Geology

The geology of Scotland for its size is very complex. The rocks we see today represent a varied assortment of geologically diverse fragments of the Earth’s crust accreted by continental drift at various times over the almost unimaginable timescale of more than 3,500 million years. Over this time, landmasses have been created and destroyed, coming together and parting to create the world we see today.

The geological map of Scotland is reproduced by permission of the British Geological Survey. © NERC. All rights reserved. IPR/95-33CW. (ipr@bgs.ac.uk)

The concept of continental drift or plate tectonics is the key to understanding the story of Scotland’s geological history. Due to rotation the earth is shaped like an orange that is slightly flattened at the poles. Its surface structure is very similar to a cracked chicken’s egg with the shell being the crust. The chicken egg analogy can be further extended with the white of the egg being the mantle and the yolk being the core.

The crust rocks ‘float’ on the surface of the denser and usually molten mantle material. The core of the Earth is believed to be about the same temperature as the sun and within this and the mantle, hot convection currents occur, disrupting and rearranging the sections of crust. Large parts of the crust are called plates. Over long periods of time movements of these plates occur and they collide with and slide past each other. Plates can also be pushed underneath other plates and can be stretched and thinned leading to the crustal rocks becoming thicker and heavier. As they sink down they are melted by the hot molten mantle rocks and become less dense in the process resulting in them ‘floating’ up to the surface again. These dynamic actions lead to earthquakes, faults, fold mountains, volcanoes and rift valleys.

Due to convection currents in the hot molten mantle rock the plates move slowly over the surface of the Earth. This process started not long after the Earth was formed and began to cool down after its formation in the planetary nebula more than 4 billion years ago, and continues today. It is an extremely slow process taking hundreds of millions of years. For example, it has been estimated that expansion in the Mid Atlantic ridge is causing the Atlantic Ocean to widen at about the same speed as a human fingernail grows. This is the underlying principle of the tectonic cycle and the cause of continental drift where the Earth’s continents move relative to each other. Continental drift was first proposed about 400 years ago. It was however not until the late 1960s that geologists finally had the firm evidence to prove it.
The appearance of the Earth’s continents, islands and landmasses today bears very little similarity to what it looked like millions of years ago. The process is dynamic and ongoing and the map below demonstrates where most of the ‘geologic action’ is presently taking place. Fortunately for Scotland today it lies in the centre of a large continental plate, far from existing active dynamic areas, very different from conditions in its history.

Scotland is really a collection of randomly mixed continental fragments, or terranes, that were assembled over time by plate tectonics. Where these various parts were positioned originally on the Earth is not clearly known. The study of rock magnetism can provide clues however to approximately where particular formations began their journey. By using this and other information from the fossil record, attempts have be made to create the likely appearance of the Earth at certain times in its past.


Chris Scotese of the University of Texas in Arlington, USA, has created the maps I have used here for the Paleomap Project. His ongoing aim is to illustrate the plate tectonic development of the ocean basins and continents as well as the changing distribution of the land and sea during the last 1100 million years. Some of these fascinating maps have been reproduced here. Each map illustrates what the continental appearance of the Earth was like in the main geological periods. The approximate position of the small piece of crust that would eventually form Scotland is also indicated thus helping to explain the prevailing conditions at that time.

 

 

Of the different terranes that eventually became Scotland the principal five are:

1. The Southern Upland Terrane, bounded to the south by the so-called Iapetus suture through northern England and to the north by the Southern Upland Fault.

2. The Midland Valley Terrane, lying between the Southern Upland fault and the Great Glen Fault

3. The Grampian Highland Terrane, between the Highland Boundary Fault and the Great Glen Fault

4. The Northern Highland Terrane, bounded on the S.E. by the Great Glen Fault and to the N.W. by a fault with a shallow easterly dip, called the Moine thrust

5. The Hebridean Terrane, to the west of the Moine thrust, embracing the Outer Hebrides and a narrow strip of land along the N.W. margin of the Scottish mainland

 


