Llechwedd quarry was a major slate quarry in the town of Blaenau Ffestiniog, north Wales. At its peak in 1884 it produced 23,788 tons of finished slate per year and had 513 employees. It continues to produces slate on a limited scale and is the location of the Llechwedd Slate Caverns tourist attraction.
The Llechwedd slate quarry was opened in 1846 by John W. Greaves a successful quarry owner. When the Ffestiniog Railway was opened in 1836, connecting Blaenau Ffestiniog with the sea at Porthmadog, Greaves travelled on the historic first train. In 1843 Greaves was elected the treasurer of the Ffestiniog Railway and in 1844 he became its chairman. By this time Greaves was running the Diffwys and Bowydd quarries under leases.
In 1846 Greaves and his partner Edwin Shelton leased pasture land nearby at Llechwedd and began prospecting for slate. They gave up their rent of Bowydd at this time. By 1847 miners located the "Old Vein" at Llechwedd and development of the quarry began under the management of John's son Ernest Greaves. An incline was built to connect the quarry with the Ffestiniog Railway which was extended from its terminus at Dinas to meet the incline. This work was completed on June 30, 1848.
In 1851 2,900 tons of finished slate was produced by Llechwedd and shipped over the Ffestiniog railway. The quarry continued to increase production in the 1850s and 1860s, with 7,620 tons shipping in 1863.
In 1890 Llechwedd converted its internal quarry machinery to electrical power, becoming the first quarry in the Ffestiniog area to do so. Because of the high rainfall in Blaenau Ffestiniog the quarry used its own hydro-electric generating station to provide this power.
In 1909 Richard Greaves, the son of Ernest, became chairman of the Ffestiniog Railway
Blaenau Ffestiniog Slate Quarries