Located in Cwm Hengae, west of the village of Aberllefenni in Gwynedd, the quarry worked exceptionally dense, deep-blue narrow-vein slate. This made it ideal for cutting into large slabs rather than splitting into roofing tiles. Contains extensive horizontal tunnels and vast underground chambers, including a huge combined cavern known as Twll Golau. Underground work ceased in 2003.

Public RoW crosses part of the tip and affords good views of the Ceunant Ddu and Hen Gloddfa quarries.