Braichgoch Slate Quarry is shown on the Ordnance Survey County Series (Merioneth XLII,12) for 1889. The quarry and adjacent workshops were amalgamated with Abercorris Slate Quarry (NPRN 96191) and Gaewern Slate Quarry (NPRN 400831), to form an extensive area of slate workings in the valley of the Afon Deri between Corris and Corris Uchaf.
At Braich Goch underground workings produced mainly slab with very large sizes capable of being cut. The quarry also offered black-coated electrical switchboards. Early use of sawing machines reputedly took place here perhaps as early as the 1830s. Total output in 1882 was 5858 tons produced by 187 men. The workforce dwindled to 148 in 1898 and 77 in 1937-8.
In 1984 the quarry was still worked by the Wincilate group, who specialize in slabs cut to regular sizes for wall facings, memorials and similar inscribed tablets.
Shortly after closure the southern part of the site was landscaped, obliterating the mills area and the workings above; the main road now runs over the site of the mills. However, traces of buildings, workings and tramroad formations - including the surface connection to Gaewen - still survive (in 1991) in the oldest, northerly, part of the area. Three adits leading to underground chambers were also open.

Sources:
Association for Industrial Archaeology: A Guide to the Industrial Archaeology of Mid-Wales (1984).
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), p.44.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.182-3.

David Leighton & Claire Parry, RCAHMW, 14 Jaanuary 2015