Cwm Hengae is a valley that runs roughly north-west from the village of Aberllefenni in Wales.

The Roman road Sarn Helen ran along the valley, connecting Aberllefenni with Dolgellau.

[1] At the point where the Nant Bychan joins the Afon Llefenni, about a third of the way from Aberllefenni to the head of the valley is Hengae House.

On the south side of the valley are the Hen Gloddfa and Ceunant Du quarries, while on the north is Foel Grochan, named after the mountain it mined.

These may have started working as early as the 14th century; the earliest confirmed date of operating is 1500 when the local house Plas Aberllefenni was roofed in slates from the mine.