Welsh Mountain Geology

Volcanic felsic tuff extrusions on the Glyderau, North Wales - photo Mark Trengove

The Geology of Wales (Cymru) – a Journey through Time

Introduction

Wales punches far above its weight in the world of geology. This science started its life in the south of England in the late seventeenth century and the proximity of Wales with an amazing diversity of rocks soon made it the formative land for geology.

There are distinct areas of rocks that give rise to different landscapes, floras, and shapes of the hills and mountains.  Below is a rough and simplified guide that will not only give an introduction to the geology of Wales but also to the origins of this scientific discipline.

Rock Types

There are three basic forms of rock:

1. Igneous Rocks

These are rocks formed inside the earth that find their way to the surface, one way or another.

If one imagines that the Earth’s crust is like the skin on a pan of two-day-old custard, a reheat of the custard, without stirring, will result in events akin to these processes. Where the material bursts forth and ejects material from underneath, as with a volcano, the ejected material, either as a hot flow of liquid or as a cloud of hot rock and ash will build on the surface of the Earth in the area. The way this material is deposited and the rate at which it cools will determine the shape of the crystals in the rock and the name a geologist will apply to it.  This type of igneous rock is known as ‘extrusive’. 

A second and more lasting form is where the material rises up inside the Earth as a hot blister of rock, like in a 1960’s lava lamp, but never actually breaks the surface. This type of igneous rock is known as ‘intrusive’. The hot blob, called either a pluton or a batholith by geologists, cools and hardens as a rock called granite. This type generally cools slowly to form a very hard rock with big crystals. It can, on occasions, be a friable granular rock instead.

The current day mountains are the eroded remains of an uplifted plain, which exposes the tops of quite a few of these features. These are a common form of mountain today. These rocks are the easiest to date by radioactive decay. Once the magma set, the radioactive clock started ticking.

2. Sedimentary Rocks

These are rocks that form as sediments, usually in a sea. As mountains wear down, the dust, grit and pebbles find their way by the forces of erosion to the sea. These materials, along with the fossils of the creatures of the era, form piled-up layers. The layering is usually obvious in the rock and is called bedding. These form horizontally, but the forces in the earth have pushed them to all sorts of angles.  However, in most sedimentary layers they are usually not far from the horizontal. There is evidence in Britain of past sedimentation layers building up to 6km thick in some cases.

This sedimentation, under modest pressure and chemical action, is reformed as rock - ready for the next time the Earth folds them to create another mountain range.

Sedimentation is not an even process. If a particular spot on the Earth is eroding away, it is a supplier of material to sediment somewhere else, and will miss out on the rock formation of that era. There can be hundreds of million years of a gap between sediments in a particular location. In this rock type, the rock can be dated radioactively from the embedded fossils.

3.    Metamorphic Rocks

These are sedimentary, volcanic, or even another type of metamorphic, rocks that hae been altered by being baked. They may be rocks originally deep in the Earth, adjacent to volcanoes or plutons, or, alternatively, receiving incredible pressure in the process of mountain-building that may have been heated up almost to the point of melting. This changes their nature and they become crystalline.

These rocks may retain the bands or layers associated with sedimentation, but are often contorted out of shape by the pressures to which they were subjected. These vary considerably, depending upon the type of original rock, the heat of the cooking process and the degree of deformation to which they were subjected. These are the most difficult to date using radioactive processes.

The Age of the Rocks

The early geologists categorised rocks by the fossils embedded in them. The initial bands were called Primary - those rocks of such an age that there were no fossils present; Secondary, with fossils of creatures that are quite alien to life today; and Tertiary, with recognisable fossils. There is also a Quaternary division for the rocks of the last couple of million years.

The above broad divisions are now called Geological Eras. These sub-divide into Geological Periods, and set out below are the main periods, time-scales and a description of the rocks formed at that time. All the earliest periods have Welsh related names and are a must for Welsh people to remember. Some of these names are used in the notes, and this table will need to be referred to in following some of the detail.