Studies have shown that around about 1,200 million years ago the terranes that ultimately formed Scotland lay in the southern hemisphere. In the last 500 million years there has been a generalized northward migration that brought all the rocks across the equator some 200 million years ago and on to their present day latitude between 54 and 61 degrees N. This stable portion of the ancient Archaen crust was formed of what are now Labrador, Greenland, the Outer Hebrides and the Northwest Highlands of Scotland. 
Rocks in the latter areas are believed to be the oldest in Europe and among the oldest in the world at between 2,900 and 3,300 million years old. They are exposed from Iona, Tiree and the Sleat peninsula in the south to Cape Wrath in the north. Due to their widespread occurrence on the island of Lewis they are known as the Lewisian. They consist largely of coarsely crystalline, striped grey and white material referred to as gneiss, the end products of a long and complex history. Within these there is also some material of igneous origin, marble and quartzites.
Due to repeated folding and recrystallization during mountain building processes the original sediments and igneous rocks were so metamorphosed into gneisses that only a guess can be made as to the original material from which they were formed. However the basic ingredients that initially went into the making of gneisses probably included sands and muds deposited in ancient seas over 2,900 million years ago. The Torridonian sandstones near Loch Torridon in the northwestern Highlands form part of these ancient rocks and were also laid down in the same period. 
The geological record becomes a little clearer about 2,400 million years ago as basaltic dykes and granitic magmas intruded and transected the Lewisian gneisses. These can be seen in the northwest Highlands. Dykes start off vertical or steeply dipping but can later be folded or rotated into other attitudes by earth movements. One of these intrusions forms the summit platform of the mountain Roinebhal on Harris in the Outer Hebrides. 
Before about 1,000 million years ago it is thought that there were many episodes involving the growth of continents, the rifting apart of continental crustal plates and the creation of new oceans. These oceans waxed through growth along constructional plate boundaries and then waned when rates of destruction along subduction zones exceeded those of growth. The resultant closure of the oceans, involving collision of the adjacent continents, uplift, folding and metamorphism is called orogenesis. One such transient ocean may have opened and closed in the interval 1,100 to 1,000 million years ago, starting with the break-up of the great continent of Paleopangaea, closing with the formation of a continent called Rodinia.

In Scotland the geological record begins to be a little more complete from about 1000 million years ago. During this time further great thicknesses of sediments were laid down on the floors of shallow seas and large river deltas flowing into these seas. Much of the landscape of the Grampian Mountains and the Northern Highlands has been carved out of rocks composed of these ancient sediments. 

This map illustrates the break up of the supercontinent, Rodinia, which formed 1100 million years ago. The late Precambrian was an “Ice House” World, much like the present day. The Red Arrow shows the approximate position of what would become Scotland. [Copyright C.R. Scotese, Paleomap Project]

About 870 million years ago it is thought that the crust thinned below the continent of Rodinia causing a major outbreak of basalt volcanism. There is no longer any direct evidence of this "llithospheric
stretching" but there are many minor intrusions north of the Great Glen fault. These "dyke swarms" probably represent evidence of fissure type volcanoes erupting on dry land or on the sea floor.

Eventually the continent of Rodinia ruptured and allowed the creation of the Iapetus Ocean, sometimes referred to as the "proto-Atlantic Ocean". The surrounding continents largely correspond to North America on one side and Europe and North Africa on the other. The "birth" of this continent generated a lot of volcanism and evidence can be seen in localities throughout the Grampian Mountains including the Tayvallich Peninsula in Argyll on the west coast. Pillow lavas here can be seen as evidence of the violence of this ocean birth.
The Iapetus Ocean had no sooner reached its maximum extent when shrinkage due to subduction of the ocean floor on both sides of the ocean occurred, leading to final closure about 455 million years ago, also associated with extensive volcanism. This led to the formation of a large mountain range that would have looked similar to the present day Himalayas. This is known as the Caledonian Orogeny and is recognized beyond Scotland and comprises much of the Appalachian Mountains in eastern North America, the Norwegian mountains and those of northeastern Greenland.

The final major tectonic event in Scotland occurred about 65 million years ago when the North Atlantic Ocean started to open. This was the Tertiary Volcanic period and was associated with extensive volcanism in Scotland. As North America and Greenland separated from Europe the Atlantic Ocean began to form. After the volcanic activity began to settle at the end of the Tertiary period, more than sixty million years later, Scotland moved into a more stable geological period. For the last 1.8 million years rock formation has not occurred in the Quaternary of Scotland. The dominant force in Scotland over this period of time has been ice.

Ben Nevis in the winter near Fort William in Lochaber. Classic U-shaped valley carved by a glacier. [Copyright Douglas R. Anderson] Raised beach between Turnberry and Girvan on the Ayrshire coast. [Copyright Douglas R Anderson]

Ice has dominated the Earth many times in the past perhaps occasionally entirely covering the globe. Ice ages have come and gone, with the last major ice sheet covering Scotland about 18,000 years ago. At this time the entire landmass of Scotland waa buried under up to a kilometer of ice. The massive overburden compressed the land and moved and flowed over it resulting in erosion, with the land being sculptured and carved by massive glaciers. As the ice eventually melted the land rose up imperceptibly slowly and created what can be seen today as "raised beaches". It also eroded mountains, cut massive U shaped valleys and scraped and eroded the rocks.


