According to Coflein [J.J. Hall, Trysor, 13 July 2005] this diminutive cairn sited near the wondrous Pistyll y Llyn is:
“The northernmost of two possible funerary cairns on Carneddau Hafod Wnog. A small cairn, probably funerary, 6m in diameter at its base and 0.75m high, and built on top of a natural outcrop...The top of the cairn is flattened, with a slight depression in its centre, filled with numerous quartz boulders. The origin of the quartz is debatable – they may have been added in the recent past”.
An excellent approach to both cairn and waterfall – the latter, to my mind, one of Wales’ finest cascades – can be made from Cwm y rhaiadr at the terminus of the road heading south-eastward into the hills from Glaspwll.