Aberllefenni Slate Quarry

Overview

A group of three connected slate workings — Foel Grochan, Hen Gloddfa and Ceunant Ddu — in Cwm Hengae, Gwynedd, worked with only brief interruption from at least the 16th century until 2003, making it the longest continually operated slate mine in the world.

History

Slate from the Aberllefenni workings is recorded roofing the local house Plas Aberllefenni as early as 1500. Commercial, organised quarrying began around 1810, and the opening of the Corris Railway in 1859 gave the quarry an efficient route to Machynlleth and beyond, helping it reach peak employment of around 190 men by 1890.

Workings

The name covers three linked underground and open workings in Cwm Hengae — Foel Grochan, Hen Gloddfa (also called Hen Chwarel) and Ceunant Ddu — worked as slate mines rather than open pits for most of their history.

Closure

Employment fell steadily through the 20th century, to around 40 men by 1950 despite post-war modernisation and new machinery at Foel Grochan; the quarry finally closed in 2003, extraction no longer being economical, with only three men still employed at the end.

What remains

A 2025 exploration report of the Foel Grochan section describes extensive intact underground chambers and equipment left in place since closure.

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.

• Aberllefenni is reputed to be the longest continually operated slate mine in the world, closing in 2003. (medium) ¹
• The quarry employed around 190 men at its peak around 1890, falling to around 40 by 1950 and just 3 at closure in 2003. (medium) 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) Aberllefenni quarries — Wikipedia
2. (observational) Foel Grochan / Aberllefeni Slate mine, July 2025 — 28DaysLater.co.uk
3. (primary) Mine plan: ABERLLEFENNI — BGS / Mining Remediation Authority mine plans catalogue

Record created: 25 August 2026 · Last researched: 25 August 2026