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
