|Came here (15.4.2010) direct from the summit of Pen y Gadair Fawr. Postie's directions sum it up, there's no easy path down here from the summit. You can't actually see the stone until you get a little way off the summit itself, although the two appear close together on the map, there is a very large difference in altitude (summit cairn at 800m , stone at 560m). If you are keen/foolish/obsessed enough to follow my route and walk here from Talgarth, directions from the summit cairn as follows:
1. Head SW off the summit cairn. The land drops away very quickly. You will see Pen Cerrig-calch and Pen Allt-mawr on the skyline ahead, and a coniferous forest below them. I didn't think this was Twyn Du at first, it looked too far away and too far below, but it is.
2. Head in the direction of the forest, taking whatever safe route you can. At first the descent is fairly gradual over tussocky grass, but as you head down the slopes get steeper and the vegetation a little higher. You also find yourself in parallel with a number of small streams that started from springs above and you may need to cross one or two of them.
3. As you get lower, the NE edge of the forest becomes visible and with it Maen Llywd comes into view! At least then you know you're in the right place. Carry on down to the stone and be grateful you haven't broken your neck or twisted an ankle getting here. Rejoice!
The stone itself is a lovely flat slab, about 7 feet tall and very narrow on its edges. It sits at the corner of the forest surrounded by a mountainous amphitheatre. Behind you is Pen y Gadair Fawr, ahead is Pen Cerrig-calch (to the SW). The distinctive skyline notch to the NW is the narrow ridge between Pen Trumau and Mynydd Llysiau (at least I think it is!). It echoes, at least slightly, the shape of the top of the stone itself. The stone is very high up - possibly higher than any other in the area, or even Wales(!). Even so, it's still dwarfed by the ridges - none of the summits are visible from this elevation.
Not a soul to be seen, just the odd sheep. Postie is right, really worth the effort. The only (minor) blemish is the tatty rusty fence around the stone, which replaces an older drystone wall intended to separate the forest from the mountainside.
Leaving to the west, the land drops further again along the northern edge of the forest (I couldn't face climbing back up onto the ridge). Although the forest is not access land, there was no sign of any forestry in progress and nothing to indicate that access was forbidden - a large area has been cleared and then left - and the easy downhill walk along forestry tracks provided a simple way back towards Lllanbedr and Crickhowell, including some very pleasant stretches along the Gargwy Fawr and then Grwyne Fechan (there's an interesting ruined Hermitage at the southern end of the forest too).
Posted by thesweetcheat
4th May 2010ce
Edited 6th May 2010ce