Scotland’s rocks and minerals

Scotland has a rich geological history and incredibly diverse geology. The geology of Scotland today may be viewed as a series of five geologically distinctive areas or foundation blocks.

These foundations blocks formed far apart hundreds of millions of years ago. They eventually came together during massive continental collisions around 480 to 425 million years ago. Once its foundations became joined, Scotland drifted northwards, across the equator to it present position. During this journey, new layers of rock formed, as sediments accumulated and were buried. This occurred in a variety of environments from deep oceans and shallow seas to deserts, swamps and lakes.

Many rare, spectacular and precious minerals occur in Scotland’s rocks. Lead, silver, gold, manganese, baryte, zinc, chromium, copper, mica and talc have all been mined in Scotland. Old mine sites also often host rare or unique minerals. Decorative, precious and semi-precious stones such as sapphire, blue topaz, Cairngorm crystal (smokey quartz), agates and amethyst have been collected in Scotland, and in some cases are still found.

Scotland’s geological foundations

The two most northerly founding blocks the Northwest sea-board and the Northern Highlands appear to have their most ancient foundations in common but are otherwise very different and must have been located far apart at one time. Similarly rocks of the Grampian Highlands are thought to be underlain by rocks comparable to those of the Northern Highlands; but the two areas must have been widely separated for much of their history.

The two most southerly foundations blocks the Central Belt and the Southern Uplands – are both products of the continental drift and continental collisions which brought Scotlands foundations together, and also connected Scotland to England.

The four major faults which divide Scotland’s foundation blocks are (from north to south): the Moine Thrust, the Great Glen Fault, the Highland Boundary Fault, and the Southern Uplands Fault. The geological boundary which separates the Southern Uplands from the underlying rocks of northern England is called the Iapetus Suture after the Iapetus Ocean that once separated England and Scotland.

Of these geological boundaries, the Great Glen Fault and the Highland Boundary Fault are the most distinctive landscape features. The Great Glen Fault forms the dramatic gash of the Great Glen, while the Highland Boundary Fault marks the distinctive change from Lowland to Highland scenery. The Moine Thrust, Southern Upland Fault and the Iapetus Suture are less obvious landscape features but are equally important geologically.

Northwest sea-board geological foundations

The geological foundations of the Northwest sea-board of Scotland, including the Western Isles, Coll, Tiree, Rum, Skye and the Northwest Highlands from Cape Wrath to Kyle of Lochalsh, are built from rocks of three very different ages. These foundations are separated from the neighbouring Northern Highlands by a fault known as the Moine Thrust.

Lewisian rocks

Lewisian rocks

The oldest rocks of the Northwest Highlands are up to 3 billion (3,000 million) years old and are known as Lewisian rocks. They are the oldest rocks in Scotland and some of the oldest in the world. The Lewisian rocks formed over a huge span of time (up to 2 billion years), through burial, compression, folding and heating of large amounts of cooled molten rock, and much smaller volumes of muddy, sandy and rare carbonate (e.g. limestone) sediments.

Torridonian rocks

By around 1,000 million (1 billion) years ago, the Lewisian rocks were already ancient and had been eroded down into a hilly landscape. Across this landscape huge rivers flowed, around 1200 to 800 million years ago, depositing layers of red sandstone, muds and pebbly conglomerates. This sequence of river sediments is known as the Torridonian, and once formed a blanket up to 7.5 kilometres thick covering the ancient Lewisian landscape.

Like the Lewisian, the Torridonian rocks were later partly eroded away. Today much of the old Lewisian landscape has been re-exhumed with hills of Torridonian rock, such as Slioch and Suilven, perched on top. Around 550 to 500 million years ago in the late Cambrian and early Ordovician geological time periods, the area was covered by a shallow sea and a sequence of pure white sands (now quartzite), some muddier sediments (now soft, nutrient rich Fucoid beds), and finally a thick sequence of carbonate rocks (now magnesium-rich limestone know as Durness dolomite) were laid down on the eroded surface of Lewisian and Torridonian rocks. These Cambro-Ordovician sediments have now also largely been eroded away. Today they are only found in a thin strip along the east edge of the area, adjacent to the Moine Thrust, and on the tops of hills such as Beinn Eighe and Foinaven.

