The West Twll (Pit) was the earliest part of Rhosydd Slate Quarry to be worked, probably sporadically in the 1830s and 1840s, and regularly from 1853. Four working levels can be seen in the pit, the lowest being Level 4 from which an adit was opened in 1857-59, reaching the surface 460 metres to the north. Traces of inclines between Levels 2 and 3, and 3 and 4 can be seen in the pit. The bottom of the pit communicates with the top of the underground incline between Levels 5 and 9.
W J Crompton, RCAHMW, 12 March 2009.