Just north of Llyn Cwm Ystradllwyn is a stone row and a quater of a kilometre east of it is a small standing stone, there are hut circles all over the place too, heres what Coflein says about them.
A row of five boulders aligned north-west to south-east and estimated at approx. 20m long including gaps. It is possible that there are two missing stones. and
A standing stone 0.5m high by 0.3m wide and 0.3m thick and triangular in plan. The stone is sited close to both a field boundary and a sheepfold and is in an upright position.