Northern Highlands geological foundations

The rocks of the Northern Highlands are bounded to the northwest by the Moine Thrust and to the southeast by the Great Glen Fault. The Great Glen Fault extends to Shetland where it is most commonly mapped as analogous to the Walls Boundary Fault. The foundations of the Northern Highlands are predominantly rocks of the Moine Supergroup or Moine rocks, named after the A Mhoine area of Sutherland. However, below the Moine rocks, are rocks that are in many cases so similar to the Lewisian rocks of the Northwest Highlands, that they are referred to as Lewisianoid (Lewisian-like) or more simply as Lewisian rocks.

The Lewisian rocks of the Northern Highlands occur in inliers where the younger Moine rocks have been eroded away. Most have a very similar history of formation, heating and compression to the Lewisian rocks of the Northwest Highlands. However, some areas show evidence of having been caught up in a mountain-building event which affected rocks of the Grenville province of east Canada, but few if any other rocks in Scotland. Despite their similarities, it is likely that the Lewisian rocks underlying the Moine were hundreds of kilometres away from the Lewisian rocks of the Northwest Highlands when the Moine rocks formed on top of them. Moine rocks were originally layers of sandy and muddy sediments deposited in a shallow sea, around 1000- 800 million years ago, on top of eroded Lewisan rocks.

The Moine rocks were heated and compressed by the later continental collisions which brought the foundations of Scotland together. This has altered them from sedimentary rocks into hard metamorphic rocks largely pale psammites (originally sandy layers) and darker pelites (originally muddy layers). Despite the alteration, original sediment layers and features associated with ripple structures are still often visible.

Grampian Highlands geological foundations

The Grampian Highlands are bounded to the northwest by the Great Glen Fault (and its extension in Shetland, the Walls Boundary Fault) and to the southeast by the Highland Boundary Fault. The foundations of the Grampian Highlands are predominantly rocks of the ‘Dalradian’ Supergroup or Dalradian rocks, named after the ancient Scots Kingdom of Dalriada in Argyll.

The Dalradian rocks were originally layers of sediment, deposited between 800 and 500 million years ago at the edge of an ancient ocean known as the Iapetus ocean. Around 800 million years ago, immense forces broke apart an ancient continent to create the new Iapetus ocean. Sediment was deposited at the edge of the ocean in water that was periodically deepened due to movement on geological faults. The Dalradian rocks formed from this sediment and include sandstone and quartzites from shallow water and coastal sands; limestone from lime chemically precipitated in coastal lagoons; and siltstones, mudstones, slate and shales from deeper-water silts, muds and clays.

The Dalradian rocks were heated and compressed by the later continental collisions which brought the foundations of Scotland together; but much of their original sedimentary structure is still visible. Rare layers of volcanic rock also occur in the Daradian Supergroup. These are related to the rifting of the ancient continent and the formation of new ocean floor as the Iapetus Ocean grew wider. There are also rock layers containing rock fragments which appear to have been transported out to sea on rafts of ice. These indicate that up to three periods of glaciation (Ice Age-type ice-cover) occurred between 800 and 500 million years ago.

The sediments which formed the Dalradian rocks must have been deposited on eroded older rocks. Within the Grampian Highlands, Lewisian-like rocks occur underneath Dalradian rocks in Islay; Moine-like rocks are found near the northern edge of the Cairngorms National Park; and areas of Moine and Lewisian rocks occur east of the Walls Boundary Fault (extension of the Great Glen Fault) in Shetland. These occurrences all suggest that the Dalradian sediments were deposited on a landscape made of eroded Moine and Lewisian rocks. However, this is likely to have been hundreds of kilometres away from the Moine and Lewisian rocks which now form the Northern Highlands.

Central Belt geological foundations

The Central Belt (also known as the Midland Valley) is bounded to the northwest by the Highland Boundary Fault and to the southeast by the Southern Uplands Fault.

The rocks which form the foundations of the Central Belt are buried by younger rocks. The exact nature of the geological foundations of the Central Belt, therefore, is uncertain. However, fragments of rock which are thought to have been eroded from the foundation rocks of the Central Belt (before they became buried) are largely volcanic in origin. Other pieces of evidence, such as rock fragments brought up from depth by later volcanic activity, geophysical profiles and even evidence from rocks in a similar geological position in Ireland, also suggest a volcanic nature the foundations of the Central Belt.

