:Dorothea quarry

The quarry commenced working in the early 1820s, though there were a number of smaller workings on or near the site before this. About 1829 it was leased by William Turner who named the workings {{anchor|Cloddfa Turner}}Cloddfa Turner. In the 1830s the quarry was generating £2000 profits per year ({{Inflation|UK-GDP|2000|1830|2016|fmt=eq|cursign=£}}). Turner's son took over as manager and renamed the quarry Dorothea, apparently after the wife of the landowner Richard Garnons. Profits began to fall in the 1840s, and in April 1848 the quarry was put up for sale, with 22 years remaining on the lease.{{cite book |first=Jean |last=Lindsay |title=A history of the North Wales slate industry |year=1974 |publisher=David & Charles}}

It was the largest quarry in the area, employing 200 men and producing 5,000-6,000 tons of finished slate a year. A group of quarrymen led by John Robinson, William Owen and John Jones purchased Dorothea from Turner for £3,000 - {{Inflation|UK-GDP|3000|1848|2016|fmt=eq|cursign=£}}. They renewed the lease of the quarry in June 1851. The quarry was put up for sale again in 1864, at which time production was given at 1,000 tons per month. No buyer was found, instead John Williams of Denbighshire gradually bought out many of the existing shareholders, and by 1879 he had amassed more than 70% of the shares.

Production peaked in 1872 at 17,442 tons, though 1875 was the most profitable year, generating £14,738 ({{Inflation|UK-GDP|14738|1875|2016|fmt=eq|cursign=£}}). In the 1930s over 350 men were employed at Dorothea. Production dropped significantly after the start of World War II and the quarry closed in 1970.{{cite book |author=Richards, Alun John |title=The Slate Regions of North and Mid Wales |pages=70–71}}

Since quarrying ended in 1970, the Dorothea quarry has flooded and become a popular site for scuba diving (even though there are no facilities provided, and diving is officially banned in the quarry); the unregulated nature and depth of the site has encouraged some divers to overestimate their capabilities – in the decade 1994–2004, 21 divers lost their lives in the quarry.{{cite web |url=http://news.bbc.co.uk/1/hi/wales/north_west/3557437.stm |title=Why Dorothea claims lives |accessdate=2007-03-16 |date=2004-03-22|work=North-west Wales News |publisher=BBC Online }}

The quarry sits at the bottom of the wide Nantlle valley and consists of six pits, the deepest dropping 106m from the surface. The slate veins here run vertically, allowing unusually deep vertical pits to be dug. Because the pits fall below the water table they needed to be constantly pumped to stay dry. A Cornish beam engine was installed in 1904 to pump the pits; it stayed in use until 1951 when it was replaced with electric pumps. This was the last new Cornish engine to be built.{{efn-lr|Another engine was installed later at Hodbarrow Mine, Cumberland, but this was an older engine, re-located.}} It remains in situ in its Grade I listed engine house.{{cite web |title=Cornish Beam engine, Dorothea Quarry, Dyffryn Nantlle |url=http://www.penmorfa.com/Slate/beamengine.html |website=The Slate Industry of North and Mid Wales |accessdate=21 February 2019}}

The quarry was one of the first users of the {{RailGauge|3ft6in}} Nantlle Railway in 1828. It later developed an extensive internal tramway system of {{RailGauge|2ft}} gauge. Dorothea was one of the first users of De Winton locomotives in 1869.

{{RailGauge|2ft}} gauge locomotives known to have worked at Dorothea.

{{Commons category|Dorothea quarry}}

{{Notelist-lr}}

{{reflist}}

{{Welsh Slate Quarries}}

{{Recreational dive sites|fresit}}

Category:Slate mines in Gwynedd

Category:Diving quarries in the United Kingdom

Category:Cornish engines

Category:Preserved beam engines

Category:Llanllyfni

Category:Nantlle Valley