Slate was quarried on a small scale at Croesor from around 1840, and the Croicer Valley & Portmadoc Freehold Slate Company Ltd was set up soon afterwards, but did not last for long.
Early phase Two companies were working the slate by 1861, the Croesor Fawr Slate Quarrying Company Ltd, and the Upper Croesor Slate Quarry Company Ltd. There was close cooperation between them, as in 1865, the Upper company allowed the Fawr company to tip slate waste on their land, and they amalgamated in the following year. At the same time, the Croesor United Slate Company Ltd was set up, with a working capital of £160,000, and a lease to look for slate on Croesor Fawr farm. They also took over the lease held by the Upper company. Hugh Beaver Roberts was in charge of the new company, while Hugh Unsworth McKie was in charge of the Upper company. Both men were directors of the Croesor & Port Madoc Railway, effectively the lower section of the
Croesor Tramway below the
incline at Carreg-hylldrem. The Managing Director of the new company was James Wyatt, formerly the agent for the
Penrhyn quarry, near
Bethesda. After 30 June 1867, the chairman was T H Wyatt, and McKie became the manager and secretary. Prior to the opening of the Croesor Tramway, slate was carried by horse and cart to
Penrhyndeudraeth station on the
Ffestiniog Railway. In order to improve the journey, a long straight road, which now forms part of the A4085, was built south from Garreg by the quarry company. When the Croesor Tramway opened in 1864, a long incline was built from the quarry to the valley floor, and carriage by railway reduced transport costs from 7s 10.5d (39p) per ton to 2s 6.5d (12.5p). The mill was constructed in the early 1860s, and was powered by a diameter
water wheel which drove about 12 slate saws. In 1866, the mill was extended, to provide another 12 or 14 saws. Power came from another water wheel, constructed underground, and fed by the tailrace of the first wheel. It was in diameter, and backup power was provided by a steam engine. The shafting for the machinery was under the floor, as was a tramway for trucks to remove the cutting waste.
Late phase Details of the next period are sketchy, although Boyd states that the quarry was bought by S Pope of Llanbedr on 7 July 1883, and became part of the Parc & Croesor Slate Quarries Company Ltd, which was formed on 21 August 1895. The amalgamation with the Park Quarry, which was located near the Garreg Hylldrem inclines further down the Croesor valley, was arranged by Moses Kellow, whose uncle, Joseph Kellow, had briefly been the agent and quarry engineer for the
Conglog quarry in the 1870s. The quarry had been managed by Thomas Williams of Bryn, Croesor for many years, and Kellow had married Williams' daughter Nell in 1892. Williams died around 1895, and Kellow took over the management of the quarry, having had plans to reopen it for some years. Parc Quarry made slate ridging, and the amalgamation was made so that rather than supplying the ridging with slates from other quarries, he could supply it with slates from his own quarry. Kellow was known as a "fearless innovator", and set about modernising the quarry operation. The first problem that he faced was that the quarry below the level of the main adit had filled with water in the 17 years that it had been closed. Twelve chambers were submerged, and there were two sumps each holding some of water. Local opinion was that a new tunnel, at the level of the lowest chamber, would need to be cut, and a London-based engineer had calculated that pulsometer pumps could empty it in about six months. As the rails on the main incline to the lower chambers were still in place, Kellow mounted a large centrifugal pump on a platform, which he lowered down the rails as water levels fell. It could pump per hour, and he drained the entire quarry in around a week. His innovation also covered working practices. Traditionally, Welsh slate mines were worked by teams of four men, consisting of two rockmen, who worked in a chamber below ground, and two mill men, who processed the slate blocks produced by the rockmen. Kellow introduced a new system, in which two-man teams of rockmen supplied slabs to the mill, where independent two-man teams worked, who were allocated slabs by ballot. This enabled the mill to be run at full capacity most of the time, with productivity rising by 25 per cent in the first month after it was introduced. Initial resistance from the quarrymen was overcome as they saw their wages rising significantly. Kellow introduced compressed
air drills, which were fed by a small water-powered compressor located near the foot of the incline to the nearby
Rhosydd quarry. He also decided to construct a low-level adit, and work began on it, using the new air drills. A section of incline was built, to be used to counterbalance the trucks removing the spoil, but the adit was not completed. The water wheels were supplemented by
Pelton wheels, and Kellow, writing in 1911, described four high-pressure turbines, producing . Water to supply the turbines was obtained from two reservoirs, one located at above sea level and covering , and a second covering at . Water was also taken from a natural lake at . The adit was worked by three
Ruston & Hornsby 4-wheeled diesel mechanical locomotives, originally built in 1939 and 1941, and obtained second-hand. Two had been scrapped or sold by 1955, and the date of disposal of the third is unknown. Several buildings were constructed in the marshalling area at the far end of the adit, and a massive haulage winch partially blocked access to the incline to the levels above. In 1971, the
Central Electricity Generating Board (CEGB), operators of the
Ffestiniog pumped storage power station, became aware of this use, and calculated that if there was an explosion underground, either of their two dams could be damaged. They therefore drained both reservoirs, and Cookes began removing about 250 tons of explosives per week, which were taken to Penrhyndeudraeth or by rail from
Blaenau Ffestiniog railway station to various other
ICI Nobel works in the United Kingdom. In the early 1970s the quarry was purchased by the Ffestiniog Slate Company with a view to reopening it as a working slate mine. Planning permission for this was not granted and in the later 1970s most of the remaining mine infrastructure was removed for use in the
Oakeley Quarry, also owned by the Ffestiniog Slate Company. == Current conditions ==