Geology of Scotland

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An early geological map of Central Scotland Scotland.central.geological.jpg
An early geological map of Central Scotland

The geology of Scotland is unusually varied for a country of its size, with a large number of different geological features. [1] There are three main geographical sub-divisions: the Highlands and Islands is a diverse area which lies to the north and west of the Highland Boundary Fault; the Central Lowlands is a rift valley mainly comprising Palaeozoic formations; and the Southern Uplands, which lie south of the Southern Uplands Fault, are largely composed of Silurian deposits.

Contents

The existing bedrock includes very ancient Archean gneiss, metamorphic beds interspersed with granite intrusions created during the Caledonian mountain building period (the Caledonian orogeny), commercially important coal, oil and iron-bearing carboniferous deposits and the remains of substantial Palaeogene volcanoes. During their formation, tectonic movements created climatic conditions ranging from polar to desert to tropical and a resultant diversity of fossil remains.

Scotland has also had a role to play in many significant discoveries such as plate tectonics and the development of theories about the formation of rocks and was the home of important figures in the development of the science including James Hutton (the "father of modern geology"), [2] Hugh Miller and Archibald Geikie. [3] Various locations such as 'Hutton's Unconformity' at Siccar Point in Berwickshire and the Moine Thrust in the northwest were also important in the development of geological science.

Arthur's Seat in Edinburgh, the eroded remains of a volcano active during the Carboniferous period Edinburgh (4571480418).jpg
Arthur's Seat in Edinburgh, the eroded remains of a volcano active during the Carboniferous period
Folded rocks near Arnisdale Bent Rock at the Crudha Ard - geograph.org.uk - 447478.jpg
Folded rocks near Arnisdale
Glyptolepis paucidens fish fossil found in mid-Devonian rocks of Scotland Glyptolepis paucidens - mid dev scotland.jpg
Glyptolepis paucidens fish fossil found in mid-Devonian rocks of Scotland
Lanarkite, susannite and macphersonite from Leadhills Lanarkite, Susannite, Macphersonite-359326.jpg
Lanarkite, susannite and macphersonite from Leadhills

Overview

The main geographical divisions of Scotland Scotland (Location) Named (HR).png
The main geographical divisions of Scotland

From a geological and geomorphological perspective the country has three main sub-divisions all of which were affected by Pleistocene glaciations.

Highlands and Islands

The Highlands and Islands lie to the north and west of the Highland Boundary Fault, which runs from Arran to Stonehaven. This part of Scotland largely comprises ancient rocks, from Cambrian and Precambrian times, that were uplifted to form a mountain chain during the later Caledonian orogeny. These foundations are interspersed with many igneous intrusions of more recent age, the remnants of which have formed mountain massifs such as the Cairngorms and Skye Cuillins. A significant exception to the above are the fossil-bearing beds of the Old Red Sandstone found principally along the Moray Firth coast and in the Orkney islands. These rocks are around 400 million years old, and were laid down in the Devonian period. [4] The Highlands are generally mountainous and are bisected by the Great Glen Fault. The highest elevations in the British Isles are found here, including Ben Nevis, the highest peak at 1,344 metres (4,409 ft). Scotland has over 790 islands, divided into four main groups: Shetland, Orkney, and the Hebrides, further sub-divided into the Inner Hebrides and Outer Hebrides.

Stac an Armin, St Kilda Stac an Armin and Boreray.jpg
Stac an Armin, St Kilda

The Hebridean archipelago outlier of St Kilda is composed of Palaeogene igneous formations of granites and gabbro, heavily weathered by the elements. These islands represent the remnants of a long extinct ring volcano rising from a seabed plateau approximately 40 m (130 ft) below sea level. [5]

The geology of Shetland is complex with numerous faults and folds. These islands are Scotland's most northerly area of Caledonian orogenic rocks and there are outcrops of Lewisian, Dalradian and Moine metamorphic rocks with similar histories to their equivalents on the Scottish mainland. Similarly, there are also Old Red Sandstone deposits and granite intrusions. The most distinctive feature is the ultrabasic ophiolite peridotite and gabbro on Unst and Fetlar, which are remnants of the Iapetus Ocean floor. [6] Much of Shetland's economy depends on oil and gas production from fields in the surrounding seas. [7] [8]

Midland Valley

Often referred to as the Central Lowlands, this is a rift valley [9] mainly comprising Palaeozoic formations. Many of these sediments have economic significance for it is here that the coal and iron bearing rocks that fuelled Scotland's industrial revolution are to be found. Although relatively low-lying, hills such as the Pentland Hills, Ochils and Campsie Fells are rarely far from view. This area has also experienced intense volcanism, Arthur's Seat in Edinburgh being the remnant of a once much larger volcano active in the Carboniferous period about 340 million years ago. As a result of ice age glaciers, drumlins were formed, and many hills have a crag and tail landform.

