Dorothea Quarry

Overview

The greatest of the Nantlle Valley slate quarries, which chased a vertical vein so far below the water table that it needed the last new Cornish beam engine ever built to stay dry. The engine is still there in its Grade I listed house; the pits it kept drained are now three lakes that have drowned more than twenty divers.

History

Quarrying at Dorothea began in the 1820s on the site of smaller earlier workings, and around 1829 William Turner took a lease and gave it his own name, Cloddfa Turner. It was profitable almost at once, clearing some £2,000 a year in the 1830s, but returns fell away and the quarry was offered for sale in 1848. In 1851 three working quarrymen — John Robinson, William Owen and John Jones — bought it for £3,000, and from 1853 it passed to the Williams family, who held it until it closed. Under them Dorothea became the dominant quarry in the valley: output peaked at 17,442 tons in 1872, the most profitable year was 1875 at £14,738, and even in the 1930s it still employed over 350 men. Nantlle never consolidated the way Penrhyn and Dinorwig did — there was no single landlord, so the valley stayed a patchwork of independent quarries — and Dorothea's scale was won by outlasting and outworking its neighbours rather than by absorbing them. Demand never recovered after the Second World War, and the quarry closed in 1970.

Geology

The Nantlle slate belt runs through Cambrian rocks whose cleavage stands close to vertical, and that single fact shaped everything about how Dorothea was worked. A vertical vein cannot be followed by driving level chambers into a hillside, the way the Blaenau Ffestiniog mines work their gently inclined beds; it has to be chased straight down. That took the workings well below the water table into ground that would flood without continuous pumping, and it is the reason a slate quarry in Caernarfonshire ended up buying Cornish mine engineering.

Geological setting: Slate and Regional Metamorphism (well documented example)

Workings

Dorothea was worked as six open pits rather than as underground chambers, the deepest reaching about 106 m. Getting slate up out of a deep pit was the valley's characteristic problem, and Nantlle answered it with the locally invented chain incline, alongside the Blondin aerial ropeways used at neighbouring quarries. Pumping was the harder problem. A 68-inch Cornish beam engine designed by Nicholas Trestrail and built by Holman Brothers of Hayle was erected from 1905 and set to work in June 1906, and it kept the pits drained until 1951, when electric pumps took over. It was among the very last Cornish engines ever built, and it survives in situ in its engine house — a clock set in the gable — which is listed Grade I. Finished slate left the valley by the Nantlle Railway, opened in 1828 with Liverpool capital and Stephenson engineering, and the first public railway built to serve the slate districts.

Closure

Dorothea closed in 1970 as the post-war collapse in slate demand worked through the valley. The pumps stopped with it, and the pits did what they had always been going to do without them: filled with water to make three deep lakes.

What remains

The site is privately owned and the ruins are open to walkers on a circuit around the water, but the pits themselves are dangerous and swimming is prohibited. A 2020 exploration report describes the engine house sealed shut, its boilers still standing at the back and its clock face badly decayed, with rows of quarrymen's slate houses, a farmhouse reduced to a shell and an office building with decorative tilework surviving as ruins. The flooded pits became a well-known technical diving site and then a notorious one: 21 divers died between 1994 and 2004, with the toll reaching around 25 by 2014. Diving was prohibited for years before a licensed arrangement with the landowner reopened the water to suitably qualified technical divers in 2021, and there have been deaths since — a diver was recovered from about 85 m in May 2025.

Substantial. The Grade I listed Cornish engine house with its engine still inside is the centrepiece, alongside pit edges, spoil runs, dressing sheds, offices and terraces of quarrymen's housing.

Redevelopment

None — Dorothea was inscribed in 2021 as part of the Slate Landscape of Northwest Wales World Heritage Site, which recognises the Nantlle valley as one of the areas that roofed the 19th-century world.

Abandoned workings can be dangerous — unstable ground, uncapped shafts, flooding and bad air are common even where a site looks safe from the surface. This entry describes history and general location only; it is not an access guide.

Recorded claims

Individual factual claims, each attached to the source(s) that support it below. Where sources disagree, both are kept.

• Quarrying began at Dorothea in the 1820s on the site of smaller earlier workings; William Turner took a lease around 1829 and the quarry was known as Cloddfa Turner. ¹
• After being offered for sale in 1848, the quarry was bought in 1851 for £3,000 by three quarrymen, John Robinson, William Owen and John Jones; from 1853 it belonged to the Williams family until closure. ¹
• Output peaked at 17,442 tons in 1872, and 1875 was the most profitable year at £14,738. ¹
• The quarry still employed over 350 men in the 1930s. ¹
• Dorothea was worked as six pits, the deepest reaching about 106 m. ¹
• The 68-inch Cornish beam engine was designed by Nicholas Trestrail and built by Holman Brothers of Hayle; work began in April 1905 and the engine was set going in June 1906. (disputed) ²
• The Cornish beam engine is recorded as installed in 1904. (disputed) ¹
• The engine pumped until 1951, when electric pumps replaced it, and it survives in situ in a Grade I listed engine house; it was the last new Cornish engine to be built. ¹
• The slate veins at Nantlle run close to vertical, so the quarries had to be worked as deep pits below the water table and required continuous pumping. ²
• The Nantlle valley developed its own chain inclines for raising slate from deep pits, alongside Blondin aerial ropeways used at neighbouring quarries; the Nantlle Railway of 1828 was the first public railway in the slate districts. ³
• The quarry closed in 1970 and the pits flooded to form three deep lakes. ¹
• Twenty-one divers died in the flooded quarry between 1994 and 2004, with the toll reaching about 25 by 2014. ^1,4
• Diving was prohibited for years before a licensed arrangement with the landowner reopened the water to qualified technical divers in 2021; a diver was recovered from about 85 m in May 2025 and an inquest opened that June. ⁵
• Dorothea forms part of the Slate Landscape of Northwest Wales, inscribed as a UNESCO World Heritage Site in 2021. ³
• A 2020 exploration report describes the engine house sealed, its boilers still in place and its clock face decayed, with terraces of slate housing, a shell of a farmhouse and a tiled office building surviving as ruins. ⁴

Photographs & further reading

This page doesn't try to reproduce everything — these are the best original sources found for this mine.

Images

Sources

1. (secondary) Dorothea quarry — Wikipedia
2. (secondary) Cornish beam engine, Dorothea — Penmorfa (Welsh slate quarrying reference)
3. (primary) Dyffryn Nantlle — Slate Landscape of Northwest Wales — Llechi Cymru / Wales Slate (Welsh Government)
4. (observational) Dorothea Slate Quarry, Nantlle Valley, July 2020 — 28DaysLater
5. (secondary) Diver found at 85 metres in Dorothea Quarry, inquest confirms — The Scuba News UK

Record created: 24 August 2026 · Last researched: 24 August 2026