Hendre quarry was located to the west of Dolwyddelan, towards the head of the Lledr valley. It was a small pit working which operated from about 1840, producing a characteristic black slate. Water power was used for haulage and pumping, and steam may have been used later. There was no mill. Material was transported down valley by cart. There is archaeological evidence here for the use of a horse-whim used for uphauling rubbish.
In 1882 six men produced 120 tons but earlier tonnages were higher. In 1898 seven men were employed. The quarry reputedly closed in the early 1900s. However, early Ordnance Survey maps portray the quarry as 'Disused' from the first edition of 1889. Perhaps, then, the quarry was used only spasmodically in its later years.
Visible remains (in 1991) include traces of buildings, wheelpits (one alongside incline), launder pillars and behind the site a shallow reservoir.

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), pp.99 & 140.
O.S. County Series maps: sheet Caernarvonshire XXIII.13, editions of 1899, 1900 & 1914.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.75.

David Leighton, RCAHMW, 12 February 2015