Introduction

I have approached a number of people to write articles, but if readers would like to contribute an article please contact me. The only two stipulations I make are that the article has to be hill related and that I don't end up in court through its publication! Otherwise the choice of subject matter is down to the Guest Contributor.

Guest Contributor – John Kirk

As well as being one of the most progressive of British hill listers; John Kirk has a passion for geology which is explored in this article

Geology of and Geological Divisions of Wales

Introduction

Wales punches far above its weight in the world of Geology. The 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 of this science. There are distinct areas of rocks that give different landscapes, floras, and shape of the hills, and below is a rough and simplified guide.

 

Rock Types

There are three basic forms of rock.

1.      Igneous Rocks

These are rocks formed inside the earth and 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 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 what names a geologist will apply to it.  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. The hot blob, called either pluton or a batholith by Geologists, cools and hardens as a rock called Granite. This generally cooled 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 British 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 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 layers and pile up. 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 be at all sorts of angles, but in most sedimentary layers they are usually not far from level. There is evidence in Britain of past sedimentation layers building up to 6,000 metres thick in some cases. This stuff, under modest pressure and chemical action, is reformed as rock; ready for the next time the Earth folds them up as 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 fossils.

 

3.    Metamorphic Rocks

These are sedimentary rocks that have been altered by being cooked. Rocks deep in the Earth, adjacent to volcanoes or plutons, or, alternatively, they receive incredible pressure in the process of mountain building and can be heated up to almost the point of melting. This changes their nature and they become crystalline. They 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 the rocks. 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
Pre Cambrian Before 600 MY	Fossil free primary rock, like The gneiss rocks of north west Scotland from 1600 to as much as 2900 MY old. The Moine schist rocks are 1500 to 1025 MY old and Torridonian rocks are from 800 to 1000 MY. Further South, the Long Mynd and the Malvern Hills near Worcester are Pre Cambrian.  This age of rock probably underlies all rocks in the UK but usually at substantial depths. As the earth crust thickens in some areas, the Pre-Cambrian will be melted and recycled.
Cambrian 600 to 500 MY	Named as these were first identified in North Wales, These are the earliest fossil baring secondary rocks. They form the Harlech 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. The rocks of this age are generally absent in other areas.
Ordovician 500 to 440 MY	Named after a Welsh tribe, the Ordovices, These rocks are found in a large part of Central Wales, the Lake District, Southern Scotland and Eastern Ireland. These rocks contain the earliest fossil fish, and consist of dark coloured shales, grits and sandstones.
Silurian 440 to 395 MY	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. 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 MY	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, a desert zone.
Carboniferous 345 – 280 MY	This was the period that Britain passed through equatorial regions, and the Caledonian Orogeny came to a close. Land areas had lush forests that eventually formed coal seams. There were shallow seas that teamed with life to lay down much limestone and at times, 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 to the south and in the South Wales Valleys area.
Permian 280 – 225 MY	This is named after the Perm district of Russia and rocks of this age are not well represented on our mountains. The exception is the Clwydian Range.
Triassic 225 – 190 MY	Named in Germany where it has three distinct beds, this period is again poorly represented in Britain, the exception again is the Clwydian Range.
Jurassic 190 – 136 MY	Named after the French mountains of Jura, this type of rock is not represented in our mountainous areas. It is present in Eastern and Southern England
Cretaceous 136 – 65 MY	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 MY	There are a variety of different recent sedimentary rocks, but these do not form Mountains. The Tertiary Volcanoes of Western Scotland were formed about 60 MY ago at the time of the opening of the Atlantic Ocean.

 

 

Wales – The Story of the rocks.

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 now 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 this early earth were in the process of forming up as one super continent which is called Pangaea by the Geologists. In this process, Avalonia was closing on another early continent to its north. This continent, of much older rocks, named Laurentia, which is now largely Eastern Canada and Greenland, also had on board land that would one day be a part of Northern Ireland and the Highlands of Scotland. The coming together of these two continental masses would take a process of some 130 million years, and form a mountain range of Himalayan scale that would extend from present day Scandinavia across Scotland and form the Appallachion Mountains of the eastern USA.  This process of mountain building was the Caledonian Orogeny. The mountain building 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 a nose out of the sea. The line of the continental “join” is now called the Highland line, which extends from the just south of Aberdeen at its north eastern end, across the southern highlands and 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, that today form parallel ranges and valleys.

