The remains at Croesor consist of a quarry track up from the village, used for road vehicle transport to the mine. Along the valley floor the Croesor Tramway ran leading up multiple inclines to Croesor and Rhosydd quarries.

The hydro-power station which was built to power the quarry was restored in 1999 and it once again produces power.

Just outside the mine gate there are a couple of buildings, probably a barracks for the miners.

Now entering the mine yard, on the left you have the remains of the winding house at the top of the incline. Just ahead of you is the remains of the fan house which ventilated the mine, and next to this is the adit.

The largest remains on site are the mill. Mostly a ruin now there are still some tunnels underneath but most have been bricked up from its time as an explosives store.

At the bottom of the tip there is remains of an old incline and a spoil heap where a lower adit was started but never finished.

Croesor was a medium-sized underground slate quarry operating primarily on the western edge of the Blaen y Ffestiniog slate vein. The quarry initially began operations in the 1850s, though with mixed success, and ultimately closed in 1878. It reopened in 1895 under the leadership of Moses Kellow, an energetic and innovative figure who significantly increased production to about 5,000–6,000 tons annually. Production declined in the early 20th century before the quarry closed permanently in 1930.

Unlike many quarries, Croesor had no significant surface workings and employed forced ventilation. It was notable for its high use of steam engines relative to its size. The quarry operated with seven underground floors connected by a network of adits and inclines. Ventilation was maintained through multiple vertical shafts and a Guibal fan installed in 1866. Water-powered mills were an important part of the operation: a 28-foot water wheel powered a mill with a dozen saws in the early 1860s; this was followed by a 39-foot wheel in 1866, supported by a steam engine. Later modifications under Kellow replaced the water wheels with a Pelton wheel and introduced electrification, including a hydroelectric station producing 350 kW AC power by 1903. The quarry also pioneered the use of one of the first AC electric locomotives by 1905.

Transportation evolved from initial horse-drawn carting of finished slate to the Ffestiniog Railway at Penrhyndeudraeth, to a long inclined tramway linking to the Croesor Tramway after 1864, significantly reducing freight costs. Several steam engines supported pumping, ventilation, and haulage throughout its operation. Various experimental ventilation and haulage techniques, including water-blast ventilation and hydro-compressor powered air drills, were trialed.

Open access. Please be careful inside as there are many call outs from people doing the through trip to Rhosydd. Many of these are because people can’t find their way out the Rhosydd side. Please be prepared before entering the mine.