Diffwys, also known as Diffwys Casson slate quarry forms part of the Ffestiniog group of slate quarries and is immediately adjacent to Maenofferen and Votty & Bowydd, occupying an east-west ridge between the Bowydd stream to the north and Afon Du-bach to the south. The quarry worked a series of slate veins of the Ordovician period and includes a mixture of above and underground workings.

Diffwys is celebrated as the mam-chwarel (mother quarry) of Ffestiniog, in that it was the first quarry, to produce for export rather than purely local needs around 1760 by Methusalem Jones of Cilgwyn quarry in Nantlle, in a lease to the Peniarth estate. Jones was the first to discover and exploit the Old Vein, which became the basis for this and other Ffestiniog quarries? prosperity. In 1800 Diffwys was purchased by William Turner and William Casson, slate quarrymen and entrepreneurs who had worked in the Lake District. Output exceeded 5000 tons per annum in the 1820s when it was the dominant producer. George Casson (nephew of William) recommended selling the quarry in 1862 for the huge sum of £120,000, reflecting the considerable investment that had been made over the previous few years in steam-powered saw-mills. Production however declined and by 1892 the company was bankrupt and the quarry on the market, this time for £3,435. At this date it employed fewer than 200 men. Extraction at the quarry continued on a small scale until 1925. Untopping work (removal of underground overburden to expose the pillars), begun in the 1980s.

The quarry to some extent was worked as four separate quarries: Hen Waith, also known as `Twll Mawr Casson? (SH 7112 4627), Penffridd (SH 7145 4639), Drum Boeth (SH 7167 4640) and Chwarel Newydd (SH 7187 4623). There was however a unified series of tips and working levels (floors) on the south side of the quarry, still clearly visible and in varying states of preservation. Remains include gwaliau for hand-processing and a series of `integrated? mills, those that include all the processes for producing roofing slate. Diffwys is thought to be one of the earliest quarry's in the industry to use ?integrated mills?. The earliest mill however directly associated with the quarry and one that produced slabs was off site at Pant yr Ynn (NPRN 28620), powered by the Afon Du-bach. Diffwys lacked a water supply of its own and therefore subsequent mills located on at least 4 of the Floors from the 1850s onwards made use of steam power.

The quarry also preserves considerable evidence for the use of internal railways and inclined planes, both counter-balanced and uphaulage. The quarry however, long refused to use the Festiniog Railway and until 1860 all slate went out by cart to boats on the Dwyryd estuary, a well-constructed track can still be followed down the hillside (NPRN 416212). An incline connection to the Festiniog Railway was finally installed in 1864.

Louise Barker, RCAHMW, October 2015.

Sources:
David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology;
Jean Lindsay ((1974), 'A History of the North Wales Slate Industry', David & Charles;
Alun Richards (1991), 'A Gazeteer of the Welsh Slate Industry', Gwasg Carreg Gwalch.
David Gwyn, (2015) Welsh Slate: the Archaeology and History of an Industry (RCAHMW)