Gloddfa Ganol (in English: Middle Quarry, also known as Mathew's Quarry) is one of the quarries that together form the Oakeley Quarry.
It was opened as an independent quarry by Nathaniel Mathew in the 1860s, on land located between Cesail Quarry and Rhiwbryfdir. The quarry was a pioneer of the use of slate dressing machines and was an early user of steam powered mills and inclines. By the mid-1870s Gloddfa Ganol was producing more than 10,000 tons of finished slate annually. 
The leaseholds for both Cesail and Gloddfa Ganol expired in 1878 and the landowner, W.E. Oakeley, consolidated the Upper and Middle quarries into a single operation: the Oakeley Quarry. Oakeley closed in 1971.
In the mid 1970s, Oakeley Quarry was re-opened, partly as a working quarry and partly as a tourist attraction. The tourist attraction used the name Gloddfa Ganol Mountain Tourist Centre and consisted of quarry tours and an extensive collection of narrow gauge railway rolling stock. The tours comprised an independent walking tour of workings within the Gloddfa Ganol area, and guided Land Rover/cap lamp tours at extra cost in the older Cesail workings higher up, leading to a viewpoint high above Glan y Pwll. The narrow gauge exhibits were owned by Narrow Gauge Enterprises, and moved to Gloddfa Ganol after an abortive attempt to create a museum at Pen yr Orsedd Quarry in Dyffryn Nantlle in 1975-6. Most of the locomotives were exhibited at first in a purpose-built building within the main complex, but were moved subsequently to a working mill towards the eastern end of the site, close to the walking tour adit; a passenger carrying railway was created between the two areas, but was only operated spasmodically. Other stock was kept outside, and a small portion of the collection deteriorated to the point of being scrapped (specifically the ex-Jaywick Railway carriages); it was also rumoured that some items of rolling stock were dumped down shafts.
The tourist operation continued in operation until 1997 when the quarry was sold to Alfred McAlpines and the tourist operation closed. The narrow gauge stock was sold off in a sealed bid auction organised by one of the directors of Narrow Gauge Enterprises, and the collection was dispersed. The FR acquired the 3' gauge de Winton vertical-boilered locomotive Llanfair, which was moved to Dinas, having been outbid for the part-restored ex-Penrhyn de Winton Kathleen.
The quarry continued to operate as McAlpine's Oakeley Quarry. Together with McAlpine's other slate interests in North Wales, it was sold to a firm named Rigcycle (linked to the Irish construction group Lagan) in December 2007.
- Richards, Alun John (1999). The Slate Regions of North and Mid Wales. Carreg Gwalch. ISBN 0863815529.
- Narrow Gauge Heaven Gallery of Gloddfa Ganol pictures
- Ben Fisher's fotopic site Joffres at Gloddfa Ganol, June 1996 gallery
- BBC Wales News 2007 sale