Geology of Scotland

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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

formations; and the Southern Uplands, which lie south of the Southern Uplands Fault, are largely composed of Silurian
deposits.

The existing

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
Folded rocks near Arnisdale
Glyptolepis paucidens fish fossil found in mid-Devonian rocks of Scotland
Lanarkite, susannite and macphersonite from Leadhills

Overview

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

Orkney, and the Hebrides, further sub-divided into the Inner Hebrides and Outer Hebrides
.

Stac an Armin, St Kilda

The Hebridean archipelago outlier 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

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

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

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

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

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]

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

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,

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.)

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

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

The Scottish landmass now formed part of the

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

Sidlaw Hills, parts of the Pentland Hills and Scurdie Ness on the Angus coast.[29][30][31]

Carboniferous period

During 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

The Old Red Sandstone Continent became a part of the supercontinent

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

As the

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
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–Palaeogene extinction event
brought the age of dinosaurs to a close.

Cenozoic era

Palaeogene period

In the early

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.

Several

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

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

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]

  • James Hutton (1726–1797), the "father of modern geology", was born in Edinburgh. His Theory of the Earth, published in 1788, proposed the idea of a rock cycle in which weathered rocks form new sediments and that granites were of volcanic origin. At Glen Tilt in the Cairngorm mountains he found granite penetrating metamorphic schists. This showed to him that granite formed from the cooling of molten rock, not precipitation out of water as the Neptunists of the time believed.[53] This sight is said to have "filled him with delight".[54] Regarding geological time scales he famously remarked "that we find no vestige of a beginning, no prospect of an end".[55]
  • John Playfair (1748–1819) from Angus was a mathematician who developed an interest in geology through his friendship with Hutton. His 1802 Illustrations of the Huttonian Theory of the Earth were influential in the latter's success.[56]
  • Edinburgh University. A president of the Geological Society from 1815 to 1817, he is best remembered for producing the first geological map of Scotland, published in 1836. 'MacCulloch's Tree', a 40-foot (12 m) high fossil conifer in the Mull lava flows, is named after him.[57][58]
Charles Lyell
Murchison's memorial tablet in Perm. "To Roderick Impey Murchison, Scottish geologist, explorer of Perm Krai, who gives to the last period of the Palaeozoic era the name of Permian."
  • Sir Roderick Murchison (1792–1871) was born in Ross and Cromarty and served under Wellesley in the Peninsular War. Knighted in 1846, his main achievements were the investigation of Silurian rocks published as The Silurian System in 1839 and of Permian deposits in Russia. The Murchison crater on the Moon and at least fifteen geographical locations on Earth are named after him.[62][63]
  • evolution.[64]
  • Andersonian College and Museum, Glasgow in 1859. His 1864 paper On the Physical Cause of the Changes of Climate during Glacial Epochs led to a position in the Edinburgh office of the Geological Survey of Scotland, as keeper of maps and correspondence, where Sir Archibald Geikie, encouraged his research. He was eventually to become a Fellow of the Royal Society.[65]
  • glaciation. He became Director-General of the Geological Survey of the United Kingdom in 1888 and was also well known for his work on volcanism.[66]
  • Arthur Holmes (1890–1965) was born in England and became Regius Professor of Geology at the University of Edinburgh in 1943. His magnum opus was Principles of Physical Geology, first published in 1944, in which he proposed the idea that slow moving convection currents in the Earth's mantle created 'continental drift' as it was then called. He also pioneered the discipline of geochronology. He lived long enough to see the theory of plate tectonics become widely accepted, and he is regarded as one of the most influential geologists of the 20th century.[67]

Important sites

Siccar Point

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

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

Skye. James Forbes and Duncan MacIntyre completed the first recorded climb of this mountain in 1836.[74]

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

The island of

Palaeogene
basalt.

Schiehallion

The Munro Schiehallion's isolated position and regular shape 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

stomata have been counted and lignin
remnants detected in the plant material.

East Kirkton quarry

A disused

acanthodian fish.[84]

Wester Ross bolide

In 2008 the ejected material from a

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

References

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  3. ^ Keay & Keay (1994) pages 415-422.
  4. ^ McKirdy et al. (2007) pages 124-31.
  5. ^ SNH Trends- seas Archived 2007-07-09 at the Wayback Machine (PDF) Retrieved 2 January 2007.
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  7. ^ a b Shepherd, Mike (2015). Oil Strike North Sea: A first-hand history of North Sea oil. Luath Press.
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  10. ^ a b c Gillen (2003) page 95.
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Cited references:

General reference:

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