In the continental coming together, the tough little continent of Avalonia took much less of a hit.  The Southern Uplands of Scotland and much of Central Wales were bucked up and a line of volcanic plumes burst forth with the pressures. These extended from The Cheviot in the North East and included The Lake District, North Wales and the Wicklow Mountains in Ireland. In this process the Harlech Dome 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 mountains.

The area of Wales was in for another big continental coming together 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 the string of plumes of molten rock in the south west of England that cooled without breaking the surface, the largest of which is now Dartmoor. 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 now at the equator on its epic journey north. On land that would one day become South Wales, tropical forests grew and 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 pushed 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 Snowdon, 30 Million years can be compared to only the topmost six metres. This is only a comparatively short period in geological time and almost at the end of the story. We know that the rocks of which our 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 10 of the last 30 million years, and, 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 were gently folded upwards, and the chalk ridges in the south east of England were uplifted. The north and west were uplifted 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 will remove any loosened debris, the wind will sandblast the surface, ice will form in cracks, and gravity will always win in the end. Soon the uplifted block started to wear. What became our mountains were not necessarily rocks that were the roots of former mountains of ages lost, but the harder bits. Soft stuff wears away faster than the hard.

The story is complicated in detail, 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, our mountains had arrived. The last million years has seen the greatest amount of sculpture taking place with four major ice ages. Ice over-deepened valleys, cut out the cwms, created hollows for llyns and created our landscape.

So, our mountains today are not the eroded stumps of mountains of ages past, they are the eroded roots. Not like a worn car tyre, but more of a worn re-cut re-tread!

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

   

The Geological divisions of Wales.

Wales falls into a number of uniquely identifiable areas based upon geology.

Anglesey and the Lleyn Peninsula

The geology of Anglesey is one of the most complex in Wales and is a series of rock types crushed on lines parallel to the Highland Boundary Fault. The land was uplifted substantially in the Caledonian Orogeny to expose the basal layers. It would be a mountainous land if it were not for the action of sea ice making its way south in the last number of ice ages. The two kilometre thick ice had the effect to planate the land almost level. Just the unusual and complex geology testifies to its origin.

A line of long extinct Caledonian Orogeny volcanoes provide a backbone for the Lleyn peninsula to project into the Irish Sea. These provided resistance to the sea ice of the recent ice ages, but the ice in turn has created the landscape of the hard volcanic plugs sculptured into steep little cones. The rock types are similar to Anglesey and the boundary is almost a straight line heading SSW from Bangor to Criccieth.

The Great Dome – Snowdonia. 

The Geological Harlech Dome of Cambrian rock is centred on the Rhinogs. The dome of Cambrian rock with some granite intrusions is about 30 km in diameter and consists of the Rhinogs and the western tops of the Arenigs. The area is then circled by the very complex rocks of Ordovician age including various volcanic rocks, ash based mudstones and slates with more granite intrusions. The surrounding arc is up to 20 km wide, but is absent on the Cardigan Bay side, probably due to ice planation. It looks like there was a sufficient mass of mountains to stop the sea ice at Penmaenmawr. Once the Conway Valley was full of ice, its progress southwards here was stopped. The incomplete Outer Ring has the following boundaries:-

West –a line from Bangor to Criccieth.

East – a line along the Conwy valley to Pentrefoelas and then ESE to Corwen

South – Along the line of the Bala Fault SW from Corwen via Bala to Tal y Llyn and the sea at Tywyn. 

   

The Denbigh Moors

This highland is composed of Silurian rocks and surrounded by carboniferous limestone that probably formed over all the area but is now stripped by the forces of erosion from the higher lands. There are no igneous intrusions. The area lies east of the line from Conwy via Pentrefoelas to Corwen as set out above, and west of the line of the River Clwyd.

