Cilgwyn Slate Quarry was located a short distance to the north-west of Nantlle village. It was a pit working dating from at least the fourteenth century. By the end of the eighteenth century it was a substantial undertaking with, by the 1820s, horse-whim haulage. The quarry was idle in the 1840s, when there was much illicit working by trespassers, but was restarted in the 1850s when it developed rapidly into a large undertaking in four pits with an output (in 1882) of 7430 tons, employing 300 men. There was a steam hauled railway system on the pit floors with tunnels connecting the pits; and steam haulage, including the use of wire-rope inclines, Blondins and a steam railway system around the south and east of the pits. The main mill was on the east with three others to the south. A long rubbish run took waste to the west, later tipping to the north via a long horseshoe loop line. Finished product was shipped at Caernarfon, originally being boated from Foryd, later carted direct; transport by rail after the Nnatlle Railway opened.
Output declined steeply during the early twentieth century with final closure by 1956.
Visible remains in 1991 included the main tramway formations (some re-used as roadway), both inclines and the 'horseshoe' line. Buildings demolished.

Sources:
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), 47-8.
RCAHMW air photos: AP94-CS 1000; AP945129/42

RCAHMW, 22 January 2015

2.

Two tips of waste rock, both part of the relict landform of Cilgwyn Slate Quarry and generated by its now-infilled pit, one extending westwards, the other northwards.   

The western tip is a significant visual feature, dominating the landscape of the Nantlle valley but also providing a vantage point into it. Many slate-makers’ shelters were erected here in the 1930s by quarrymen working in the informal economy, not only stark symbols of the economic devastation the slate industry of north-west Wales faced in this period but also illustrating a return to the small-scale minimal capitalisation that marked the very early phases of slate quarrying, and local resourcefulness when times were hard.  

 The northern tip was formed by constructing a railway on a distinctive alignment, a sinuous curve, to enable it to gain height, reflecting the need to find space to tip waste rock as well as the typical slate quarry narrow gauge railway technology of sharp curves and steep alignments. 

This site is part of the Slate Landscape of Northwest Wales World Heritage Site, Component Part 3: Nantlle Valley Slate Quarry Landscape. Inscribed July 2020.   

Sources:   

Louise Barker & Dr David Gwyn, March 2018. Slate Landscapes of North-West Wales World Heritage Bid Statements of Significance. (Unpublished Report: Project 401b for Gwynedd Archaeological Trust)   

Tirwedd Llechi Gogledd Orllewin Cymru / The Slate Landscape of Northwest Wales. Nomination as a World heritage Site (Nomination Document, January 2020)   

Wales Slate World Heritage Site https://www.llechi.cymru/    

H. Genders Boyd, RCAHMW, January 2022