Cwm Eigiau Slate Quarry is located at the head of the valley of the Afon Eigiau, a tributary of the Conwy. It is a small open quarry which opened before 1840, closing by the end of the 1900s. It was connected by an 1863 extension of the Cedryn tramway which served the nearby Cedryn Quarry (NPRN 400572), succeeding an earlier cart road, linking the quarries to the Conwy river.
Visible remains (in 1991) include the workings themselves which were conducted on five terraces with an incline to a mill, with possible evidence of an earlier incline and of direct tramming from a low level subsequent to use of incline; ruins of up to four separate mills; a massive leat embankment and some small supply reservoirs and leatwork; and various ruined buildings including workshop and powder house. The barracks have also been identified and despite dilapidatation its plan is thought to reflect the influence of the Rigby Foundry at Hawarden in north-east Wales, tenants of the quarry from 1827.
Despite significant capitalisation in the 1850s and 1860s, it is estimated that production cannot have reached 1000 tons per annum.

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), pp.26, 54, 56 & 187-8.
Ordnance Survey County series six-inch maps: sheet Caernarvon. XIII SW, editions of 1892, 1901, 1920 & 1953.
RCAHMW air photos AP945123/47.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), 70-1.

David Leighton, RCAHMW, 27 January 2015