Small-scale working of the site began in the 1830s, but was hampered by the remote location, and the lack of a transport system to carry the slates to markets.

Transport was made more difficult by the attitude of the Cwmorthin quarry, through whose land the most obvious route to the Ffestiniog Railway ran.

A solution was found in 1864, with the opening of the Croesor Tramway, to which the quarry was connected by one of the longest single-pitch cable-hauled inclines in Wales.

Huge amounts of money were spent on development work, and the company, unable to make adequate returns, went into voluntary liquidation in 1873.

The job of opening up new areas was spearheaded by Evan Jones, who nearly succeeded, but was hampered by a slump in the slate industry and the onset of the First World War, when the quarry was "non-essential" and was mothballed.

The result was that Rhosydd was part of the Cwmorthin Ucha estate, and the Ormsby Gores issued take-notes and later leases.

Harper briefly acted as managing director, but was soon replaced by a naval captain called Richard Oliver.

On 27 June 1853, the partnership became the Rhosydd Slate Company, which negotiated the conversion of the take-note into a lease in September.

[3] As the quarry developed, and workings were opened up to the north, a more obvious route was that down Cwm Orthin to the east, which reached the Ffestiniog Railway at Tanygrisiau.

[4] However, Cwmorthin objected, and restricted the use of this route by prohibiting the use of carts, which they enforced by erecting a gate at Ty Gwyn, about 25 yards (23 m) above the Ffestiniog Railway.

Construction began in 1862 without an Act of Parliament, by obtaining wayleaves for the route from Porthmadog to the foot of the first incline at Garreg Hylldrem.

[12] The first four wagons loaded with a total of 7.4 tons of slates descended the incline on 1 August 1864, the day on which the Croesor Tramway officially opened.

Adequate returns could not be made on the large amount of money sunk into development, and on 27 June 1873, it went into voluntary liquidation.

[14] A sale notice produced in 1874 in preparation for an auction of the quarry and plant claimed that development work at Rhosydd had cost £150,000.

Evan Jones took on the job of opening up new workings in 1906, when the previous agent, William W Morris, was retired, having shown that he was unable to complete such a task.

They had also bought the Groesyddwyafon quarry in 1920, and continued to work both much as before, but struggled to find markets for the product, and Rhosydd closed on 13 September 1930.

Between 1949 and 1954, a few slatemakers from Llanberis worked on the site, making roofing slates from the slabs which had been used to build the walls of the mill.

Each chamber was begun by miners, who cut a roofing shaft, which they then widened and made a trench on one side, to allow the rockmen to free blocks of slate.

The floor 4 adit was 1,501 feet (458 m) long, and was constructed from three faces, created by digging two vertical shafts to enable the work to proceed faster.

[21] The level 9 adit was the longest built at Rhosydd, measuring 2,221 feet (677 m) from the mouth to the haulage system at the inner end.

During its construction, an internal incline was built downwards from floor 5, so that rock from higher levels could be lowered down to the mine exit.

A screw handle was used to attach the front megryn to the rope, both on the line of full trucks in the mine and on empty ones outside the adit.

They fanned out into double tracks at both ends, and for 490 feet (150 m) in the middle, so that trucks moving in opposite directions could pass one another.

At the bottom of the incline, the cradle descended into a pit, to that its top was level with the floor, to enable the loaded trucks to be off-loaded easily.

[24] By 1883 the quarry had become one of the largest underground workings in Wales outside of Blaenau Ffestiniog, with 170 chambers and shipping 5,616 tons of finished slate.

[26] Although productivity was somewhat lower than the main quarries in Blaenau Ffestiniog, and it appears that the slates did not split quite as well, Rhosydd won a prize at the International Exhibition held in 1862, and promotional material indicated that its slates were used to roof the Royal Mint, Chatham Dockyard, buildings on the Blenheim Estate, Morris Motors' Oxford car works and the bus garage at Barking.