The lake occupies a hollow excavated in the sandstones of the Brownstones Formation of the Old Red Sandstone by glacial action during the succession of recent ice ages. Its northern margins are rimmed by a large curved late-glacialmoraine.
Access
The lake is within open country and readily accessible to walkers. A popular path up to Pen y Fan runs southwards up Cwm Llwch, passing by the outlet of the lake. The lake is seen to advantage from the peaks of both Pen y Fan and Corn Du and indeed from the spur of Pen Milan to its west.
Llyn Cwm Llwch viewed from the peak at Corn Du
References
^Ordnance Survey Explorer map OL12 'Brecon Beacons National Park: western area'