Penarth slate quarry is located on the south side of the Dee valley above Carrog. It began as terrace and open pit workings from about 1868 - with the arrival of the railway - but later developed underground. It may have a much longer history.
The initial working used hand dressing, material carted to the road. Water power may have been used after workings developed underground in the mid-nineteenth century (though no traces have been found). An incline connection was made to the railway before 1875 (NPRN 309749). At its foot was a sawmill, possibly water powered, which may have utilised hand saws. By 1903 all sawing was done at this 'Wharf' mill. From the mill product was transported from the nearby 'Penarth Slate Wharf' on the railway (309749).
In about 1868 the quarry employed 150 men reducing to only 10 in 1883 but with an output of almost 500 tons. The quarry closed in 1890 but re-opened five years later, employing 50 men in 1898. It finally closed in 1932 when it was producing only slab (1000 tons per annum).
Remains of several buildings survive on site (in 1991), including the mill with remnants of machinery, weigh-house, and the incline with drumhouse.
The pit and terrace workings survive, with ruinous buildings. Underground there are extensive tunnels on three levels.

Sources:
Ordnance Survey County Series 25-inch maps: sheet Merioneth XVI.2, editions of 1875 & 1901.
RCAHMW air photos: 955025/70; 955026/44-5.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), pp.206-7.

David Leighton, RCAHMW, 26 February 2015