Visited 23rd March 2003: This stone stands to the south of the mountain road between Tregaron and Beulah, and may not be prehistoric. The only reference I've yet found for it (Cambria Archaeology) suggests it may be a post-Medieval milestone or standing stone. They list it without a name, so I've named it after the stream that runs nearby.
Having visited the stone, I'm not sure whether it's prehistoric or not. It looks to me like it's too far off from the road to be a milestone, but it could be a boundary marker (a small one at that though). Could it be a kerb stone from a cairn? Not the most awe inspiring site in itself, but enigmatic, and beautifully positioned.