Looking NE. The block of forestry to the NW of the cairn shown on my 1/25000 map has been felled, allowing the full light of day to shine on the site once more.
Coflein doesn’t mention anything structural, but this looks awfully like a cist – note in particular the two upright slabs placed at right angles, at the top of the picture.
The walk up to Carn-y-Wiwer from the floor of the Rhondda Fawr valley via Cwm Lluest passes a lovely series of waterfalls. Lost on the cairn-builders? Doubtful.