Geological Period

Rock types and distribution in Britain

Pre-Cambrian

Before 600 MYA 

Fossil-free primary rock, like the gneiss rocks of north-west Scotland from 1600 to as much as 2900 MYA. In Scotland, the Moine schist rocks are 1500 to 1025 MYA, and Torridonian sandstone rocks from 800 to 1000 MYA. Further south, in England, the Long Mynd and the Malvern Hills near Worcester are Pre-Cambrian.  This age of rock probably underlies all rocks in Britain, but usually at substantial depths. As the earth crust thickens in some areas, the Pre-Cambrian became melted and recycled.  

Cambrian

600 to 500 MYA

So-named as these were first identified in North Wales. These are the earliest fossil-bearing secondary rocks. They form the Harlech Dome peaks of North Wales, the base rock of Anglesey and the quartz caps on mountains in the far north-west of Scotland. A thin strip of Cambrian rock also forms Stiperstones in Shropshire, England. The rocks of this age are generally absent in other areas. 

Ordovician

500 to 440 MYA

Named after a Welsh Celtic tribe - the Ordovices. These rocks are found in a large part of Central Wales, the Lake District, Southern Scotland and Eastern Ireland. They contain the earliest fossil fish, and consist of dark coloured shales, grits and sandstones. 

Silurian

440 to 395 MYA

This 45 million year period is named after another Welsh tribe, the Silures, and the rock occurs in a large part of Central Wales, much of Southern Scotland, the Southern Lake District and the Howgill Fells in England. These rocks contain fossils of the earliest land animals and the early ammonites. This period was at the start of the Caledonian Orogeny with mountain-building rather than deposition further north. 

Devonian

395 – 345 MYA

Found originally in Devon, hence the name, and much of the English West Midlands, and is the signature rock of South Wales. This is Old Sandstone with comparatively few fossils. At the time these rocks were laid down, Britain was in the region of 30 degrees south of the equator in a desert zone.

Carboniferous

345 – 280 MYA

This was the period when Britain passed through equatorial regions, and the Caledonian Orogeny came to a close. Land areas had lush forests that eventually formed coal seams – hence the name of the period – ‘carbon-bearing’. There were shallow seas that teamed with life, laying down from their shells and skeletons limestone deposits. The central area of England was part of the delta of a great river, depositing the material for millstone grit. The Late Carboniferous was also the time of the Hercynian Orogeny, as mountains were built in the south-west, and in the South Wales Valleys area.

Permian

280 – 225 MYA

This is named after the Perm district of Russia. Rocks of this age are not well represented in Britain. The exception is the Clwydian Range in north-east Wales.  

Triassic

225 – 190 MYA

First named in Germany where it has three distinct beds, this period is again poorly represented in Britain, the exception again being the Clwydian Range.  

Jurassic

190 – 136 MYA

Named after the Jura Mountains in France and Switzerland, this type of rock is not represented in the mountainous areas of Britain. It is, however, present in the lowlands of Eastern and Southern England.

Cretaceous

136 – 65 MYA

This comes from the Latin word for chalk, and the extensive chalk and weald areas of south-east England are of this age.

Tertiary

65 – 2 MYA

There are a variety of different recent sedimentary rocks of this period in Britain, but these do not form uplands. The Tertiary volcanoes of Western Scotland (e.g. the Isle of skye) were formed about 60 MYA ago at the time of the opening of the Atlantic Ocean.

Wales – the rock story

We will start the story of Wales some 600 million years ago. At this time the area of the earth that would become Wales was located on the margins of a small continent called Avalonia. This land was located somewhere south of the present South Africa and heading north, inexorably, at a few inches a year. It is still going north at about the same speed.

At about this time all the continents of the early earth were in the process of forming up as one super-continent called Pangaea by geologists. In this process, Avalonia was closing in on another early continent to its north. This continent, of much older rocks, is named Laurentia.  This is now largely comprised of eastern North America and Greenland, but also the land that would one day be a part of Scandanavia, Northern Ireland and the Highlands of Scotland.

The coming together of these two continental masses took a process of some 130 million years, and formed a mountain range of Himalayan scale that extended from present day Scandinavia across Scotland, and also the Appalachian Mountains of the eastern USA.  This process of mountain-building is called in the UK the Caledonian Orogeny. The process was slow, and one can only speculate on how high the mountains got as the forces of erosion were at work from the moment the first peak put its top out of the sea.