Southern Uplands

The Southern Uplands Scotland Southern Uplands01 2002-08-16.jpg
The Southern Uplands

The Southern Uplands are a range of hills almost 200 km (120 mi) long, interspersed with broad valleys. They lie south of a second fault line running from Ballantrae towards Dunbar. [10] The geological foundations largely comprise Silurian deposits laid down some 4-500 million years ago. [11] [12]

Post-glacial events

The whole of Scotland was covered by ice sheets during the Pleistocene ice ages and the landscape is much affected by glaciation, and to a lesser extent by subsequent sea level changes. [13] [14] In the post-glacial epoch, circa 6100 BC, Scotland and the Faroe Islands experienced tsunamis up to 20 metres high caused by the Storegga Slides, a series of immense underwater landslides off the coast of Norway. [15] [16] Earth tremors are infrequent and usually slight. The Great Glen is the most seismically active area of Britain, but the last event of any size was in 1901. [17]

Chronology

Archean and Proterozoic eons

The oldest rocks of Scotland are the Lewisian gneisses, which were formed in the Precambrian period, up to 3,000 Ma (million years ago). They are among the oldest rocks in the world. They form the basement to the west of the Moine Thrust on the mainland, in the Outer Hebrides and on the islands of Coll and Tiree. [18] These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma. [19] One of these intrusions forms the summit plateau of the mountain Roineabhal in Harris. The rock here is anorthosite, and is similar in composition to rocks found in the mountains of the Moon. [20]

Suilven is formed of Torridonian sandstone, sitting on a landscape of Lewisian gneiss. Suilven looking east.jpg
Suilven is formed of Torridonian sandstone, sitting on a landscape of Lewisian gneiss.

Torridonian sandstones were also laid down in this period over the gneisses, and these contain the oldest signs of life in Scotland. In later Precambrian times, thick sediments of sandstones, limestones muds and lavas were deposited in what is now the Highlands of Scotland. [21] [22]

Palaeozoic era

Cambrian period

Further sedimentary deposits were formed through the Cambrian period (541–485 Ma), some of which, along with the earlier Precambrian sediments, metamorphosed into the Dalradian series. This is composed of a wide variety of materials, including mica schist, biotite gneiss schist, schistose grit, greywacke and quartzite. [23] The area that would become Scotland was at this time close to the south pole and part of Laurentia. Fossils from the north-west Highlands indicate the presence of trilobites and other primitive forms of life. [21] [24]

Ordovician period

The proto-Scotland landmass moved northwards, and from 460 to 430 Ma, sandstone, mudstone and limestone were deposited in the area that is now the Midland Valley. This occurred in shallow tropical seas at the margins of the Iapetus Ocean. The Ballantrae Complex near Girvan was formed from this ocean floor and is similar in composition to rocks found at The Lizard in Cornwall. Nonetheless, northern and southern Britain were far apart at the beginning of this period, although the gap began to close as the continent of Avalonia broke away from Gondwana, collided with Baltica and drifted towards Laurentia. The Caledonian orogeny began forming a mountain chain from Norway to the Appalachians. There was an ice age in the southern hemisphere, and the first mass extinction of life on Earth took place at the end of this period. [21] [25]

Silurian period

The collision of Avalonia, Baltica and Laurentia (The names are in French.) AVALONIA.jpg
The collision of Avalonia, Baltica and Laurentia (The names are in French.)

