The impressive three-storeyed Ynysypandy slate-slab mill, and its surrounding yard, rail-access and water-supply system, was built to serve Gorsedda quarry in 1856-7 by the local contractor Evan Jones of Garndolbenmaen, probably to the design of the distinguished engineer James Brunlees. It is ingeniously planned so that the natural fall of the site assisted the manufacturing process. An internal pit accommodated an overshot water wheel, supplied by the Henwy stream, and on the south side a long curving ramp brought branches of the railway from Gorsedda quarry into the mill at two different levels, serving the middle and upper floors. The grand, round-headed openings are closely spaced like a Roman aqueduct, and derive from foundry practice. The eastern gable is surmounted by a decorative feature incorporating a false chimney stack, and the windows were at one time framed. It is one of very few architecturally ambitious buildings in the slate industry of Wales.

The mill specialised in the production of slate slabs for floors, dairies, troughs, urinals, etc. In its heyday, in 1860, it was producing over 2,000 tons per annum, but seven years later production was down to 25 tons per annum (due to the poor quality of the quarried slate) and the business went into liquidation in 1871.  

The building provided a venue for eisteddfodau until the roof was removed around 1906. It was conserved by the Snowdonia National Park in 1981 and is accessible to visitors.

Statement of Significance:

Ynysypandy is a spectacular if untypical structure within the slate industry of north-west Wales, an architecturally ambitious multi-storey slate-slab mill, water-powered, built to serve the short-lived Gorsedda Quarry (NPRN 40557). Though it resembles a ruined abbey, its form derives from established iron-foundry practice, and represents a transfer of technology from the growing public railway network of the 1850s, through James Brunlees, the Gorsedda engineer. Functional linkages with the quarry, water resources and the railway system (NPRN 34633) are evident, though its majestic mountain setting only serves to emphasise the futility of this industrial undertaking.

This site is part of the Slate Landscape of Northwest Wales World Heritage Site, Component Part 4: Gorseddau and Prince of Wales Slate Quarries, Railways and Mill. 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/ 
• Haslam, Orbach and Voelcker (2009), The Buildings of Wales: Gwynedd. Pevsner Architectural Guide, page 362.

Hannah Genders Boyd, RCAHMW, January 2022