Plassddu Quarry

Overview

A slate quarry in the Nantlle Valley, one of at least 37 quarries that once worked this west-Wales slate district on a small, independently-run scale rather than as part of a handful of large integrated enterprises.

History

Plassddu Quarry was one of at least 37 slate quarries at work in the Nantlle Valley during the 19th and 20th centuries, part of a long belt of Cambrian slate, some 500 million years old, running from the Nant Ffrancon valley in the east to Nantlle in the west -- a belt that also produced roofing slates for the nearby Roman fort of Segontium in antiquity. Unlike the large integrated enterprises at Bethesda, Llanberis and Blaenau Ffestiniog, the Nantlle Valley's quarries -- Plassddu among them -- were characteristically run independently and on a much smaller scale, in part because, unlike those other districts, no single landlord owned the whole valley. Nantlle's own slate beds lay beneath the floor of the valley itself, so quarrymen here dug pits down to reach them, rather than working open hillside galleries as at Penrhyn and Dinorwic further east; slate from the valley's quarries was carried by the Nantlle Railway, opened in 1828, down to the port of Caernarfon.

Workings

Worked by digging a pit down through the valley floor to reach the slate beds beneath, rather than by open hillside galleries.

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.

• The Nantlle Valley had at least 37 operating slate quarries at one time, of which Plassddu was one, run independently and on a much smaller scale than the large integrated enterprises at Bethesda, Llanberis and Blaenau Ffestiniog -- in part because, unlike those districts, no single landlord owned the whole valley. ¹
• Nantlle's own slate beds lay beneath the floor of the valley, so quarrymen dug pits down to reach them rather than working open hillside galleries; slate was carried out by the Nantlle Railway, opened in 1828, to the port of Caernarfon. ²

Photographs & further reading

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

Sources

1. (primary) Dyffryn Nantlle — Slate Landscape of Northwest Wales — Llechi Cymru / Wales Slate (Welsh Government)
2. (secondary) Slate industry in the Nantlle Valley — Wikipedia

Record created: 30 August 2026 · Last researched: 31 August 2026