Partly underground quarry.**

Pen y Ffridd is notable for what are thought to be some of the earliest underground slate workings in Wales, now set within dense forestry. Almost all surface structures have disappeared, with the only substantial survivor in reasonable condition being a building interpreted as the former smithy. Although a set of wheels on an axle has been found on the site, the quarry is reported never to have adopted railed transport, suggesting reliance on barrows or sledges instead.

The underground remains form a striking series of chambers dipping down across five levels, linked by extremely slender pillars that create the illusion of one vast cavern. Many of these spaces show clear signs of rockfall from former working faces and old roof collapses.

Pen y Ffridd began as an open working but soon developed underground sections as quarrymen pursued the slate veins in depth. In its early years it produced around 1,000 tons of slate annually, with output doubling during the 1820s before a long decline set in. Over roughly the same period, the workforce fell from about 50 men to only a few, and the quarry finally closed in 1865. Most of the finished product left the site by cart along the road through Llanrychwyn to Trefriw quay, where it was loaded for further transport.