Amalgamation of Holland, Gloddfa Ganol and Welsh Slate workings. Also linked underground to Cwm Orthin. Extensive underground workings. A lot of untopping work done. Was the Gloddfa Ganol show mine.*

The site is in continual flux due to untopping operations. The large pit is deep, and roads and tipping areas obscure much of the original quarry layout. In the southern part of the quarry, old buildings survive and are reused as sheds, including the Bonc Shafft mill. Vestiges of a steam-powered tipping incline and traces of the vertical water balance also remain visible. At higher levels, such as Bo 5 and Gloddfa Ganol, buildings from the Mathew era, cottages, and a modern mill still stand alongside ruins of other historic structures. The 1906 incline drum house, featuring zig-zag steps, remains, as do vestiges of Samuel Holland’s first steam mill and the early inclines. The historic Horse Tunnel passes through the site, with portions of early top-level workings open, including tramway formations, weigh houses, and tipping points. Connections to Nyth y Gigfran via a tunnel with chambering exist as well. Other notable features include a powder house and early buildings possibly dating back to Holland’s original quarrying works. Future tip exploitation is expected to further erode remaining historic features.

Formed through the amalgamation of several smaller quarries, including Rhiw, Cesail, Cwm Orthin, and Nyth y Gigfran. The original Rhiw quarry was started by Samuel Holland in 1818 and was an early adopter of steam power for its mills. At its peak, the quarry produced approximately 14,000 tons of slate annually, employing over 500 men. The quarries were connected to the Ffestiniog Railway by a 2-foot gauge tramway and a gravity incline through the Horse Tunnel. Nathaniel Mathew’s Rhibrydfydir Company (working from the 1830s to the 1870s) pioneered steam-powered dressing machines and used the Nant Ystradau mill. By the mid-1870s, the combined output of the constituent quarries exceeded 50,000 tons per year, supporting a workforce with salaries in the four-figure range. In 1882–83, reckless underground working caused major collapses, leading to legal actions and the eventual full amalgamation of the quarries under Oakeley.

The combined workings extended from sea level up to 1,500 feet across 30 floors, with around 50 miles of underground track and nine mills (plus three at Cwmorthin), operating roughly 200 saw tables. Power was supplied by water wheels, vertical water balances, and about 20 steam engines powering mills, pumping, and underground haulages. The quarry operated multiple locomotives from the 1870s onward, and electricity was introduced after the 1906 opening of the Cwm Dyli hydroelectric station. Rubbish disposal was managed through backfilling, dumping, and via a viaduct-connected tip for the Penybont mill. The quarry community included about 30 cottages and a hospital, with payrolls exceeding 1,000 before World War II. Oakeley closed in 1970 but later reopened as two separate yet associated workings, eventually being sold in 1997, a transition which led to the loss of many historic artifacts.