Rhosydd Quarry

Overview

Rhosydd, perched in the high pass between Cwm Croesor and Cwmorthin, was one of the largest underground slate workings outside Blaenau Ffestiniog itself — remote enough that its quarrymen lived on site in barracks, and, in a rare piece of underground geography, its workings run right through the mountain into the neighbouring Croesor quarry.

History

Slate was discovered in the vicinity of Rhosydd in the early 1830s, reputedly by two men from Croesor who sought permission from the local estate owner, William Turner, to work the rock; slate mining proper began there in 1840. The Rhosydd Slate Company was formed in 1853 and became a limited company in 1856, developing the site as one of the largest underground slate workings outside Blaenau Ffestiniog, with 170 chambers recorded by 1883 across fourteen underground levels. Because the site was too remote for a daily commute from any village or town, many of its quarrymen lived on site in barracks — a southern block built around 1886 housing stables, an office and quarrymen's quarters, with a larger northern block, Barics Fawr, added in the 1890s. At its peak in 1885 the quarry employed 207 men and produced around 6,484 tons of finished slate that year. In 1864 a narrow-gauge tramway was built from the quarry's mills to a cable-hauled incline roughly 700-750 feet long — among the two steepest single-pitch inclines in Wales — connecting down to the Croesor Tramway and on to the port of Porthmadog. The original company went into voluntary liquidation in 1873 after heavy development costs, and the New Rhosydd Slate Quarry Company Ltd, dominated by local Welsh shareholders, bought the site in 1874 for £29,500. A major underground collapse in 1900, remembered locally as the 'Great Fall', destroyed a significant section of the workings and forced the company to develop new ground elsewhere in the mine; no fatalities have been documented in connection with it. Production and profitability declined steadily after the fall and through the slate trade slump before the First World War; the mustard-manufacturing Colman family bought the quarry in 1921 but failed to find adequate markets, and it finally closed in 1930. Captain John S. Matthews bought the site in 1947 hoping to reopen it, but the attempt failed and the pumps were switched off for good in 1948.

Geology

The quarry worked the Old Vein, one of five distinct slate veins in the Blaenau Ffestiniog district, here around 108 feet thick and dipping north at 20-29 degrees; the veins are separated by bands of poorer-quality rock within the Cambrian Ffestiniog Flags, fine volcanic ash and mudstone later compressed and cleaved by mountain-building pressure into workable slate.

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

Workings

By 1883 the quarry had developed 170 underground chambers, each around 50 feet wide separated by supporting pillars roughly 30 feet wide, across fourteen levels reaching down to level 9 at the lowest point of access. A narrow-gauge tramway carried slate from the mills to a cable-hauled incline of around 700-750 feet, one of the two steepest single-pitch inclines in Wales, connecting down to the Croesor Tramway for onward transport to Porthmadog.

Closure

Profitability declined after the 1900 'Great Fall' collapse and the pre-war slump in the slate trade; the quarry closed in 1930, and a final attempt to reopen it under Captain John S. Matthews from 1947 failed, with pumping stopped for good in 1948.

What remains

The ruined barracks blocks, including the larger Barics Fawr, survive on site along with the remains of the mill buildings and the quarry's inclines; underground, the workings are recorded by cave/mine explorers as still connecting through to the neighbouring Croesor quarry.

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.

• By 1883 the quarry had developed 170 underground chambers across fourteen levels; at its peak in 1885 it employed 207 men and produced around 6,484 tons of finished slate that year. ¹
• A narrow-gauge tramway built in 1864 connected the quarry's mills to a cable-hauled incline of around 700-750 feet, among the two steepest single-pitch inclines in Wales, linking down to the Croesor Tramway to Porthmadog. ^1,2
• A major underground collapse in 1900, the 'Great Fall', destroyed a significant section of the workings and forced development of new ground elsewhere in the mine; no fatalities have been documented in connection with it. ¹
• Exploration reports document a passable underground 'through trip' connecting Rhosydd's workings to the neighbouring Croesor quarry. ¹

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) Rhosydd quarry — Wikipedia
2. (secondary) Area 12 Cwm Croesor and Rhosydd Slate Quarries and Incline System — Heneb - Trust for Welsh Archaeology
3. (primary) Rhosydd Slate Mine plans (surface plan, plan and section of workings) — British Geological Survey / Cumbria Amenity Trust Mining History Society

Record created: 23 August 2026 · Last researched: 23 August 2026