It is currently thought most likely that the foundation rocks of the Central Belt are the eroded remains of a chain of volcanic islands. These islands collided with the Scottish landmass to the north at the start of the series of continental collisions which eventually brought all the geological foundations of Scotland together.

Southern Uplands geological foundations

The broad rolling hills of the Southern Uplands of Scotland parallel the Scotland-England border. They are bounded on the north by the Southern Uplands Fault and their southern edge coincides with one of Britain’s most fundamental geological divides. This is the so-called Iapetus Suture, the trace of the ancient Iapetus Ocean, which once divided two continents. To the north was the continent of Laurentia, containing northern Scotland, the north of Ireland, Greenland and North America. To the south lay Eastern Avalonia the continental landmass that included England, Wales and southern Ireland. Around 500 million years ago the ocean was more than 1000 kilometres wide and over the next 80 million years it closed with the drifting of the Earth’s tectonic plates.

As the two landmasses drifted together closing the ocean, the ocean crust sunk downwards into the Earths mantle. At one point however, a portion of the oceanic crust instead of sinking downwards, was heaved upward onto the continental margin where it can be seen today at Ballantrae. As collision continued, layers of sediment that had accumulated on the ocean floor were scraped off and piled up against the Laurentia continental margin. This pile of sandy and muddy sediment, which was later compressed to become greywacke (coarse, muddy sandstone) and shales, forms the foundations of the Southern Uplands.

The Iapetus Ocean finally closed around 425 million years ago, bringing together Laurentia and Eastern Avalonia. The trace of the junction lies approximately along the line of the Solway and the Scotland-England border.

Scotland’s geological foundations

The foundations of Scotland are five distinct geological blocks that were brought together in a series of continental collisions around 480 to 425 million years ago. Prior to this period of continental collision, Scotlands foundation blocks lay far apart.

The Northwest Sea-board lay to the west with the Northern Highlands some 100 km or so to the east. The beginnings of the Grampian Highlands were even further away, lying perhaps 200 km to the south. All these areas were part of the edge of a large continent called Laurentia.

South of Laurentia was the wide Iapetus Ocean, which formed around 500 million years ago, but was getting steadily narrower due the forces of continental drift. As the Iapetus Ocean closed, three land masses collided with Laurentia. This series of continental collisions (480-425 million years ago) is know as the Caledonian mountain-building event or Caledonian Orogeny. It formed a mountain range, of Alpine, or even possibly Himalayan proportions, known as the Caledonian Mountains. The remains of this mountain range stretch from Norway to the Appalachian Mountains of North America.

The Caledonian Orogeny began when a chain of volcanic islands, formed in the Iapetus, collided with Laurentia (the Grampian Event). The remains of this island chain are believed to form the foundations of the Central Belt. The second stage of the Caledonian Orogeny was the collision of Baltica (Scandinavian Europe) with the northeast edge of Laurentia (the Scandian Event). This brought the Northern Highlands and Northwest Sea-board together. It also started the process of side-ways movement which eventually brought the Northern Highlands and Grampian Highlands together.

The final stage of the Caledonian Orogeny was the less violent collision of Eastern Avalonia (containing England) to the south. This involved the scrapping up of a huge pile of sea-floor sediments which now form the Southern Uplands and occurred around 425 million years ago. Thus by around 425 million years ago, Scotlands geological foundations had come together.

Iapetus Ocean and Caledonian Orogeny

Six hundred million years ago, the Northwest Highlands, the Northern Highlands and the Grampian Highlands lay at the edge of a continent called Laurentia. The Dalradian rocks of the Grampian Highlands were still only layers of sediment, accumulating in the shallows of the vast Iapetus Ocean which lay to the south of the continent of Laurentia.

The Iapetus Ocean began to open around 800 million years ago as a large, ancient continent was pulled apart by the forces of continental drift. Sometime around 500 million years ago, the forces of continental drift began to close the Iapetus Ocean again. It was in the Iapetus Ocean, as it closed, that the volcanic islands, thought to be the foundations of the Central Belt formed.

