Plassddu Quarry

Also known as	Plas Du Quarry, Plas-ddu Quarry
Location	Nantlle Valley, Gwynedd — 53.0453, -4.2532 (approximate)
Commodity	Slate
Type	Slate
Worked	from By 1855
Maximum depth	Unknown.

Overview

A small slate quarry on the Llanllyfni side of the Nantlle Valley, noticed from 1855, one of the district's many independent pits.

History

Plas Du Quarry lies on the Llanllyfni side of the Nantlle Valley at SH 490 521, among a cluster of small workings: the East and West Dolbebin, Nant y Rallt, Rhos Du and Tanrallt quarries and Chwarel William Owen Jones. It is first noticed in the Mining Journal of 1855, appears in the Carnarvon and Denbigh Herald of 17 January 1863 and in the Llanllyfni rate books, and is entered in Alun John Richards's Gazetteer of the Welsh Slate Industry.

It was one of at least 37 quarries at work in the valley in the nineteenth and twentieth centuries, a district of small, independently run pits: no single landlord owned the whole valley, and because the slate beds lay beneath the valley floor the quarrymen dug pits down to them rather than working hillside galleries as at Penrhyn and Dinorwic. The product went by the Nantlle Railway, opened in 1828, to the port of Caernarfon.

Geology

Cambrian slate of the Nantlle Valley belt, which lies beneath the valley floor and was reached by pits sunk through it.

Workings

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

Closure

Unknown.

What remains

Unknown.

Nearby mines

Tymawr West Quarry — 0.4 miles (0.7 km) NE
Cloddfacoed Quarry — 0.6 miles (1.0 km) N
Tal-y-sarn Quarry — 0.8 miles (1.3 km) N
Cilgwyn Quarry — 1.3 miles (2.0 km) NE
Pen-y-Bryn Quarry — 1.4 miles (2.2 km) NE

Straight-line distance, within 9 miles (15 km). Show on the map.

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
Wilkinson's Gazetteer of the Mines and Quarries of Wales enters Plas Du Quarry as a slate quarry in Llanllyfni parish at SH 490 521, first noticed in the Mining Journal of 1855, with references in the Carnarvon and Denbigh Herald of 17 January 1863, the Llanllyfni rate books and Richards's Gazetteer of the Welsh Slate Industry, and with the East and West Dolbebin, Nant y Rallt, Rhos Du and Tanrallt quarries and Chwarel William Owen Jones as its neighbours. ³

Photographs & further reading

Nantlle Valley Slate Quarry Landscape (Llechi Cymru) (academic)

The fullest account available of the Nantlle Valley's slate quarries as a group, including Plassddu.

View source

Wilkinson's Gazetteer and Bibliography of the Mines and Quarries of Wales (reference)

The searchable gazetteer whose entry for Plas Du Quarry gives the grid reference and references.

View source

Sources

(primary) Dyffryn Nantlle — Slate Landscape of Northwest Wales — Llechi Cymru / Wales Slate (Welsh Government)
(secondary) Slate industry in the Nantlle Valley — Wikipedia
(secondary) Plas Du Quarry, Llanllyfni: entry in Wilkinson's Gazetteer and Bibliography of the Mines and Quarries of Wales (hendrecoed.org.uk) — Jeremy Wilkinson / Dave Linton (hendrecoed.org.uk)

Record created: 30 August 2026 · Last researched: 24 September 2026