King Arthur's
Labyrinth

Geology

Welsh Slate
Wales is world-famous for its slate products. Beds of slate, running along the sides of valleys, around hills or even through them, have been mined for centuries. But what is slate? Slate is basically squashed mud!

The rather complicated journey through geological events that culminate in the formation of slate begins, at some point back in geological time, with an ancient sea-basin, somewhere like the Irish Sea of today, with rivers feeding in sand, gravel and mud along its coast. The further away from the shoreline, the smaller the sediment particles that will make it out there: sands and gravels tend, being relatively heavy, not to travel so far and accumulate closer to the coastline.

So, "out there", on the deep-sea bed, there will be slowly accumulating layers of mud. This is what happened at various times in Lower Palaeozoic Wales, 550-410 million years ago: in between there were periods of volcanic activity, or periods of shallower seas with more sandy rocks being deposited. But it is the mud that interests us here.

Mud contains large amounts of what are called clay minerals. These are extremely fine-grained, so that each crystal is invisible to the naked eye. Clay minerals belong to the mineral family known as sheet-silicates. Other sheet-silicates include common minerals like mica. Mica sometimes occurs in large crystals that can be peeled away into lots of thin sheets.

Mica splits easily due to the mineral's internal chemical structure. The atoms of aluminium, potassium, silicon, oxygen and hydrogen making up mica are arranged in layers. Hence the term sheet silicates.

Minerals, like mica and clay minerals, which may be split in one or more preferred directions like this are said to have a cleavage. When mud settles to the seabed, the clay minerals do not all lie flat. They point in all directions, like this:

Of course mud cannot be split into thin sheets to cover your roof.

Old mud, which lay on the Lower Palaeozoic seabed hundreds of millions of years ago, now forms hillsides in North Wales where the slate-quarries are, so what has happened to turn it into slate? There is a clue here. It was on the seabed, now it is halfway up a mountain. Something pretty big must have happened to it! About 400 million years ago great earth-movements, involving the collision of whole continents, produced tremendous pressure that forced the old sea-bed upwards, like putty in a vice. The mud, which had already been turned into mudstone, was now turned into slate. But how?

We have seen that the clay minerals which of which mud is full are splittable, like mica, into thin sheets, and we have seen how in ordinary mud the mineral grains are aligned all over the place.

During the massive earth-movements that uplifted the seabed into the Welsh mountains, the rocks were subjected to tremendous directional pressures in a rather hot environment deep underground. Such pressures are sufficient to actually force mineral grains to recrystallise in the direction of minimum pressure. Or, to put it simply, if the rocks are being squashed in a giant imaginary vice, the minerals will recrystallise in an alignment parallel to the vice-jaws - and at right angles to the force that the vice-jaws are applying. Put even more simply, it's a bit like flattening something by standing on it!

So the recrystallised clay minerals are now all aligned in the same direction and the rock that has been created now has a cleavage, like the mica. It, too will split into thin sheets to go on your roof! What was originally mud on a seabed hundreds of millions of years ago is now a vein of beautiful Welsh Slate running across the Snowdonia mountainside.

Such a process, starting with one kind of rock and ending up with another, is called metamorphism. Geologists recognise many kinds of metamorphism, and the type that produces slate is broadly referred to as low-grade. But within this category there are varying sub-grades, from simple recrystallisation of one or two minerals through to significant changes in which many new sheet-silicates have formed from the chemical elements making up the old ones in the mud. The higher the grade of slate-formation, the better the quality of the slate in terms of its suitability for roofing. Thus, the distinctive purplish slates of Bethesda and Llanberis, formed from mudstones of Cambrian age, are the highest-grade in Wales and were largely used for roofing purposes. The slates of the Blaenau Ffestiniog area are also high-grade but are a bit younger, belonging to the Mid-Ordovician Nant Ffrancon Formation. Then we have the lower-grade slates of the Corris-Machynlleth area. These are of Upper Ordovician-Lower Silurian age and, while less useful for roofing, they make superb large slabs, ideal for snooker tables, mantelpieces, window lintels and a multitude of other uses.

Only rocks that originally contain a lot of clay-minerals will develop a good cleavage. If you want to make slate, start with mud, not sand. This explains why the slate in North Wales occurs in actual beds - sometimes with uncleaved sandstones in between.

Slate mines are a frequent sight throughout the mountains of North Wales. The Welsh Slate industry was once a very important part of Wales' economy. Fewer mines are active today but the tradition continues at several places from Aberlefenni near Corris up to Penrhyn near Bethesda. Why not visit the Welsh Slate Museum at Llanberis, to find out about this industry, which was responsible for the birth of whole towns where once there were only a few isolated farmsteads? Or go underground in a slate mine - there are the famous Llechwedd caverns at Blaenau Ffestiniog, and the old tunnels and caverns of the Braich-Goch mine at Corris, now the mystical King Arthur's Labyrinth.