As the Iapetus Ocean closed, three land masses were effectively heading on a collision course towards Laurentia. Nearest, and heading towards the proto-Grampian Highlands was the chain of volcanic islands, which had formed in the Iapetus Ocean. Behind this, lying on the far shore of the Iapetus Ocean, was a continent called Eastern Avalonia which contained England. The final landmass was a continent known as Baltica which contained present-day Scandanavian Europe. This lay further east, also on the far shore of the Iapetus Ocean. However, it was on a collision course, not with the proto-Grampian Highlands but with the foundations of the Northern Highlands.

The series of collisions which followed formed a massive mountain range, of Alpine, or even possibly Himalayan proportions, known as the Caledonian Mountains. The Caledonian mountain-building event or Caledonian Orogeny occurred in three of stages. The first stage occurred around 480-460 million years ago when the chain of volcanic islands collided. This is called the Grampian Event. The second stage occurred around 440 million years ago when Baltica collided. This is called the Scandian Event. The third and final stage occurred when Eastern Avalonia soft docked around 425 million years ago. This completed the joining of Scotland’s geological foundations and also joined Scotland and England together

The Grampian Event of the Caledonian Orogeny

The first stage of the Caledonian Orogeny, the mountain-building event that brought the geological foundations of Scotland together, occurred 480-460 million years ago. In this period, a chain of volcanic islands, formed in the Iapetus Ocean, collided with the edge of the area we now know as the Grampian Highlands.

This is collision known as the Grampian Event. It created a range of high mountains and formed the Dalradian rocks from the sedimentary rocks which had accumulated at the edge of the ocean, through burial, heating and compression. The Dalradian rocks of the Grampian Highlands that we see today are the eroded roots of these ancient mountains.

Distinctive and varied rocks known as the Highland Border Complex are found along the line of the Highland Boundary Fault. These are believed to be partly the youngest parts of the Dalradian rock sequence, and partly pieces of ocean floor caught up in the collision of the Grampian Highlands and the volcanic island chain.

During and after the Grampian Event, molten rock formed and pushed its way into the deformed rocks of the Grampian Highlands. Much of this was granite which now forms distinctive mountain masses such as the Cairngorms, Lochnagar and Ben Nevis. At least some of this molten rock erupted as volcanoes at the surface. The most famous example is a caldera (or cauldron) volcano, the remains of which form the dramatic scenery of Glencoe.

The Scandian Event of the Caledonian Orogeny

The second stage of the Caledonian Orogeny occurred when Baltica (Scandanavian Europe) collided with the foundations of the Northern Highlands around 440 million years ago. This collision pushed the Northern Highlands westwards onto the foundations of the Northwest Sea-board and is known as the Scandian Event. It also formed the hard metamorphic Moine rocks, originally layers of sea-floor sediment. The movement westwards occurred along zones of shearing in the rock mass (shear zones) and also along brittle fractures or faults. The shear zones can still be traced in the Moine rocks of the Northern Highlands and are sometimes referred to as slides.

The most significant brittle faults associated with the Scandian Event are those of the Moine Thrust Belt (or Moine Thrust Zone). This marks the furthest west where rocks occur that were affected by the Scandian Event. The Moine Thrust, itself marked the western extent of the Moine rocks and thus the edge of the Northern Highlands. However, as the Moine rocks were pushed over the rocks of the Northwest Sea-board along the Moine Thrust, the rocks below were dragged along with them. As a result, these rocks at the edge of the Northwest Sea-board were fractured by large faults and in some cases altered by heat and pressure. This zone of faulting below the Moine Thrust is now exposed to the west of the Moine Thrust by erosion, and is known as the Moine Thrust Belt (or Moine Thrust Zone).

The main phase to the Scandian Event, which pushed the Northern Highlands westwards, was followed by a phase of lateral movement along faults, including the Great Glen Fault and the Walls Boundary Fault in Shetland. This started the process of bringing the amalgamated Northwest Sea-board and Northern Highlands together with the Grampian Highlands. The lateral movements probably continued until around 410 million years ago, by which time the Northern Highlands and Grampian Highlands had come together.

