Nantlle Valley (Dyffryn Nantlle) Slate Quarries

Six miles south-west from Llanberis are the quarries of the Nantlle Valley, where the slate outcrops on the floor of the valley, and whilst the quarries at Penrhyn and Dinorwig are worked in open terraces, at Dorothea and Penyrorsedd Quarries they are worked in deep pits. Around 80% of the population of the Nantlle Valley speak Welsh as their first language. There were a number of quarries in the valley, the largest being the Dorothea Quarry and Penyrorsedd. The horse-drawn Nantlle Railway served the quarries from 1865 to 1963.

The Nantlle Valley is part of the long belt of Cambrian slate formed about 500 million years ago. This stretches from the Nant Ffrancon valley in the East to the Nantlle valley in the West. In the East the slate was won by open quarries using the gallery method, while at the West end, the slate beds were found beneath the floor of the valley. It was this depth of the slate beneath the valley floor which influenced the quarrying techniques of Nantlle. The only way to obtain the rock was by digging down and creating large pits. These can be found in many locations in the Nantlle valley. Some of the difficulties of pit working included the need to pump out any water entering the workings and the hauling out of waste as well as good rock. Both of these increased costs. Nantlle slate was always at a disadvantage compared to the more easily won rock at Penrhyn and Dinorwig further East and was always one of the first to suffer from any downturn in the market.

A complex legal dispute between Lord Newborough and the Crown agent, Robert Roberts of Caernarfon over who actually had the right to quarry in the area, dragged on for over 30 years. In the meantime, the quarry at Tal-y-sarn started production in 1802, Dorothea Quarry in 1829 and Fron Quarry in 1830.

The oldest quarry in the Nantlle district was Cilgwyn quarry, this is situated to the north of Dorothea on the hillside and is now a landfill site. It is thought to have been first worked in the fourteenth century and it is believed that some of Edward the 1st's Welsh castles were roofed with Cilgwyn slate. Dorothea itself opened in 1820 and remained in production until 1970.

In 1828 the Nantlle Railway opened giving the quarries of the valley a route to the sea. By the 1840's production at Dorothea had built up to about 5,000 tonnes per annum and had reached over 17,000 tonnes by the 1870's. From 1872 the tramway ran only as far as Talysarn where connection was made with the national rail network. Much of its route is traceable today as far as the easterly terminus at Penyrorsedd Quarry. Dorothea Quarry used the Nantlle Railway to despatch slate from 1829 until 1959.

The Penyrorsedd slate quarry, close to the village of Talysarn, Llanllyfni, Caernarfonshire, was first developed in 1816. In 1862 the quarry was connected to the Nantlle Railway. In 1882 the Penyrorsedd Slate Quarry Co. employed 230 men. In the late 19th century, the quarry was worked by 450 men and despite the decline in the slate industry, by 1937 there were still 351 workers employed at the site. The quarry was connected to the Nantlle railway, which carried slate to the quay at Caernarfon. The quarry finally closed in the late 1970s. The Pen-yr-orsedd quarry was one of the major slate producers of the Nantlle Valley, supplying over 10,000 tons of product in 1864. It was also the last quarry in the vale that commercially produced slate, closing in 1979.

Other large quarries in Dyffryn Nantlle also closed - Pen y Bryn in 1950 and Cilgwyn (now a landfill site) in 1956. Today the only significant quarry still in production in the valley is Penyrorsedd, now owned by the Macalpine group, and exploiting a newly found seam of green rock.

North Wales Slate Quarries