During the Silurian period (444–419 Ma) the continent of Laurentia gradually collided with Baltica, joining Scotland to the area that would become England and Europe. Sea levels rose as the Ordovician ice sheets melted, and tectonic movements created major faults which assembled the outline of Scotland from previously scattered fragments. These faults are the Highland Boundary Fault, separating the Lowlands from the Highlands, the Great Glen Fault that divides the North-west Highlands from the Grampians, the Southern Uplands Fault and the Iapetus Suture, which runs from the Solway Firth to Lindisfarne and which marks the close of the Iapetus Ocean and the joining of northern and southern Britain. [21] [26] [27]

Silurian rocks form the Southern Uplands of Scotland, which were pushed up from the sea bed during the collision with Baltica/Avalonia. The majority of the rocks are weakly metamorphosed coarse greywacke. [10] The Highlands were also affected by these collisions, creating a series of thrust faults in the northwest Highlands including the Moine Thrust, the understanding of which played an important role in 19th century geological thinking. Volcanic activity occurred across Scotland as a result of the collision of the tectonic plates, with volcanoes in southern Scotland, and magma chambers in the north, which today form the granite mountains such as the Cairngorms. [28]

Devonian period

The Old Red Sandstone Continent in the Devonian Euramerica en.svg
The Old Red Sandstone Continent in the Devonian

The Scottish landmass now formed part of the Old Red Sandstone Continent and lay some 25 degrees south of the equator, moving slowly north during this period to 10 degrees south. The accumulations of Old Red Sandstone laid down from 408 to 370 million years ago were created as earlier Silurian rocks, uplifted by the formation of Pangaea, eroded and were deposited into a body of fresh water (probably a series of large river deltas). A huge freshwater lake - Lake Orcadie - existed on the edges of the eroding mountains stretching from Shetland to the southern Moray Firth. The formations are extremely thick, up to 11,000 metres in places, and can be subdivided into three categories "Lower", "Middle", and "Upper" from oldest to youngest. As a result, the Old Red Sandstone is an important source of fish fossils and it was the object of intense geological studies in the 19th century. In Scotland these rocks are found predominantly in the Moray Firth basin and Orkney Archipelago, and along the southern margins of the Highland Boundary Fault.

Elsewhere volcanic activity, possibly as a result of the closing of the Iapetus Suture, created the Cheviot Hills, Ochil Hills, Sidlaw Hills, parts of the Pentland Hills and Scurdie Ness on the Angus coast. [29] [30] [31]

Carboniferous period

During the Carboniferous period (359–299 Ma), Scotland lay close to the equator. Several changes in sea level occurred and the coal deposits of Lanarkshire and West Lothian and limestones of Fife and Dunbar date from this time. There are oil shales near Bathgate around which the 19th-century oil-processing industry developed, and elsewhere in the Midland Valley there are ironstones and fire clay deposits that had significance in the early Industrial Revolution. Fossil Grove in Victoria Park, Glasgow contains the preserved remains of a Carboniferous forest. More volcanic activity formed Arthur's Seat and the Salisbury Crags in Edinburgh and the nearby Bathgate Hills. [32] [33]

Permian period

Map of Pangaea, during the Triassic period, 249 million years ago 249 global.png
Map of Pangaea, during the Triassic period, 249 million years ago

The Old Red Sandstone Continent became a part of the supercontinent Pangaea in the Permian (299–252 Ma), during which proto-Britain continued to drift northwards. Scotland's climate was arid at this time and some fossils of reptiles have been recovered. However, Permian sandstones are found in only a few places - principally in the south west, on the island of Arran, and on the Moray coast. Stone quarried from Hopeman in Moray has been used in the National Museum and Scottish Parliament buildings in Edinburgh. [34]

At the close of this period came the Permian–Triassic extinction event in which 96% of all marine species vanished [35] and from which biodiversity took 30 million years to recover.

Mesozoic era

Triassic period

During the Triassic (252–201 Ma), much of Scotland remained in desert conditions, with higher ground in the Highlands and Southern Uplands providing sediment to the surrounding basins via flash floods. This is the origin of sandstone outcrops near Dumfries, Elgin and the Isle of Arran. Towards the close of this period sea levels began to rise and climatic conditions became less arid. [36]

Jurassic period

Pangaea was formed by the convergence of multiple continental masses and later broke apart to form two primary continents, Laurasia and Gondwana Laurasia-Gondwana.png
Pangaea was formed by the convergence of multiple continental masses and later broke apart to form two primary continents, Laurasia and Gondwana

As the Jurassic (201–145 Ma) started, Pangaea began to break up into two continents, Gondwana and Laurasia, marking the beginning of the separation of Scotland and North America. Sea levels rose, as Britain and Ireland drifted on the Eurasian Plate to between 30° and 40° north. Most of northern and eastern Scotland including Orkney, Shetland and the Outer Hebrides remained above the advancing seas, but the south and south-west were inundated. There are only isolated sedimentary rocks remaining on land from this period, on the Sutherland coast near Golspie and, forming the Great Estuarine Group, on Skye, Mull, Raasay and Eigg. This period does however have considerable significance. The burial of algae and bacteria below the mud of the sea floor during this time led to the formation of North Sea oil and natural gas, much of it trapped in overlying sandstone by deposits formed as the seas fell to form the swamps and salty lakes and lagoons that were home to dinosaurs. [7] [37] [38]

