Slate was discovered at the highest parts of this site in the early 1830s, but not worked seriously until 1852/3 when the Rhosydd Slate Co was established. The earliest workings were at the southern end of the site, in two open pits which, as they were deepened, were drained by successively deeper adits which also served to remove the slate and waste. In 1862-71 a 675 metre-long adit was driven at what became Level 9, with an underground trwnc incline (one with a moving horizontal platform) between Levels 5 and 9.

The quarry closed in 1873, was sold in 1874 and a new company, the New Rhosydd Slate Quarry Co Ltd, established. This company continued to work Floor 9 and above until c1890, and Floors 10 and 11 during the 1890s. In 1900 a huge rock fall destroyed much of the eastern workings. Mining continued in the west and on Floors 11, 12 and 13. The quarry was closed 1914-19 and the New Rhosydd Company was wound up in 1921. Thereafter new owners worked the mine until 1930, beginning Floor 14, then it was mothballed until 1947, the machinery scrapped in 1948, and from 1949 to 1954 slates were made from the demolished walls of the mill on Level 9. The grid reference refers to the Level 9 complex, which was the main surface working area of the quarry.

W J Crompton, RCAHMW, 12 March 2009.