During and after the Scandian Event, molten rock formed and pushed its way into the deformed rocks of the Northern Highlands. Masses of cooled molten rock, known as igneous intrusions, in the Northern Highlands are not as abundant as in the Grampian Highlands. The largest igneous intrusions occur around Strontian, Strath Halladale and Helmsdale, with smaller ones elsewhere including within the Moine Thrust Belt.

Collision with England in the Caledonian Orogeny

The third and final stage of the Caledonian Orogeny was the collision of Eastern Avalonia with Laurentia. This is thought to have occurred around 425 million years ago. Unlike the collision that lead to the Grampian Event and the Scandian Event, this collision was relatively soft and did not cause huge amounts of rock deformation. This process of soft collision is sometimes referred to as soft docking.

As the Iapetus Ocean closed, ocean-floor was pushed down (subducted) below the southern edge of Laurentia. Sediment scraped off this ocean-floor created a huge pile or accretionary prism which now forms the Southern Uplands. The northern edge of the continent of Eastern Avalonia, marked by the Iapetus Suture, lies buried under younger sediments just south of the Scottish-English border.

Around 400 million years ago molten magma derived as a consequence of the collision and loss of the ocean, gave rise to the granites of Criffel and Cairnsmore in the Southern Uplands.

Rocks formed after Scotland’s foundations came together

When its geological foundations came together, Scotland lay south of the equator. Since then, the forces of continental drift have moved Scotland northwards on a journey to its present position. Scotland’s story, during this journey, is one of a continuing cycle of erosion and sediment deposition as it drifted north over the equator.

Much of the rock formed during this time was sedimentary in nature, formed from sediments deposited in rivers, lakes and seas. The type of rock formed at any given time (e.g. anything from sandstone to limestone and coal) depended on the environment Scotland was experiencing at the time. A vast lake in a relatively arid climate produced muddy sediments with layers rich in dead fish. These are now the flagstones of Caithness, full of amazing fossils. A humid, tropical climate which existed when Scotland lay on the equator, resulted in great thicknesses of dead vegetation accumulated in swamps. This now forms the coal of the Central Belt.

Volcanic rocks also formed, initially as a result of stretching centred on the Central Belt, and later as a result of upwelling hot material and stretching associated with the opening of the North Atlantic Ocean. These rocks are usually harder than sedimentary rocks so often form hills or mountain ranges.

Devonian Scotland 416-359 million years ago

Between 416 and 359 million years ago, during the Devonian time period, Scotland lay in the sub-tropical region south of the equator, and had a correspondingly hot climate. Rivers eroding the Highland areas carried sediment into lowland areas where it was deposited. Sediment accumulated in lakes and trapped remains of fish and plants between sediment layers. During this time large deposits of sediment were laid down, these are often termed the ‘Old Red Sandstone’ sediments. The remains of Devonian plants and fish can be found as fossils today, preserved between these ancient sediment layers.

For much of the Devonian, a huge inland lake existed over the areas which are now Orkney and Caithness. At times this ‘Orcadian Lake’ extended to what is now the south shore of the Moray Firth. Much of the ‘Old Red sandstone’ makes good building stone and is a distinctive feature of buildings in areas where it occurs locally, such as Strathmore from Blairgowrie and Coupar Angus to Forfar and Edzell.

Devonian rocks now blanket Scotland’s older geological foundations in Orkney, Caithness, around the Dornoch and Moray Firths and along the northern edge of the Central belt, south of the Highland Boundary Fault. Smaller areas of Devonian rock occur elsewhere from Shetland to the Southern Uplands.

Carboniferous Scotland 359-299 million years ago

The Devonian time period is followed by the Carboniferous, during which Scotland lay at the equator. The Midland Valley area was covered in rainforest and the surrounding tropical seas had abundant fauna and thriving coral reefs. Rocks formed from the sediments and corals of the near-shore shallow-marine environments of the Carboniferous occur in the Lothians, Borders, Fife, Ayrshire and along the northern coast of the Solway Firth.

