Geology of Scotland
The geology of Scotland is unusually varied for a country of its size, with a large number of different geological features. 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.
The existing bedrock includes very ancient Archean gneiss, metamorphic beds interspersed with granite intrusions created during the Caledonian mountain building period, 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, Hugh Miller and Archibald Geikie. 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.
File:Edinburgh.jpg|thumb|Arthur's Seat in Edinburgh, the eroded remains of a volcano active during the Carboniferous period
File:Lanarkite, Susannite, Macphersonite-359326.jpg|thumb|upright|Lanarkite, susannite and macphersonite from Leadhills
Overview
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. 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. 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.File:Stac an Armin and Boreray.jpg|thumb|left|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 below sea level.
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. Much of Shetland's economy depends on oil and gas production from fields in the surrounding seas.
Midland Valley
Often referred to as the Central Lowlands, this is a rift valley 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 are a range of hills almost long, interspersed with broad valleys. They lie south of a second fault line running from Ballantrae towards Dunbar. The geological foundations largely comprise Silurian deposits laid down some 4-500 million years ago.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. 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. 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.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. 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. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma. 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.File:Suilven looking east.jpg|thumb|upright|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.
Palaeozoic era
Cambrian period
Further sedimentary deposits were formed through the Cambrian period, 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. 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.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.Silurian period
During the Silurian period 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.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. 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.
Devonian period
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. 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.