Hendre-ddu slate quarry is remotely situated at the head of the Angell valley, north-west of Aberangell. This quarry operated from the 1850s to the 1940s. It was a fairly extensive site, part underground, which had a substantial mill and other buildings. Material went out via the Hendre-ddu incline and tramway, connected to the privately financed Mawddwy Railway. Output in 1883 was 878 tons from 31 men, but is unlikely to have exceeded 1000 tons. The quarry employed 25 men in 1898 and 17 in 1837-8. It closed in 1946.
The site is now mostly enveloped in forestry. Visible remains in 1991 included workings on four levels, a compact mill area, adits associated with massive spoil heaps (containing sizeable blocks), traceable tramroad formations and inclines, and a well-preserved reservoir at the top of the site. The quarry is known to have had a ward for injured workers attached to it.

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.176.
Ordnance Survey County Series 25-inch maps: sheet Merionethshire XXXVII.14, editions of 1889 & 1901.
RCAHMW air photos: 94-CS 1774-6 & 945180/49.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.171.

David Leighton & Claire Parry, RCAHMW, 12 February 2015