lies at the junction of two valleys and its visibility from some distance suggests that it could possibly be a territorial marker or it could mark an ancient trackway, guiding travellers across the watershed, in a way similar to the stone north of Maen Mawr circle. In the Forties, some faint Latin and Ogam inscriptions were still visible on the stone's surface.
   A legend says that whenever a cock crows, the stone moves off to drink in the River Nedd. According to another story, the stone visits the River Mellte on Midsummer morning.

http://www.stonepages.com/wales/maenllia.html