Part of NPRN 40590. A row of four open pits with precipitous sides, in a line trending north-east to south-west, in the hillside at the head of Cwmorthin. They were cut from below, from a working level which was denoted Level C (1) from which an adit, now collapsed, ran out to surface behind the incline drum house (NPRN 408350). Level B (2) was 18 metres below Level C with five chambers wholly underground in the north vein and was reached by an adit emerging just below the track to Rhosydd Quarry at SH6679746596. Lower again was Level A (3) which did not reach the slate veins after 105 metres, and in the rear face of one of the open pits is Level D, developed between 1891 and 1903 and exposed when that pit was quarried up to daylight.
W J Crompton, RCAHMW, 16 October 2008.