Penrhyngwyn slate quarry is located to the south-west of Dolgellau, on the north-western slopes of Cadair Idris. It was originally a small, unmechanised open quarry which later developed on several levels underground, probably at different times. Some sawing was done on site and a mill was built but possibly never completed. Material was carted down to the nearby road. The quarry was in use in 1883 when 27 men were employed producing 250 tons. By 1889, when the Ordnance Survey first-edition maps appeared, the quarry was out of use. Although it is shown as disused on the second-edition map of 1901, records show that 15 men were employed in 1898.
Several levels of workings can be identified associated with dressing sheds, weigh-houses, forge, mill and other buildings, incline and tramroad formations. Large tonnages of bad rock lie all over the site hinting at reasons for the quarry's early demise. Aerial survey in the 1990s indicated a quarry complex about 444m E/W and 127m N/S. Well preserved complex of spoil heaps, roads and quarry buildings sloping steeply downhill from main quarry hole to E, about 60m x 50m. A central area of fan-shaped spoil heaps bisected by E/W incline about 152m long, and central track or road along which a row of huts sit; three separate blocks, together about 49m x 6m. The most westerly structure a triangular terraced yard about 73m x 29m containing two ruined buildings, placed downslope at the foot of the incline.

Sources:
OS County Series 25-inch maps: sheet Merinethshire XXXVII.6, editions of 1889 & 1901.
RCAHMW air photo collection, oblique images.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p160.

David Leighton & Toby Driver, RCAHMW, 26 February 2015