Of the two quarries, Bellstone was the first to be opened up. Originally known as the Prescelly Quarry it was first worked in December 1825. By 1840 it had expanded to such an extent that it was necessary to divert the track, running north below the quarries to Bwlch Pennant, which was being overrun by the spoilheaps. Working continued intermittently to the 1860s when the name Bellstone was first used, and on a more intensive level through the 1870s and 80s when it had the benefit of the railway built to serve the Rosebush Quarry. The last company to work Bellstone was dissolved in 1891.
Rosebush Quarry had commenced operating by 1842 and was developed extensively from the 1860s onwards. The lessee of the quarry was instrumental in the construction of the railway from Clynderwen, through Maenclochog, to the site in the 1870s; and the working peak probably came in the years immediately after its opening in 1876. The development of the settlement of Rosebush, with its terrace of cottages and manager's house, also dates from this period. Intermittent working then followed through to the end of the century. The railway subsequently became part of the North Pembrokeshire line to Fishguard, and the railway buildings remained in use for a number of years after the closure of the quarry.
Workings on the southern part of the Bellstone Quarry are of a more ordered nature; with a deep pit drained by adit, still partially open and effective, and terraces developed on the hillside to the east. Spoil from the terrace workings has been tipped on the southern side in a long narrow heap necessary to avoid encroaching the Rosebush boundary. Other spoil has been tipped to the north-west on a large heap which exhibits a well defined 'fan' pattern through the use of tramways to extend the area of the heap. This heap has been eroded on its western side by the removal of slate as hard-core. No structures now remain on the primary dressing floor as a result of stone robbing by Preseli Pembrokeshire District Council in 1991. Some sheds below the quarry, and west of the trackway, do remain in use for domestic purposes associated with Quarry House. Quarry House itself is currently occupied as a dwelling.
The full development of terraced levels is exhibited in the southern working. Again, a deep pit is drained by adit (2 on the attached map); above this there are five further working levels. Access to the lowest of these was by an adit (1 on the map) partially cut through the country rock and part as a structure, roofed with old tram-rails, through the spoil. A tramway in this adit was used to tip spoil on the western side of the track and railway which were crossed by a bridge. Much of the spoil has now been removed from the dump west of the railway for use as road fill. Two of the bridge piers survive although that on the western side has recently (September 2002) been robbed of some corner stones which could compromise its structural integrity. Access to, and removal of slate from, the upper levels was by means of self acting inclines, the structure of which is still largely intact. At the head of each incline were drum houses which have survived in part although heavily robbed of stone by the district council in 1991. At each level the rude shelters used by the slaters survive as collapsed walling.
Adjacent to the trackway to the west of southern workings are the remains of two large structures. Both are roofless but retain the vast majority of their walls. On the east side of the trackway is the locomotive shed connected with the standard gauge railway. To the west is the dressing shed, on the northern end of which is the remains of the turbine house. The turbine was apparently removed by the American forces occupying the site during the 2nd World War.
The windmill, which stood on Penfelinwynt and supplied power to the quarry, has not survived and its site is now lost in the forestry above the quarry.
As a group, the four major working areas of Bellstone and Rosebush Quarries illustrate the various stages of development in slate quarrying; from the primitive excavations at the northern end, to the extensive terraced levels on the south. Whilst individually they are probably not of national importance, with a number of better examples to be found in North Wales. However, as a group they are of local and regional importance in South Wales where there has only been limited slate extraction.
The surviving structures on the site are concentrated around the most recent workings on the southern part of Rosebush Quarry. Here they are easily accessed and can assist in the interpretation of the various stages of slate production, from the quarrying of the rock to the sawing and splitting of slabs and roofing slates. Again they are not necessarily of national importance but, as a group, provide a unique local example. This is particularly important as they provide the only example of a direct interface with a standard gauge railway, devoted to serving the quarry, and an associated quarrying settlement.
In the long term the quarries could provide an excellent example for the interpretation of the slate industry in Pembrokeshire. as such they should be protected until such time as resources are available to develop such a scheme.