Pen-yr-Orsedd opened in 1816 under William Turner, who already owned the much larger Dorothea quarry next door and a third operation called Diphwys Casson at Blaenau Ffestiniog. Turner's quarrying empire stretched across the slate districts of North Wales. In 1854 he sold Pen-yr-Orsedd to John Lloyd Jones, who in turn sold it on in 1862 to the Darbishire Company -- the family business that owned the great granite quarries at Penmaenmawr on the North Wales coast. The Darbishires invested 20,000 pounds in expanding the quarry, but the early returns were modest: by 1871 Pen-yr-Orsedd was producing only 500 tons of finished slate a year. William Darbishire took direct management that year and turned the operation around. By 1882 production had risen sixteenfold, to almost 8,000 tons annually. Pen-yr-Orsedd had become one of the major slate producers of the Nantlle Valley, employing hundreds of quarrymen and supporting a substantial workforce of dressers, splitters, blacksmiths and engine men.

The Welsh slate industry's long decline began in the early twentieth century. Slate roofs gave way to tiles, then to asphalt, then to industrial roofing systems. The First World War interrupted production; the interwar depression hit demand hard; the Second World War redirected labour and materials. Quarry after quarry closed across Gwynedd. Dorothea, the giant next door, stopped working in 1969, its great pit slowly filling with water to become the famous flooded quarry of Welsh diving lore. Pen-yr-Orsedd carried on. The quarry kept producing into the 1970s, smaller each year but commercially viable enough to operate. When it finally closed in 1979 it had been working continuously for 163 years. With its closure, the commercial slate industry of the Nantlle Valley ended. The quarrymen retired, found other work, or moved away. The stone they had cut continued to keep cities dry from London to Toronto.

In 1963 a railway enthusiast called Rich Morris began collecting narrow-gauge industrial locomotives -- the small engines that had once shunted slate, granite and china clay across British industrial sites. By the mid-1970s his collection had outgrown his home in Longfield, Kent, and Morris needed somewhere to store and exhibit it. In 1976 he came to an agreement with the Festiniog Slate Group, who held the lease on Pen-yr-Orsedd, to move many of his locomotives to the quarry site. The idea was a museum dedicated to industrial narrow-gauge railways, telling the story of the small engines that had run the British extractive industries. Several of his locomotives had quarry pedigrees of their own. Britomart, a Hunslet built in 1899, had worked at Pen-yr-Orsedd itself before being sold in 1965; it now runs on the Ffestiniog Railway. Sybil, a Hunslet from 1903, was restored to working order at the Brecon Mountain Railway. Una, another Hunslet, from 1905, runs at the Welsh Slate Museum in Llanberis.

The Pen-yr-Orsedd workings cover hundreds of acres on the northern slopes of the Nantlle Valley. The quarry pit is staircased -- cut in great horizontal terraces called galleries, each a working level, joined by inclines down which the slate trams ran. The mill buildings, the engine sheds, the inclines themselves and miles of trackbed survive in various states of preservation. The site is technically dangerous and access is restricted; walkers along the Nantlle Ridge and the Llanllyfni mountain paths look across at the quarry's terraces from above. In 2021 the slate landscape of north-west Wales -- which includes Pen-yr-Orsedd along with Dorothea, Dinorwic, Penrhyn and other major quarries -- was added to the UNESCO World Heritage list, recognising the global significance of Welsh slate to the construction history of the modern world. Pen-yr-Orsedd cut roof slates for Westminster Abbey, for warehouses in Liverpool, for tenements in Glasgow, for civic buildings in Buenos Aires. The pit is silent now, the engines on display at museums, the quarrymen's grandchildren speaking Welsh in the same villages where their grandfathers learned to read it.