Pant Mawr Slate Quarry was opened in the 1840s and operated in conjunction with Moelwyn Slate Quarry on the east side of Moelwyn Mawr. A series of thirteen levels with their attendant spoil heaps form a line across the hillside. From 1853 slates were carried by pack animals down Cwm Maesgwm. In 1863 a tramway (NPRN 408876) was constructed across the south-west face of Moelwyn Mawr to the head of a double-pitch incline (NPRN 408877 and 408878) falling 340 metres to Cwm Croesor where it joined the Croesor Tramway.
The mill was probably built in the 1850s on a terrace part-excavated and part supported by a massive retaining wall, at an altitude of 530 metres above sea level. The mill was built of roughly-squared country rock and measured 48 by 12 metres, but only fragments of its walls remain. Two inclines connected the mill level with some of the adits at a higher level (NPRN 408874), and to the 1863 tramway below (NPRN 408875). Pant Mawr was amalgamated with Fron Boeth Quarry (NPRN 408879) in 1886 and surface operations ceased.
W J Crompton, RCAHMW, 25 March 2009.