Aberllefenni quarry is the collective name of three slate quarries, Foel Grochan, Hen Gloddfa (also known as Hen Chwarel) and Ceunant Ddu, located in Cwm Hengae, just to the west of Aberllefenni, Gwynedd, North Wales.

Foel Grochan is the quarry on the north side of the valley, facing Ceunant Ddu and Hen Gloddfa on the south; all three were worked as a single concern throughout their history.

The earliest confirmed date is 1500 when the local house Plas Aberllefenni was roofed in slates from the mine.

[3] In 1873, the quarry appointed Robert Hughes as manager, and he quickly modernised working practices there.

There was some improvement during the late 1930s, with 140 workers employed in 1940,[2] but the Second World War brought further drops in production, with the number of men fell to 40 in 1944.

Slate extracted from the narrow vein at Aberllefenni is deep blue and extremely hard and dense.

Foel Grochan mine consists of eight near-horizontal tunnels at approximately 60 feet (18.3 m) vertical separation.

As more slate was extracted, several of the upper chambers were joined vertically to form an extremely large cavern known as Twll Golau which is open at the top.

After 1948, the short tramway connecting the quarries to the slate cutting shed at Aberllefenni continued in use.

Underground, the 2 ft 3 in (686 mm) gauge tramway continued in use, operated by battery electric locomotives until the end of working in 2003.