The line of the continental “join” in Britain is now called the Highland boundary fault line, which extends from the just south of Aberdeen at its north-eastern end, across the Southern Highlands of Scotland and into Northern Ireland. The line crosses southern Loch Lomond along a chain of islands. The rocks on each side are completely different from each other. The angle of contact between the continents has set the “grain” of the Highlands, with a series of parallel rock groups across the north, today forming parallel ranges and valleys.

In this continental slow-crash, the tough little continent of Avalonia took much less of a hit.  The Southern Uplands of Scotland and much of Central Wales were buckled up, and a line of volcanic plumes burst forth with the pressures. These extended from The Cheviot Hills in the north-east England, and included The Lake District, North Wales and the Wicklow Mountains in the east of Ireland. In this process the Harlech Dome in what is now Wales was the centre of a large uplift, surrounded by very contorted metamorphic rocks and small volcanic vents. These form the basic blocks of today’s Welsh mountains.

The area that is now Wales was in for another big continental crash around 300 million years ago, forming a further set of mountains in what is called the Hercynian Orogeny. This event buckled up the existing Devonian Sandstone in South Wales and Southern Ireland to form two parallel sandstone ridges, and set off a string of plumes of molten intrusive rock in the south-west of England that cooled without breaking the surface. The largest of these is now Dartmoor in Devon. In the Devonian period Wales was about 30 degrees south of the equator and in a desert region. By the period of the Hercynian the country was at the equator on its epic journey north. On land that would one day become South Wales, tropical forests grew. These would eventually form coal.

All of these mountain ranges were doomed to the forces of erosion over the abyss of time. By 200 million years ago, the great Caledonian peaks had been reduced to a rolling plain and the super-continent of Pangaea was starting to break up. Great forces were at play, tearing the land apart. Over a period of 100 million years, the line that now is Scotland’s Great Glen saw the northern section of Scotland slide 65 miles south-west. Rifts and faulting saw the central valley of Scotland sink, with much matching volcanic activity, and the area of North Wales was uplifted along the line of the Bala Fault.

Only 30 million years ago, that which is now Wales was a level plain on the margins of the new European continent. If one compares the age of the earth to the height of current-day Yr Wyddfa (Snowdon), the highest mountain in Wales at 1085 metres, 30 million years can be compared to just the topmost six metres of the peak. This is is a short period in geological time and almost at the end of the story. We know that the rocks of which these mountains are made are a lot older than that, so what happened? 

It was at this time, 30 million years ago, that another mountain-building process started. Africa was moving in on southern Europe and the Alps were about to be formed. This process took the first ten of the last 30 million years.  As a by-product, it resulted in the western margins of Europe being raised, almost as a block, by up to two kilometres. There was some buckling: the Pennines in northern England were gently folded upwards, and the chalk ridges in the south-east were uplifted. The north and west of what became later the island of Britain were uplifted the most.

Almost immediately the forces of erosion started to work. This is a process of attrition - the sun shines on the rocks by day expanding the material, they cool again at night, the rain removes any loosened debris, the wind sandblasts the surface, ice forms in cracks, and gravity always prevails in the end. Soon the uplifted block started to wear. What became Britain’s mountains were not necessarily rocks that were the roots of former mountains of ages lost, but just the harder bits on that previous plain on the edge of what became mainland Europe. Soft rock wears away faster than hard.

The story is complicated in detail, as there was later “down-warping” of the western margins of Europe. This lost some material to the continental shelf and created the North Sea Basin. In broad principle, however, the Welsh mountains had arrived.

However, the last million years have seen the greatest amount of sculpturing taking place, with four major ice ages. Ice scour-deepened valleys, cut out the cwmau (hanging valleys), and created hollows for llynau (lakes) and created the Welsh landscape as seen today.

The mountains of Wales, and all of Britain and Ireland, are not the eroded stumps of mountains of ages past, they are the eroded roots - not like a worn-down car tyre, but more a worn re-cut re-tread!

We, as human beings, occupy a different timescale to the mountains. For us, time is just now. We see the mountains as unchanging but their story is excitingly dynamic within their frame of time. As far as the mountains are concerned, a human lifetime is a flash, less than the thickness of the whitewash on top of a painted trig point.