Cretaceous period

In the Cretaceous (145–66 Ma), Laurasia split into the continents of North America and Eurasia. Sea levels rose globally during this period and much of low-lying Scotland was covered in a layer of chalk. Although large deposits of Cretaceous rocks were laid down over Scotland, these have not survived erosion except in a few places on the west coast such as Loch Aline in Morvern [39] [40] where they form a part of the Inner Hebrides Group. At the end of this period the Cretaceous–Palaeogene extinction event brought the age of dinosaurs to a close.

Cenozoic era

Palaeogene period

In the early Palaeogene period between 63 and 52 Ma, the last volcanic rocks in the British Isles were formed. [41] As North America and Greenland separated from Europe, the Atlantic Ocean slowly formed. This led to a chain of volcanic sites west of mainland Scotland including on Skye, the Small Isles and St. Kilda, in the Firth of Clyde on Arran and Ailsa Craig and at Ardnamurchan. [42] Sea levels began to fall, and for the first time the general outline of the modern British Isles was revealed. [43] At the beginning of this period the climate was sub-tropical and erosion was caused by chemical weathering, creating characteristic features of the Scottish landscape such as the topographical basin of the Howe of Alford near Aberdeen. [44] The vegetation of the period is known from Palaeocene sedimentary deposits on Isle of Mull. The rich flora here included temperate-climate tree species such as plane, hazel, oak, Cercidiphyllum , Metasequoia and ginkgo. [45]

Neogene period

Miocene and Pliocene epochs

In the Miocene and Pliocene epochs further uplift and erosion occurred in the Highlands. Plant and animal types developed into their modern forms. Scotland lay in its present position on the globe. As the Miocene progressed, temperatures dropped and remained similar to today's. [44] [46]

Quaternary period

Pleistocene epoch
The 'Barns of Bynack' - tors on the summit plateau of Bynack More, Cairngorms National Park. Barns of Bynack.jpg
The 'Barns of Bynack' - tors on the summit plateau of Bynack More, Cairngorms National Park.

Several ice ages shaped the land through glacial erosion, creating u-shaped valleys and depositing boulder clays, especially on the western seaboard. The last major incursion of ice peaked about 18,000 years ago, leaving other remnant features such at the granite tors on the Cairngorm mountain plateaux. [47] [48]

Holocene epoch

Over the last twelve thousand years the most significant new geological features have been the deposits of peat and the development of coastal alluvium. Post-glacial rises in sea level have been combined with isostatic rises of the land resulting in a relative fall in sea level in most areas. [49] [50] In some places, such as Culbin in Moray, these changes in relative sea level have created a complex series of shorelines. [51] A rare type of Scottish coastline found largely in the Hebrides consists of machair habitat, [52] a low lying dune pasture land formed as the sea level dropped leaving a raised beach. In the present day, Scotland continues to move slowly north.

Geologists in Scotland

James Hutton, painted by Abner Lowe James Hutton.jpg
James Hutton, painted by Abner Lowe

Scottish geologists and non-Scots working in Scotland have played an important part in the development of the science, especially during its pioneering period in the late 18th century and 19th century. [1]

Charles Lyell Charles Lyell.jpg
Charles Lyell
Murchison's memorial tablet in Perm, Russia. "To Roderick Impey Murchison, Scottish geologist, explorer of Perm Krai, who gives to the last period of the Palaeozoic era the name of Permian." Memorial tablet of Roderick Murchison at the School 9 in Perm.jpg
Murchison's memorial tablet in Perm, Russia. "To Roderick Impey Murchison, Scottish geologist, explorer of Perm Krai, who gives to the last period of the Palaeozoic era the name of Permian."

