A cairn of uncertain age, but Coflein suggests it may be prehistoric:
Situated on the summit of the ridge above the Nant yr Ychen and the Grwyne Fechan. Stone built and roughly circular on plan, measuring about 8m in diameter and up to 0.5m in height. Probably a routemarker, but some stones eroding out of the vegetation suggest that it could be older than post medieval. The cairn has a deep hollow, presumably the result of antiquarian investigation or robbing. The resulting spoil now forms the walker’s cairn that overlies the south-east side.
It didn’t grab my attention as I passed it.