Holland's Quarry, also called Upper Quarry or Cesail Quarry, was begun in 1827 as a surface quarry. It developed as an underground slate mine after 1840 (featuring pillared workings) and became a substantial undertaking with inclines bringing material to the surface. A steam-powered mill was built on Holland's Level from c.1860 and, unusually, some gas lighting was used underground. From 1839 finished slates were lowered down a gravity incline to the Festiniog Railway, the quarry an early user of this facility. A notable feature was the 1000 yard long 'Horse Tunnel' which gave connections to the mill from the inclines down from above. In 1882 output was 13753 tons with 534 men employed. The site became part of the Oakeley Quarries complex in 1878 (NPRN 404307).

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015).
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.132.

David Leighton & W J Crompton, RCAHMW, 12 February 2015