Important sites

Siccar Point

Angled Devonian and Silurian strata of 'Hutton's Unconformity' at Siccar Point, Berwickshire Siccar Point.jpg
Angled Devonian and Silurian strata of 'Hutton's Unconformity' at Siccar Point, Berwickshire

Siccar Point, Berwickshire is world-famous as one of the sites that proved Hutton's views about the immense age of the Earth. Here Silurian rocks have been tilted almost to the vertical. Younger Carboniferous rocks lie unconformably over the top of them, dipping gently, indicating that an enormous span of time must have passed between the creation of the two beds. When Hutton and James Hall visited the site in 1788 their companion Playfair wrote: [10] [68]

On us who saw these phenomenon for the first time the impression will not easily be forgotten...We felt necessarily carried back to a time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited, in the shape of sand or mud, from the waters of the supercontinent ocean... The mind seemed to grow giddy by looking so far back into the abyss of time; and whilst we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events, we became sensible how much further reason may sometimes go than imagination may venture to follow. —John Playfair (1805) Transactions of the Royal Society of Edinburgh, vol. V, pt. III. [69]

Knockan Crag

The Moine Thrust in Assynt is one of the most studied geological features in the world. Its discovery in the 1880s was a milestone in the history of geology as it was one of the first thrust belts in the world to be identified. [28] Investigations by John Horne and Benjamin Peach resolved a dispute between Murchison and Geikie on the one hand and James Nicol and Charles Lapworth on the other. The latter believed that older Moine rocks lay on top of younger Cambrian rocks at Knockan Crag, and Horne and Peach's work confirmed this in their classic paper The Geological Structure of the North-West Highlands of Scotland , which was published in 1907. [70] [71] A statue to these two pioneers of fieldwork was erected at Inchnadamph near the hotel there which played a prominent part in the annals of early geology. [72] This area is at the heart of the 'North West Highlands Geopark'. [73]

Dob's Linn

Lapworth also had a prominent role to play in the fame of Dob's Linn, a small gorge in the Scottish Borders, which contains the 'golden spike' (i.e. the official international boundary or stratotype) between the Ordovician and Silurian periods. Lapworth's work in this area, especially his examination of the complex stratigraphy of the Silurian rocks by comparing fossil graptolites, was crucial in to the early understanding of these epochs.

Skye Cuillin

Sgurr nan Gillean and the Pinnacle Ridge from Basteir gorge, Skye. James Forbes and Duncan MacIntyre completed the first recorded climb of this mountain in 1836. Pinnacle ridge&gillean2.jpg
Sgurr nan Gillean and the Pinnacle Ridge from Basteir gorge, Skye. James Forbes and Duncan MacIntyre completed the first recorded climb of this mountain in 1836.

The Skye Cuillin mountains provide classic examples of glacial topography and were the subject of an early published account by James Forbes in 1846 (who had become a fellow of the Royal Society of Edinburgh aged only nineteen). [75] [76] He partnered Louis Agassiz on his trip to Scotland in 1840 and although they subsequently argued, Forbes went on to publish other important papers on Alpine glaciers. [77] In 1904 Alfred Harker published The Tertiary Igneous Rocks of Skye, the first detailed scientific study of an extinct volcano. [78] [79]

Strontian

In the hills to the north of the village of Strontian the mineral strontianite was discovered, from which the element strontium was first isolated by Sir Humphry Davy in 1808. [80]

Staffa

Fingal's Cave Fingal's Cave, Staffa - geograph.org.uk - 906890.jpg
Fingal's Cave

The island of Staffa contains Fingal's Cave made up of massive hexagonal columns of Palaeogene basalt.

Schiehallion

The isolated position and regular shape of Schiehallion, a Munro in Perthshire, led Nevil Maskelyne to use the deflection caused by the mass of the mountain to estimate the mass of the Earth in a ground-breaking experiment carried out in 1774. Following Maskelyne's survey, Schiehallion became the first mountain to be mapped using contour lines. [81]

Rhynie

The village of Rhynie in Aberdeenshire is the site of an important sedimentary deposit - Rhynie chert. The bulk of this fossil bed consists of primitive plants that had water-conducting cells and sporangia but no leaves, along with arthropods: Collembola, Opiliones (harvestmen), pseudoscorpions and the extinct, spider-like Trigonotarbids. This fossil bed is remarkable for two reasons. Firstly, the age of the find (early Devonian circa 410 Ma) [82] [83] makes this one of the earliest sites anywhere containing terrestrial fossils, coinciding with the first stages of the colonisation of land by plants and animals. Secondly, these cherts are famous for their exceptional state of ultrastructural preservation, with individual cell walls easily visible in polished specimens. For example, stomata have been counted and lignin remnants detected in the plant material.

