Limestone is a sedimentary rock. This means that it is formed as a sediment, usually in layers or ‘strata’ on the seabed, and is soft to medium in hardness. It is likely to contain fossils. Slate is a metamorphic rock (formed by intense heat or pressure). Slate is formed from soft sedimentary rock, made extremely hard by intense pressure which almost obliterates any traces of layers or fossils. A third category, igneous rocks, includes rocks formed from hot molten material. These are usually extremely hard, and never contain fossils.

Carboniferous limestone

Malham Cove

The main type of rock around Malham in north Yorkshire is grey limestone, which is quite hard for a sedimentary rock. It is called ‘carboniferous’ because it was formed in the geological period known by that name.

It has very obvious layers or strata, as well as vertical joints which tend to divide it into huge rectangular blocks. Water does not get through the blocks themselves, but it does seep down the joints and along the horizontal gaps between strata.

Rain picks up the natural carbon dioxide in the atmosphere, becomes acidic, and eats into the joints in the rock, splitting the whole area into blocks. Limestone is soluble in rainwater, so in a limestone landscape, water disappears underground. This is an example of chemical erosion.

In the early phases of the evolution of a limestone landscape, rainwater percolating downwards through some of the joints starts to dissolve more of the rock, forming vertical channels as it moves down. These channels are called potholes: they are the gateways to limestone’s underground world.

Once underground, the water dissolves the rock and creates a network of tunnels and caves which extend for many kilometres. It also erodes by abrasion, because there is a lot of water, which carries sharp stones and pebbles, flowing in some tunnels. Sometimes huge caverns are formed, as at Gaping Ghyll. When the roofs get very thin, they often collapse.

Gaping Ghyll

The appeal of North Wales slate

A slate quarry

Nearly all blue-grey slate roofs in Britain, and many more throughout the world, are made from Welsh slate. This is a metamorphic rock, formed from mudstone under conditions of immense temperature and pressure during a period of mountain building. Unlike limestone, this rock is completely unaffected by rainwater, so it is not subject to chemical erosion. It’s also so dense and compacted that there is hardly any physical erosion.

The slabs of rough slate brought in from the quarry need powerful machinery to handle and process them. Slate is very heavy: a piece the size of a telephone directory weighs about 6 kg. First, the rough edges are taken off the raw lumps and they are shaped into clean-cut blocks. The blades used have to be tipped with industrial diamonds. The next stage makes use of a unique property of slate. Although it is difficult to cut, it splits easily into thin sheets, which are still very tough, impermeable, and highly resistant to weathering.

Slate being split

The slate-mining industry and its landscape

These peculiar qualities of slate, and the skills to work with it, have been known to the people of north Wales since Roman times; but it was only two centuries ago that slate really came into its own. As the new factory towns of the Industrial Revolution spread across the landscape, it was slate that was used to roof the houses. As a barrier and as a protection against the elements it was matchless.

The market for slate eventually spread worldwide. The industry that grew up in response to the enormous demand for it at home and abroad changed the landscape for ever. Slate production was a wasteful business. Only one-tenth of the rock blasted out of the ground is usable. As a result, the old mines and quarries are surrounded by mountains of waste material, which is immune to both chemical and physical erosion.

Most slate occurs in remote places. But nowhere was too difficult for the fast-growing mining industry of the Industrial Revolution. The profits to be made were so high that companies opened up mines and quarries in the mountains high above the normal altitudes for agriculture and settlement. For example, Rhosydd village is 450 m above the height at which crops can be grown. But it was here, in 1830, that the Rhosydd slate mines were opened and a whole village built for the people who worked here. This place still feels like the middle of nowhere.

There were once 200 families living in Rhosydd. The men and the boys spent their whole working lives underground. People started work at the age of 12. Like all the other mines in the area, the way this one worked was determined by the position of the slate on and beneath the ground. The slate on the surface was the first to be mined. When that ran out, the mines had to follow the layers of slate underground through hundreds of kilometres of tunnels, some of which reached nearly 500 m below the surface.

Between 1880 and 1900, when the industry was at its peak, over five million tonnes of finished roofing slates were produced in north Wales. But slate mining was often a rough and ruthless business — people were often killed in the mines; there were many strikes; and mines often had to be closed as new roofing materials came onto the market.

Rhosydd was closed in 1930 — exactly 100 years after it started production. Apart from slate there was no other reason for the existence of the village, so the end of the mine was the end of the community. But the scars in the landscape left behind by the slate industry remain. It would cost a fortune to restore this land.

Deserted village of Rhosydd

At one time there were 614 working mines in north Wales. Today, there are only two in regular use, which still produce roofing slates. But because it is more expensive than modern roofing materials, slate is usually only in demand when an important new public building is erected.