During the Carboniferous and also during the later Permian period, approximately 360-250 million years ago, Central Scotland was affected by north-south directed extensional stresses (or stretching). The stretching was linked to movement of the Earth’s tectonic plates. Centred around what we know as the Central Belt, the stretching resulted in rifting and sinking of the continental crust to form Scotland’s ‘Midland Valley’. This is why the old geological foundations of the Central Belt have sunk down and are now buried by younger Devonian, Carboniferous and Permian rocks. The rifting of the crust was similar to that which is occurring in east Africa (the East African Rift) today. Rifting results in thinning of the crust, the thinning causing upwelling of magma from the underlying mantle.

Most of the magma came to the surface in the Central Belt along faults, fractures in the Earth’s crust which had formed to accommodate the stretching. Erupted lava covered a large area forming lava fields such as the Campsie Fells. Arthurs Seat in Edinburgh and Dumbarton Rock represent the remains of early Carboniferous volcanoes.

Later in the Carboniferous, successions of sediments were laid down alternately in shallow seas and in rivers, in response to changes in sea level. Limestones, shales and sandstones testify to the changing environmental conditions. In estuary and swamp environments that became widespread in later Carboniferous times, coal deposits were laid down in the steamy tropical climate.

Permian to Cretaceous Scotland 299-65 million years ago

The Permian period, between approximately 299 and 250 million years ago, was warm and dry. Scotland had drifted to the sub-tropical zone north of the equator, and desert conditions prevailed. Sediment was blown around by the wind and re-deposited in the form of sand dunes. There was also a lot of active faulting in the Permian in response to continued extension of the Earth’s crust in southern Scotland. The desert conditions extended into the Triassic, but by the Jurassic around 200 million years ago (when Scotland lay at a similar latitude to the Mediterranean Sea today), a shallow tropical sea covered most of Scotland. There was an abundance of sea life, including corals and ammonites. On the surrounding land there was an ecosystem that included dinosaurs.

During most of the Cretaceous (145 to 65 million years ago), Scotland was dry land. However, by the Late Cretaceous around 75 million years ago, all but the very highest areas of Scotland was submerged beneath a tropical sea. Thick layers of chalk were deposited on the sea floor. We do not see much evidence of the chalk today as much of it has since been eroded away.

Today areas of Permian, Triassic, Jurassic and Cretaceous rocks in Scotland are relatively small. The largest areas of Permian and Triassic rock are found in the Southern Uplands (Permian) and on Arran (Permian and Triassic). Other significant areas occur in Ayrshire (Permian), the south shore of the Moray Firth (Permian and Triassic) and Lewis (Permian and Triassic), with smaller outcrops elsewhere including Rum, Mull and Applecross.

The small areas of Jurassic and Cretaceous rocks that exist in Scotland are confined to the east and west coasts. The largest areas are around Helmsdale on the east coast and Skye – ‘Scotland’s Dinosaur Isle’ – on the west coast.

Palaeogene and Neogene Scotland 65-2.6 million years ago

The Cretaceous was followed by the Palaeogene and Neogene. Together these lasted from 65 to 2.6 million years ago, and during this time Scotland drifted northwards to it present latitude. By the end of the Cretaceous, much of the huge ‘Caledonian’ mountain chain formed during the continental collisions that closed the Iapetus Ocean, had been reduced to a landscape of low relief near to sea level.

At the start of the Palaeogene, there was doming and stretching of the crust along Scotland’s western margin and a period of short but intense volcanic activity associated with the opening of the North Atlantic. This gave rise to extensive lava fields and a chain of volcanoes that included the areas we know today as St Kilda, Skye, Ardnamurchan, Mull and Arran. Much of the Highlands was rapidly ‘uplifted’, with some areas rising as much as 1 kilometre. It is this uplift, which probably continued at a more modest rate through the Neogene, which gave the high elevation Scotland’s Highlands.

The early Palaeogene was also a time of strong global warming and the climate in Scotland was similar to the hot and humid subtropics in parts of western Africa today. This provided ideal conditions for deep chemical weathering of the bedrock.

During the Neogene (23 to 2.6 million years ago), rivers running from the high ground of the west carried sediment eastwards into the North Sea basin. The climate cooled progressively, with cooling intensifying around 3 million years ago to culminate in the start of the Ice Age 2.6 million years ago. This marks the end of the Neogene and the start of the Quaternary geological period (2.6 million years ago to present).

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