East Kirkton quarry

A disused quarry at East Kirkton in the Bathgate Hills is the location where the Carboniferous fossil of Westlothiana lizziae (aka 'Lizzie') was found in 1984. This lizard is one of the earliest known ancestors of the reptiles. The specimen was purchased in part by public subscription and is now on display in the National Museum of Scotland. The site was originally discovered in the early 19th century and has also provided fossil eurypterids, sharks and a variety of primitive acanthodian fish. [84]

Wester Ross bolide

In 2008 the ejected material from a meteorite impact crater was discovered near Ullapool in Wester Ross. Preserved within sedimentary layers of sandstone, this is the largest known bolide impact from what are now the British Isles. [85]

See also

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The North West Highlands Geopark is a geopark in the Scottish Highlands. Awarded UNESCO geopark status in 2004, it was Scotland's first geopark, featuring some of the oldest rocks in Europe, around 3,000 million years old. The park contains many notable geological features, such as the Moine Thrust Belt and Smoo Cave and covers an area of around 2,000 square kilometres (770 sq mi).

<span class="mw-page-title-main">Geology of Yorkshire</span>

The Geology of Yorkshire in northern England shows a very close relationship between the major topographical areas and the geological period in which their rocks were formed. The rocks of the Pennine chain of hills in the west are of Carboniferous origin whilst those of the central vale are Permo-Triassic. The North York Moors in the north-east of the county are Jurassic in age while the Yorkshire Wolds to the south east are Cretaceous chalk uplands. The plain of Holderness and the Humberhead levels both owe their present form to the Quaternary ice ages. The strata become gradually younger from west to east.

Hutton's Unconformity is a name given to various notable geological sites in Scotland identified by the 18th-century Scottish geologist James Hutton as places where the junction between two types of rock formations can be seen. This geological phenomenon marks the location where rock formations created at different times and by different processes adjoin. For Hutton, such an unconformity provided evidence for his Plutonist theories of uniformitarianism and the age of Earth.

<i>Skolithos</i> Trace fossil

Skolithos is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow with a distinct lining. It was produced globally by a variety of organisms, mostly in shallow marine environments, and appears as linear features in sedimentary rocks.

<span class="mw-page-title-main">Geology of Orkney</span>

The geology of the Orkney islands in northern Scotland is dominated by the Devonian Old Red Sandstone (ORS). In the southwestern part of Mainland, this sequence can be seen to rest unconformably on a Moinian type metamorphic basement.

<span class="mw-page-title-main">Moine Supergroup</span> Supergroup in Highland, Scotland, UK

The Moine Supergroup is a sequence of Neoproterozoic metamorphic rocks that form the dominant outcrop of the Scottish Highlands between the Moine Thrust Belt to the northwest and the Great Glen Fault to the southeast. The sequence is metasedimentary in nature and was metamorphosed and deformed in a series of tectonic events during the Late Proterozoic and Early Paleozoic. It takes its name from A' Mhòine, a peat bog in northern Sutherland.

<span class="mw-page-title-main">Lewisian complex</span> Suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland

The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 billion years (Ga). They form the basement on which the Torridonian and Moine Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with a minor amount of supracrustal rocks. Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.

<span class="mw-page-title-main">Highlands controversy of Northwest Scotland</span> 19th-century geological controversy

The Highlands controversy was a scientific controversy which started between British geologists in the middle of the nineteenth century concerning the nature of the rock strata in the Northwest Highlands of Scotland. The debate became contentious, even acrimonious, because of some of the personalities involved and because it pitted professional geologists of the Geological Survey against academic and amateur geologists. An initial resolution was achieved by about 1886 but the great complexity and scientific importance of the discovery of the Moine Thrust Belt and the geological processes involved in its creation led to field work continuing for a further twenty years culminating in the 1907 publication by the Geological Survey of a book of fundamental geological significance: The Geological Structure of the North-West Highlands of Scotland.

<span class="mw-page-title-main">Geology of the Isle of Mull</span>

The geology of the Isle of Mull in Scotland is dominated by the development during the early Palaeogene period of a ‘volcanic central complex’ associated with the opening of the Atlantic Ocean. The bedrock of the larger part of the island is formed by basalt lava flows ascribed to the Mull Lava Group erupted onto a succession of Mesozoic sedimentary rocks during the Palaeocene epoch. Precambrian and Palaeozoic rocks occur at the island's margins. A number of distinct deposits and features such as raised beaches were formed during the Quaternary period.

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General reference: