Two major veins of commercial quality slate run through the Dyfi hills, from Aberllefenni in the west, to Dinas Mawddwy in the north-east. Several commercial quarries worked these veins during the 19th and the first half of the 20th centuries. Major quarries in the area include Ratgoed on Mynydd Llwydiarth and Cymerau on Mynydd Cymerau in the west; Hendre Ddu on Mynydd Hendre-ddu, Gartheiniog on Mynydd Gartheiniog in the centre of the hills; and Minllyn on Moel Dinas just south of Dinas Mawddwy.