Cwm Eigiau Quarry

Overview

A small open slate quarry at the head of the Afon Eigiau valley in Snowdonia, worked before 1840 and served by its own tramway down to the river Conwy -- a valley whose name later became notorious for an entirely separate catastrophe, the 1925 collapse of the Eigiau reservoir dam.

History

Cwm Eigiau slate quarry was a small open working at the head of the Afon Eigiau valley, a tributary of the Conwy, worked before 1840. Slate was originally carried out by packhorse before a tramway extension was built in the 1860s down to the river Conwy, connecting to the neighbouring Cedryn quarry's own tramway. By 1874 the two operations had combined as the Caedryn & Cwm Eigiau Slate Co. Ltd, but the company was dissolved just eleven years later, in 1885, once the slate proved to be of poor quality; the quarry itself had closed by the end of the 1900s. Decades later, the same valley's name became attached to one of Wales's worst civil engineering disasters: the Aluminium Corporation Limited built a dam at Eigiau from 1907 to power its aluminium processing plant at Dolgarrog, and a short extension to the old Eigiau Tramway was built to assist the dam's construction and later reconstruction work. On 2 November 1925, after five days of exceptionally heavy rain, the Eigiau dam failed, and the resulting flood killed 16 people in the village of Dolgarrog below; the disaster led directly to the Reservoirs (Safety Provisions) Act 1930, which introduced statutory safety requirements for large dam construction and inspection nationally.

Workings

A small open quarry worked across five terraces with an incline down to a mill, alongside up to four separate mill buildings, a substantial leat embankment, and small supply reservoirs feeding the works.

Closure

The Caedryn & Cwm Eigiau Slate Co. Ltd was dissolved in 1885 after its slate proved poor quality; the quarry itself had closed by the end of the 1900s.

What remains

Ruined mill buildings, a workshop and a powder house survive on the terraced quarry site, along with the leat embankment and small reservoirs.

Five worked terraces, ruins of up to four mills, a workshop, a powder house, and a substantial leat embankment with small supply reservoirs.

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 1874 the Cedryn and Cwm Eigiau slate operations had combined as the Caedryn & Cwm Eigiau Slate Co. Ltd, but the company was dissolved just eleven years later, in 1885, once the slate was found to be of poor quality. ¹
• The quarry was worked across five terraces with an incline to a mill, with up to four separate mill ruins, a substantial leat embankment, small supply reservoirs, and buildings including a workshop and powder house surviving on site. ²
• On 2 November 1925, after five days of exceptionally heavy rain, the Eigiau dam -- built from 1907 by the Aluminium Corporation Limited to power its Dolgarrog processing plant, and served in part by an extension of the old quarry tramway -- failed, and the resulting flood killed 16 people in Dolgarrog; the disaster led to the Reservoirs (Safety Provisions) Act 1930. 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) Eigiau Tramway — Wikipedia
2. (primary) Cwm Eigiau Slate Quarry — Coflein (RCAHMW)
3. (observational) Quarry workings in Cwm Eigiau — Geograph Britain and Ireland (Patrick)

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