Metamorphism means transformation. In geology it entails the application of increased temperature and pressure to change the shape and alignment of mineral grains. If the conditions are sufficiently extreme the minerals themselves may change. The slates of north Wales and the Lake District must have begun their lives as mud deposits or suspensions of volcanic ash in water. A mild degree of metamorphism produced a shale, and then in time flat silicate crystal sheets formed perpendicular to the direction of the pressure. This process occurred during several geological periods. The result is a rock that that shows cleavage, the capacity to be split into sheets. Sheets that can form the tops of snooker tables, sheets that take the place of plates in swanky restaurants, and sheets that can be used for roofing. Especially for roofing.
I don’t believe that there is a significant quantity of Welsh slate being produced at present. To the best of my knowledge the only active slate mining still being undertaken in the UK is at Honister in the Lake District. Characteristic Westmorland green roofing slate has been produced for centuries although even here there have been recent periods of closure. Until very recently I had not asked myself for exactly how long this material has been used. Two days ago we visited a Roman excavation at Ravenglass which has recovered large quantities of local slate. At least one substantial fragment we saw had a nail hole adjacent to a short edge, which is explicable by its use on a roof. The common Roman roofing material consisted of thick ceramic tiles (tegulae). These were enormously heavy and must have required massive roof timbers to support them. Lighter slates were much to be preferred I imagine. At present I am not certain how far the use of Roman Lake District slate extended, but in more recent times it got as far as Bradford where it forms our own roof.
Purple, grey and blue Welsh slates were also exploited in Roman times. They were moved by pack-horse in the post-mediaeval period, but as a universal roofing material slate was a child of the railways. It became widely used in Britain, and indeed Europe, around the mid-nineteenth century when that form of transport became readily available. There was a beautiful purple Cambrian period slate quarried south of Caenarfon and Bangor, where massive spoil tips can still be seen. A grey Ordovician slate was mined at Blaenau Festiniog, once the largest slate mine in the world. Slate prices rose rapidly and the Victorian industry was highly profitable to the owners.
The owners perhaps but not the miners. To those there were the obvious dangers from explosives and roof falls. The slate dust to which they were exposed produced the lung disease silicosis, a hazard that may have actually been worsened by the adoption of mechanical, rather than manual, methods for extraction and production. At Llechwedd slate caverns, now developed as a tourist attraction, we were told that the men worked six days each week, and were then expected to attend chapel twice on their one day of rest. Few men survived their forties and after the boom time of the 1860s-70s the industry was beset by labour disputes. Yesterday we spent the day with our friend Wendy. She also has visited the Llechwedd caverns; she and I both had exactly the same unforgettable experience. We were both told that the underground workers had an hour’s break around midday and built small slate cabans in which to eat and rest. The tradition was to occupy the time with the discussion of a current political or religious issue. The fact that, despite the adverse circumstances, minutes were taken to record such discussions is as remarkable a testimony to unconquerable human spirit as I can imagine. The men were as tough and remarkable as the metamorphic rock they worked.