As the ice retreated north from Scotland, conditions on the land slowly improved, with a warming climate that would eventually sustain a myriad of colonizing animals and plants. It was into this newly formed Scotland that the earliest human hunter-gatherers made their way up from the south into this land that had been created for them by such turbulent geological processes.

Geological Periods

Distinct fossils first made their appearance around 540 million years ago when marine organisms developed shells or carapaces. Major changes in the evolution of life, as indicated by animal fossils, provide the basis for useful time breaks. There are three principal time divisions, or Eras, primarily based on this paleontological evidence, collectively known as the ‘Phanerozoic’

The Palaeozoic - extended from about 540 to 250 million years ago

The Mesozoic - from about 250 to 60 million years ago

The Caenozoic - from the end of the Mesozoic to the present day

Each of these ‘Eras’ is sub-divided into a number of ‘Periods’ (See diagram below).

It is mainly in the Palaeozoic that we find a long and complex history of volcanic eruptions in Scotland. This is divided into six periods. They are, in decreasing order of age, Cambrian, Ordovician, Silurian composing the Lower Palaeozoic and the Devonian, Carboniferous and Permian periods making up the ‘Upper Palaeozoic’ There is no evidence of volcanic activity in the Mesozoic Era, which consists of the Triassic, Jurassic and the Cretaceous periods. In the Caenozoic Era volcanic activity in Scotland is almost totally confined to the early part, the Tertiary Volcanic period.

This timescale is used by geologists to describe the timing and relationship between events that have occurred during Earth's history. The table on the next page is a very much simplified version that agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy. Different spans of time scale are usually determined by major geological and paleontological events such as mass extinctions.

The Cambrian Period - 543 to 490 mya (Palaeozoic)
The Cambrian period is recognized as the time when life appeared in abundance in the seas of the world. At that time there was no life on the land. This did not occur until the end of the Silurian period some 100 million years later. All the organisms in the sea in the Cambrian were invertebrate and included trilobites, small arthropods that lived in shallow seas. Fossils from these early forms of life can be found in Cambrian rocks in Sutherland.
The Cambrian Earth. This map shows the newly formed continent of Gondwana near the South Pole. Shallow seas were flooding the continents. In these seas life was abundant and animals with hard shells first made an appearance....this was the "Cambrian explosion of life". During this time Scotland continued to move northwards from aboput 30 degrees below the equator. Scotland marked with Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

 

In the early Cambrian Scotland lay well inside the southern hemisphere on the southern margin of a continent called Laurentia partially on the continental margin and partially in the surrounding shallow seas. Sand and mud was washed in by rivers to the north leading to the formation of sandstones, limestones and mudstones. These rocks can be found around Assynt, south of Durness and on the Isle of Skye. Fossils from these sediments have been found in the northwest Highlands.

An interesting example of a "trace fossil" called Pipe Rock can be found here. This is a quartzite characterised by abundant long cylindrical burrows (pipes) also known as Skolithus. These trace fossils represent the escape burrows of an animal living in these ancient sediments.

Olenellus Lapworthi, Fucoid Beds (Cambrian) a'Ghubhais, Kinlochewe, Rosshire. Length 30mm. Reproduced from Peach, 1894)

"Pipe Rock" showing the vertical white Skolithus burrows, Assynt. (Specimen 220 x 230mm)

 

The Ordovician Period - 490 to 443 mya (Palaeozoic)
Scotland continued to move further north from about 30 degrees south of the equator. The Iapetus Ocean was closing by subduction and the sediments that would form the Southern Uplands mudstones and sandstones were being laid down. At the beginning of the Ordovician the ocean was at its widest, possibly 5000 km across.
The Ordovician Earth. The end of the Ordovician was one of the coldest times in the Earths history. Ice covered much of the southern region of Gondwanda. Scotland eventually lay close to the equator and was thus spared the worse of the climate. Ancient oceans separated the barren continents of Laurentia, Baltica, Siberia and Gondwanda. At this time there was no life on the land. Scotland approximate position marked with Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

As discussed earlier Scotland is made up of discrete fragments, or terranes, assembled progressively by strike-slip movements and welded together during the collision of Laurentia with Avalonia and Baltica during the Caledonian Orogeny. This process can be
broken up into a number of separate events, spanning the period from the early Ordovician to the early Devonian, a period of about 100 million years. The subduction zones on either side of the Iapetus Ocean caused all the movements. Fold mountains of Himalayan proportions, as evidenced by the deep rock structure and fold amplitudes now exposed, were formed over Scotland.


Rare volcanic rocks in the north part of the Southern Uplands and in the Highland Boundary Fault area represent the volcanic island arc, which must have occurred to the northwest. The well-known Ballantrae Complex of the Ayrshire coast contains pillow lavas and associated volcanics and sediments and represents an ophiolite.

The spectacular pillow lavas at Downan Point near Ballantrae, South Ayrshire

The Ordovician seas teemed with life and trilobite fossils are found in Ordovician mudstones in Ayrshire. In Southern Scotland near Moffat fossil graptolites can also be found.


The end of the Ordovician period about 439 million years ago was marked by a mass extinction, although less severe than later end Permian or end Cretaceous extinction events. The trilobite-dominated communities of the Cambrian disappeared to be replaced by Palaeozoic groups with articulate brachiopods, crinoids, bryozoans and some corals. These groups lived attached to the sea floor and filtered food out of the water. There was no, or very little, life on the land at this stage. Two discrete extinction events separated by perhaps 500,000 to 1 million years occurred extinguishing 60% of marine genera and 26% of marine families.

Calymene Drummuckensis (65mm x 45mm). Trilobite from Upper Ordovician, Girvan, Ayrshire

Cybeloides Girvanensis (35mm x 30mm) Trilobite from Upper Ordovician, Girvan, Ayrshire Graptolite from Dobbs Lynn near Moffat, South of Scotland. (Specimen 35mm in length)

It is thought that glaciation and global cooling were the most likely cause of these events. The extinctions began with a rapid glaciation and consequent drop in sea levels, and then as the glaciers melted, sea levels rose rapidly and delivered cold anoxic water into shallow seas causing the second pulse of extinction. Brachiopods suffered particularly heavy extinction, as did trilobites, echinoderms, bivalves and some corals. One of the most enduring curiosities of this episode is why it had such little long-term ecological impact. The various surviving groups produced many new species over the following 1-2 million years, and the Earth’s Silurian communities do not appear to be much different from those before the extinction.

The Silurian Period - 443 to 417 mys (Palaeozoic)

The closure of the Iapetus Ocean finally brought Scotland and England together to produce the Caledonian Orogeny lasting from about 500 to 360 million years ago. By this definition, most Caledonian igneous rocks in Scotland range in age from about 500 million (early Ordovician) to around 390 million years ago (early Devonian) with related activity continuing to around 360 million years (end Late Devonian) in Orkney and Shetland.

The Silurian Earth. Laurentia collides with Baltica closing the northern branch of the Iapetus Ocean and forming the "Old Red Sandstone" continent. Coral reefs expand and land plants begin to colonise the barren continent. Scotland was near the Equator. Scotland marked with Red Arrow. [Copyright C.R. Scotese, Paleomap Project]
At the end of the Caledonian Orogeny Scotland was part of a huge continent straddling the equator called the Old Red Sandstone Continent. The Caledonian mountain range lay along the southern margin and included Scotland. The rapid erosion of these mountains provided the sediments for the formation of the Old Red Sandstone rocks of the succeeding Devonian Period.
The Devonian Period - 417 to 354 mya (Palaeozoic)

During the Devonian period Scotland lay about 10 degrees south of the equator and Laurentia was joined as one landmass to what would become North America and Greenland. Most of Scotland was made up of high mountainous areas of Himalayan or Alpine proportions with a few freshwater basins fringing this super-continent.

The Devonian Earth. By the Devonian the early Paleozoic oceans were closing, forming a "pre-Pangea". Freshwater fish were abole to migrate from the southern hemisphere continents to North America and Europe. Forests grew for the first time in the equatorial regions of Artic Canada. Scotland marked with Red Arrow. [Copyright C.R. Scotese, Paleomap Project]
Great thicknesses of continental sediments derived from the erosion of the Caledonian Mountains accumulated in internal basins, freshwater lakes, rivers and in the subsiding Midland rift valley. In Shetland 10 km were formed, 5 km at Caithness and up to 9 km in the Midland valley itself. Associated with these sediments were volcanic lavas up to 3000 metres in the Midland Valley, which probably covered a much larger area than we see today and may have been interconnected with volcanic cauldron subsidence in the southern Highlands.

 

The Devonian period was an important one for the evolution of life. We see the appearance of land plants, marine and fresh-water fish, and towards the end of this period, amphibians. Insects also diversified to include the first winged forms, and so the land and the air were colonised.

Near what is now the village of Rhynie in Aberdeenshire, a unique deposit of chert was formed in which the remains of some of the earliest plants to colonise the land have been found. It is a remarkable example of how early life forms have been preserved in an instant of geologic time. An ancient marsh plant and animal community thrived close to a number of hot springs, which overflowed occasionally and overwhelmed the adjacent area with silica rich boiling water that later cooled rapidly trapping animals and plants within it thus preserving them. This time capsule of primitive Devonian life has been studied for many years and due to the detailed preservation extensive information has been learned about the insects, and plants that lived on the large continent of Laurentia 400 million years ago.

 

A piece of Rhynie chert with several horizons, including upright standing Aglaephyton major axes.
Artist impression of the area around the Rhynie Hot Spring in the Devonian. From an original painting by Merlin Currie
The Carboniferous Period - 353 to 290 mys (Palaeozoic)

By the end of the Devonian the land had become low enough for the Rheic Ocean to spread its shallow continental shelf seas over many areas at the start of the Carboniferous Period. Scotland lay on the equator and therefore had a tropical climate similar to central Africa today. Global sea levels oscillated in response to growth or shrinkage of the polar ice sheets and in the warmer intervals shallow seas encroached across the lowlands of the Midland valley.

The Carboniferous Earth. By the late Carboniferous the continents that make up modern North America and Europe had collided with the southern continents of Gondwana to form the western half of Pangea. Ice covered much of the southern hemisphere. Vast coal swamps formed on the equator. At this time Scotland was on the equator where its coal measures were being formed. Within these forests early four-legged vertebrates were evolving. Scotland marked with Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

The early part of the Carboniferous was marked by vast outpourings of lava across the Midland valley creating new features including the Campsie Fells and the Gargunnock Hills, Arthur’s Seat and Salisbury Crags in Edinburgh and also the nearby Bathgate Hills. Apart from occasional marine inundations these generally stood higher than the sea level. The local volcanoes were not thought to be large, perhaps only one to two hundred metres high. Around them in the Midland valley were tropical swamps where trees and ferns grew. The main forest trees with trunks up to 50 cms across, were ancestral forms of modern tree ferns and conifers (pteridosperms and gymnosperms). Forest fires, indicated by horizons of wood ash, were clearly a common occurrence since the oxygen content of the Carboniferous atmosphere was higher than it is today and may have been as much as 35% as compared to the modern level of about 21%. The build-up of oxygen was mainly due to the burial of vast amounts of carbon locked into biological material that would later form into the extensive coal measures. Due to this high oxygen level the insects and other Arthropoda were able to grow much larger than similar animals today.

 

An example of this can be seen on the island of Arran in the Firth of Clyde where the track of a large Myriapod (Arthropleura) has been preserved. This animal was about 2 metres long and would have been a formidable sight as it roamed the forest feeding off vegetation and leaf litter.


The forests supported a wide and prolific fauna but flying insects and flowering plants had not evolved in the early Carboniferous so the forest canopy would have been rather silent as well as devoid of colour. In the later Carboniferous flying insects were abundant with some dragonflies having wingspans of up to 35 cms.

Track of a giant Myriapod near Laggan, Arran. Track about 350mm wide. Hammer head for scxale 170mm long
Artists impression of life on what would become Arran in the forests of the Carboniferous Period. The giant Myriapod Arthropleura to the left of the scene. From an original painting by Merlin Currie

 

As the trees in this ancient forest died they would fall into the swamp eventually to form layers rich in organic carbon. Over time the remains of this carbonised vegetation were deeply buried and compacted eventually forming the coalfields of Scotland. By the end of the Carboniferous period Scotland lay north of the equator and its climate became more arid.

Stem of the Carboniferous Lepidodendron from an opencast mine near Dalmellington, Ayrshire. This is an extict genus of plant related to club mosses (Lycopsids). hammer head 165mms.
The Permian Period - 290 to 248 mya (Palaeozoic)

Throughout the Permian Scotland lay within a great continent called Pangaea. This came into existence as a result of plate movements that brought several continental masses into conjunction. The late Carboniferous as well as Permian and Triassic rocks of Scotland were formed deep in the interior of Pangaea in tropical latitudes north of the equator in conditions of extreme aridity.
The higher ground, which would eventually become the Grampian Mountains and Northern Highlands, was a rocky upland desert while vast sand seas accumulated in some of the fringing lowlands forming aeolian dune-bedded red sandstones, beautifully exposed today on the east coast of Arran and in some gorge sections of the river Ayr near Mauchline. Such sandstones were quarried extensively around Mauchline and Dumfries and used in the building industry. Many of the beautiful red sandstone buildings in Edinburgh, Glasgow and Dumfries owe their beginning to these Permian deserts.

The Permian Earth. Vast deserts covered western Pangea during the Permian as reptiles spread across the face of the supercontinent. About 95% of all life perished during the extinction event that marked the end of the Palaeozoic Era. Scotland position marked with a Red Arrow. [Copyright C.R. Scotese, Paleomap Project.]
Ballochmyle Quarry near Mauchline, Ayrshire. Photographs taken in the early 1800s showing the Permian red desert sandstones with obvious dune-bedding. This cross-bedding of the Permian sand dunes can still be seen along the River Ayr near Mauchline, Ayrshire (Note the lack of safety gear!). This desert environment in the Permian in Scotland was very similar to that in the Namib Desert today.
Cross-startified Permian sandstone near Ballochmyle, on the River Ayr, Ayrshire. (photograph Steven Pierotti) Brick-red cross-stratified Permian sandstone beautifully exposed at Ballochmyle, near Mauchline on the River Ayr.

Because there are few fossils in desert deposits it is, in Scotland at least, very difficult to identify a break between the Permian and the Triassic periods. The desert deposits of the Permian and Triassic are accordingly collectively regarded as composing the New Red Sandstones (in contrast to the Old Red Sandstones that accumulated in late Silurian and early Devonian times). Thus the transition between the Palaeozoic and the Mesozoic is hidden at some level within the New Red Sandstone formations. These are mostly preserved in the Hebrides and southwestern Scotland with one small outcrop in the northeast near Elgin. Volcanism in the Permian was almost entirely related to faulting and consequent pressure relief on the underlying mantle and can be categorized as wholly “intra-plate”.
Pangaea was no sooner formed than it began to experience extensional stresses leading eventually to its disintegration in the Mesozoic and early Caenozoic. The magmatism tended to be low key and the volcanoes, although numerous, were small ones, which did not give rise to any dramatic landscape features in Scotland.

The End Permian Extinction

Scotland with its desert conditions in the Permian and Triassic is not the best place to study the fossils of the time. However in other parts of the world the fossil record shows evidence for the biggest mass extinction of all time at the end of the Permian. This caused major changes in fauna and flora and about 95% of all life was extinguished, thus marking the end of the Palaeozoic Era.
Of the five mass extinctions recorded in Earth’s geologic record the one at the end of the Permian period and the start of the Triassic was the most catastrophic. The Permian-Triassic (P-Tr) extinction event was so severe that up to 96% of all marine life and up to 70% of terrestrial vertebrate, insect and plant species became extinct. Sometimes called the ‘Great Dying’ it defines the boundary between the Permian and the Triassic and occurred 251.4 mya (million years ago).
For many years little was known about the Permian-Triassic extinction but starting in the 1990s studies of what might have happened have stirred great controversy. There have been several proposed mechanisms for the extinction event. These can be easily categorized into catastrophic or gradualistic processes. The former include large or multiple bolide impact events, increased volcanism, sudden releases of methane hydrates from the sea floor and even gamma ray bursts [GRB] from the collapse of super-massive stars in the local region of our galaxy.
It is probable that no single event caused the extinction. Rather it appears to have resulted from the combination of a number of disadvantageous conditions. The uniting theme for both marine and terrestrial extinctions seems to be global warming, exacerbated by volcanism, methane hydrate release and the relative inefficiency of the global carbon sinks. Whether further research will support the concept that a bolide impact may have aggravated matters remains to be seen. It may be that volcanism and an impact event is required to tip the Earth into such a critical state. It is interesting to ponder what might have been if no such extinction had taken place. The earth today would be the same but undoubtedly the animal life alive on it would be different. Some of the organisms thriving in the Permian if they had not been wiped out may have gone on to better things but we will never know.

The Triassic Period - 248 to 206 mya (Mesozoic)

The Triassic period comprised the start of the Mesozoic Era with its new reptiles and early mammal groups and the later fast evolving ammonites in the seas of the Jurassic. However, during the Triassic much of Scotland remained in desert conditions with higher ground in the Highlands and Southern Uplands and flash floods providing sediments to the surrounding basins. These deserts were home to early dinosaurs and mammal-like reptiles such as the 0.35m long Rhynchosaurus or the 1 metre long Cheirotherium. In the sandstones deposited around Elgin, fossils of Elginia have been found. This was a 0.75 metre long reptile that probably lived around watercourses with dune fields in between.

The Triassic Earth. The supercontinent of Pangea, mostly assembled by the Triassic, allowed land animals to migrate from the South Pole to the North Pole. Life began to rediversify after the great Permo-Triassic extinction and warm-water faunas spread across the Tethys Ocean. Scotland marked with Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

At Golspie on the northwestern shore, late Triassic sandstones pass up into early Jurassic fresh-water conglomerates. Further south actual fossils were very poorly preserved in these desert conditions but early reptile tracks have been found in a number of localities near Dumfries.

At the end of the Triassic sea levels rose again and climatic conditions became less arid.

The Jurassic Period - 206 to 144 mya (Mesozoic)
During the Jurassic period Scotland usually lay beneath a warm shallow sea called the Tethys Ocean. This ocean was teeming with life such as shellfish, corals, ammonites, belemnites and marine reptiles including icthyosaurs and plesiosaurs. The climate was hot and sub-tropical as Scotland lay about 40 degrees north of the equator. In these shallow seas great thicknesses of sandstone, limestones and mudstones were formed, now containing abundant fossils. The topography would have
been very flat with numerous lagoons but the sea level
fluctuated and at times the land was exposed.
The Jurassic Earth. By the early Jurassic south central Asia had assembled. The wide Tethys Ocean separated the northern continents from Gondwana. Though Pangea was intact, the first rumblings of continental break up could be "heard". Scotland marked with a Red Arrow. [Copyright C.R. Scotese, Paeomap Project]

Plant and flesh-eating dinosaurs roamed the land and fossils of these animals have been found in the Jurassic sediments on the east side of the Trotternish peninsula in the Isle of Skye.


Fossils of Cetiosaurus, a plant eating land-dwelling dinosaur and Stegosaurus have been found. Footprints of a number of large meat-eating dinosaurs have also been found at Staffin on the Isle of Skye. Other Jurassic outcrops occur on the Isle of Raasay with some scattered deposits in Ardnamurchan.


On Skye and Raasay an almost complete sequence of Liassic ammonite zones occurs within a 400 m sequence in limestones laid down in the early Jurassic. These pass up into shales and sandstones with the important Raasay Ironstone deposits at the top of the sequence. These were mined from about 1910 to 1942. The middle Jurassic comprises about 530 m of marine sandstones followed by 250 m of the Great Estuarine Series of freshwater sandstones with bivalves, oolites, algal beds and dolomites. On Skye the Upper Jurassic comprises 200 m of shales and sandstones, the Staffin Shales, with abundant ammonite faunas.

Fossil limb bone of a large sauropod, Staffin Bay, Isle of Skye
Footprint of a large, possibly carniverous, dinosaur, from Staffin Bay, Isle of Skye. Artists impression of a Scottish Cetiosaurus that would have roamed the Jurassic landscape of Scotland. From an original painting by Merlin Currie

On the Sutherland coast of the Moray Firth Liassic rocks are found at Golspie, overlying the Rhaetic. Middle Jurassic rocks are found at Brora where coal occurs within sandstones and shales. Between Brora and Helmsdale Upper Jurassic deposits of shales and limestones occur followed by 700 m of the Kimmeridge Clays with bituminous marine shales containing ammonites. Within these clays are boulder beds with large blocks of Old Red Sandstone, Helmsdale granite, gneisses and eroded Jurassic sandstones. These boulder beds formed next to a rising fault escarpment on the edge of the North Sea basin. This basin subsided along the Helmsdale fault, a branch of the Great Glen fault suggesting that there was late Jurassic fault movement on the edge of the North Sea basin. The thick Kimmeridge Clay deposits with organic matter provided a good deal of the oil found today in sandstone reservoirs.


During this period there was very little volcanic activity and there are therefore no Jurassic volcanic rocks in Scotland.

The Cretaceous Period - 144 to 65 mya (Mesozoic)
The Cretaceous Earth. During the Cretaceous the South Atlantic opened. India separated from Madegascar and raced northwards on a collision course with Eurasia. Note that North America was connected to Europe and that Australia was still joined to Antarctica. Scotland is now more recognisable and is marked with a Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

 

In the Cretaceous period sea levels rose globally and much of low lying Scotland was covered by a warm shallow sea. Within this sea up to 400m of chalk was laid down. Although much of Scotland was covered in this layer of sediment and chalk most of these rocks were eroded during the later Tertiary and Quaternary periods. Today only very small outcrops of Cretaceous rocks occur. Marine Upper Cretaceous sediments are found on Mull and at Morvern, where 13 metres of Cenomanian green sands are overlain by 8 metres of pure white sandstone. These pure quartz sands are mined at Lochaline for high quality glass manufacture.


