Oakeley quarry is located on the immediate north-west side of Blaenau Ffestiniog. It was formed by the amalgamation of the Upper(Hollands) (NPRN 415776) and Middle (Gloddfa Ganol) (404307) quarries in 1878 and the Lower (Welsh Slate) quarry in 1883. the subsequent merger with Cwm Orthin and Nyth y Gigfran quarries made this the largest quarry in the area. with an output of almost 60,000 tons per annum employing 1700 men. After Penrhyn (40564) and Dinorwig (40538), this was the third largest quarry in the industry. Despite problems caused by the 'great collapses' of 1882-3, the quarry eventually comprised workings extending almost from sea level to almost 1500 ft (457m) on 26 levels (the lowest lettered 'R') with a stated 50 miles of rail tracks underground and extensive locomotive worked surface tramways. There were some 12 mills, the main concentrations being the Middle and Vertical Shaft mills that between them had 53 dressing machines and 53 saw-tables. About 30 workers cottages stood on the site and it had its own hospital. Much use was made of steam power, including the haulage of rubbish. The old Hollands quarry workings (415776) were used for dumping as well as considerable backfilling.
Apart from surface garvity inclines there were a number of underground uphaulage inclines mainly of the table type; one incline had six tracks. Unusually, a vertical shaft haulage was also used. From 1906 when electricity became available from Cwm Dyli, it gradually replaced steam.
Finished product was lowered directly by incline to the Ffestiniog Railway. From 1934 the LMS Railway was also used - there were inclines from both the main site and the Penybont mill to the railway. In 1937-8 the quarry employed 752 men.
The quarry closed in 1970 but re-opened in 1978 as two separate but associated undertakings, Gloddfa Ganol (with its tourist visitors centre) and Oakeley quarry, both mainly working untopping. In 1987 a pulverising plant was installed to produce powder for moulded slates. Two parts of the Oakeley underground workings were opened to visitors and the working mill could also be viewed. The commercial operation, under the name Ffestiniog Slate Co. reworked parts of the previously underground mine as an opencast quarry. This was sold to McAlpine in 1998 and the tourist side of the business was closed. The quarry continued to operate using heavy earth moving equipment to expand the previous open-cast operation.

Sources:
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.136-7.
http://en.wikipedia.org/wiki/Oakeley_Quarry

RCAHMW, 25 February 2015