The Clwydian Ridge

This ridge is geologically special, the 225 MY old rocks are from the Permian / Triassic periods and the main representation of this age of rocks in British hills. There are some lowland examples in the Cheshire gap and east of the Pennines. The boundary is the River Clwyd throughout its length, and The Bala Fault east to the English Border.

Mid Wales

By far the largest geological area of Wales, The northern boundary is the line of the Bala Fault all the way from the English border at Pulford north of Wrexham SW to Corwen, via Bala to Tal y Llyn and the sea at Tywyn. The southern border is a line, almost parallel to the Bala Fault from Knighton, SW to Crossgates, SSW to Builth Wells then a line SW along the lines of the Rivers Irfon, Afon Bran and the Towy to the sea at Carmarthen. A South Western Border is a line from Carmarthen to Newport on the Cardigan Bay Coast.   

This large area is predominantly Silurian with some older Ordovician rocks (Plynlimon) showing through. Compared to most of Wales the area has relatively undisturbed rock with gentle folding. There are a few odd exceptions. The Berwyn range has a number of very hard Rhyolite dykes and intrusions that produce an odd result. Besides them being hard enough to ensure the survival of quite high hills that should have been worn down, They also create the highest waterfall in Wales and create “wrong sided” hills. Normally the south side of a hill ridge is smooth as the sun stops glaciers developing. On a normal ridge, such as the Glyders or Nantle, the south side is relatively smooth, the north dramatic. The Berwyn demonstrates a rocky crag of rhyolite facing south and a smooth grassy north side.

In the east of the area within sniffing distance of the English Border are some volcanic plugs, West of Oswestry, Moel y Golfa near Welshpool and Corndon Hill. Just east of Corndon Hill into England are some very unusual exposures of ancient rocks at The Long Mynd and Stiperstones. 

   

 West Wales

This area is all south and west of a line from Carmarthen to Newport on the Cardigan Bay coast. After the large areas of dull Silurian mudstones of Mid Wales, this little area is an action packed very diverse area with the stumps of some ancient volcanoes and swarms of dykes of igneous rocks, one of the more famed is the intrusion of Andesitic lava that provided blue stones for Stonehenge. The area has two east – west orientated ridges of Devonian Sandstone, a continuance of the South Wales formation. These two ridges can be traced across the sea and across southern Ireland where they form Ireland’s highest mountains. The projecting sandstone ridges form the jaws of St. Bride’s Bay. 

Sandstone South Wales

 

This is another large patch of Wales and represents the area of Old Red Sandstones of the Devonian Period. The northern border is the line set out above from Knighton to Carmarthen and is bounded on the east by the English Border to the sea. The only exclusion is the next and final area, The Valleys of South Wales.

The sandstone area has several distinct areas formed by the angle of bedding of the sandstone. The rock is fairly level in the Eastern Black Mountains and Radnor Groups, but has a dip and scarp formation further west, the northern block of Mynydd Eppynt with a steep scarp towards the northern boundary of the area and a long dip slope to the south, and the matching structurally but far superior landscape value of the great sandstone wave that crosses the area of the Brecon Beacons, Fforest Fawr and Carmarthen Fan.

The Valleys of South Wales

Last and probably least of the Welsh Geological Areas is the Carboniferous area known as the valleys of South Wales. This is an area where rocks of Carboniferous age have been laid down on top of the underlying Devonian Sandstones, and is the best example in Britain of river course imprinting. The rocks were formed when Wales was on its long journey north in the tropics, and was a rich shallow sea when the limestone was formed, and when out of the sea, the tropical forest produced the coal measures. Later the area was part of the “Proto-Rhine” delta, and much mudstone developed. All was laid down level, but was raised in the last 30 million years with the formation of the Alps. As the land rose, rivers in place maintained their courses, resulting in a set of very deep valleys. The Boundary is a bit of an oval shape, From Kidwell south of Carmarthen, heading NE parallel to the Sandstone border until the hills develop. The line then continues about 10 km south of the crest of the Great Sandstone Wave to Blaenavon and then SW to Cardiff.

John Kirk