At the end of this period the Cretaceous-Tertiary extinction occurred when 65% of all life on Earth was extinguished.

Lochaline Sand Mine, Morvern.
The Tertiary Period - 65 to 1.8 mya

The Tertiary Period brought a return to volcanic activity in Scotland and is thought to have started as a disturbance in the mantle below the landmass that is now North America, Greenland and Europe. Hot magma from the mantle began to rise, creating a “mantle plume” causing the crust to thin and then spread apart initiating the volcanic activity that culminated in the formation of the Atlantic Ocean. The plume was given the name of the Iceland Plume although at the time of its origin Iceland did not exist. It is further thought that the volcanic activity in Iceland today is related to this plume tail.

The Tertiary Earth. 50 - 55 million years ago India began to collide with Asia forming the Tibetan plateau and the Himalayas. Australia, which was attached to Antarctica, began to move rapidly northwards. Scotlandmarked with a Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

Tertiary volcanism gave rise to some of the most rugged scenery in the Inner Hebrides and on the Scottish mainland and led to a chain of volcanic centres including Skye, the Small isles, St Kilda, Mull, Ailsa Craig and Arran in the Firth of Clyde and on the mainland at Ardnamurchan.

One other interesting feature within the scenery of the Tertiary volcanics is MacCullochs tree in the
Wilderness on the island of Mull. Here a tree that is thought to have been a cypress tree standing about 12 metres high was overwhelmed by lava and still stands preserved as rock within the lava.


It should also be noted that in many scientific circles the combined period of the Palaeogene and the Neogene is also known as the Tertiary Period.

McCullochs Tree preserved in the Wilderness, Burg, Isle of Mull. Hexagonally jopinted basalts on Burg beach, The Wilderness, Isle of Mull
Fingal's Cave on the uninhabited island of Staffa in the Hebrides. The cave is formed entirely from hexagonal columns of jointed basalt within a single Tertiary lava flow. Carsaig Arches on the Ross of Mull south of Bunessan on the Isle of Mull. A little visited example of spectacular Tertiary scenery
The Quaternary Period - 1.8 mya to the present
The Quaternary Earth. When the Earth is in its "ice house" climate mode, there is ice at the poles. The polar ice sheet expands and contracts because of variations in the Earth's orbit (Milankovitch cycles). The last expansion of the polar sheets took place about 18,000 years ago. Scotland marked with Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

The period between the end of the Neogene and the modern day is known as the Quaternary. This spans 1.8 million years and due to the fact that most of man’s evolution took place in that time it is also known as the ‘Age of Man’, comprising only a very small fraction of the geological history of the Earth.

 

The Geology of Agates in Scotland

 

Most of the agates displayed on this website occur in the volcanic rocks of Scotland. Although these are widely distributed, agates do not occur in all of them.


The most productive rocks are the Old Red Sandstone (ORS) lavas mainly occurring in the Midland valley of Scotland. The Old Red Sandstone is a term used in Scotland to describe mainly sandstone rocks that were deposited in Devonian times but also in the late Silurian and as recently as the early Carboniferous. This reflects the difficulties in ‘fixing’ the base or top of the Devonian Period in the Scottish rocks. There were at the time numerous volcanic centres producing basaltic lava flows. This extensive andesitic volcanic activity characterises the Lower Old Red Sandstone in the MidlandValley and the Lorne areas as well as that in the Rhynie area.


The volcanic activity lasted intermittently for many millions of years depositing many lava flows one on top of the other. In between volcanic activity weathering and alteration of the rock would take place before activity resumed again. Eventually all volcanic activity ceased and the lavas cooled down.

Old Red Sandstone agate-bearing lavas occur in a band stretching from north of Montrose in the northeast to Maidens in the southwest of Scotland. Similar lavas can also be found in the Cheviot Hills in the Scottish Borders and the area around Oban in the southwestern Highlands and they even crop out in Shetland on the Esha Ness peninsula. Agates have been found in all these areas but the area around Oban in the southwestern Highlands has, in recent years, not been so productive. This may be due to less material to find or that the area has not been adequately explored

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The other main agate-bearing lavas in Scotland are from the Tertiary period (about 65 million years old) of the western Islands stretching from Ailsa Craig in the south to Skye in the northwestern Highlands. Within this area the most productive areas are on the islands of Mull and Rum and also on the western cliffs of Skye. Agates have also recently been recorded on the island of Eigg.
The lavas of the Lower Palaeozoic (470 million year old) and Permo-carboniferous periods (320 million years old) are rarely agate-bearing (Macpherson H. G., Agates, British Museum (Natural History), 1989).

 

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