The geology of the Isle of Skye in Scotland is highly varied and the island's landscape reflects changes in the underlying nature of the rocks. A wide range of rock types are exposed on the island, sedimentary, metamorphic and igneous, ranging in age from the Archaean through to the Quaternary.
The oldest rocks found on Skye are gneisses of the Lewisian complex that were formed about 2,800 million years ago during the Archaean. [1] These gneisses outcrop on the southeastern coast of the Sleat peninsula and were originally granitic igneous rocks. Near Tarskavaig, Neoproterozoic metasediments of the Tarskavaig Group are found above strongly deformed Lewisian rocks. The Lewisian and Tarskavaig sequences are thrust over another sequence of unmetamorphosed Neoproterozoic sediments, the Torridonian, along the Moine Thrust Zone. The Torridonian on Skye comprises two conformable sequences, the older Sleat Group and the younger Torridon Group. Both groups consist dominantly of sandstones and were deposited mainly by alluvial fans and rivers.
A sequence of Cambrian to Lower Ordovician sediments of the Eriboll and Durness Groups lie above the Torridonian with an angular unconformity. The Lower Cambrian Eriboll Group comprises a basal quartzite, locally with a basal conglomerate, followed by the distinctive Pipe Rock Member, a quartz arenite with white weathering skolithos trace fossils. The Pipe Rock is overlain by the Salterella Grit, a coarse sandstone, and the Fucoid Beds, a sequence of calcareous sandstone and siltstone. The overlying Durness Group comprises a series of dolomites of Upper Cambrian to Middle Ordovician in age. The lowest unit is the Ghrudaidh Formation, followed by the Eliean Dubh Formation, the Sailmhor Formation and the Sangomore Formation, all consisting of dolomites with chert. The two main exposures of Cambro-Ordovician sediments are the "Ord Window" (a gap in the Kishorn Thrust sheet through which the sequence beneath the thrust can be seen) on the northern coast of Sleat and the area between Broadford and Loch Slapin. These beds are affected by thrusting in both areas and by contact metamorphism from Palaeogene granite intrusions in the northern outcrop, locally forming marble, such as at Torrin. [1]
Sedimentary rocks of Mesozoic age underlie most parts of the island north of the Sleat Peninsula. They are hidden beneath Palaeogene volcanic rocks over most of this area, being exposed only on the eastern and northern coasts of the Trotternish peninsula, on the Strathaird peninsula and between the Red Hills and Sleat. [1] Triassic rocks of the Stornoway Formation are found near Broadford, a sequence of sandstones and conglomerates deposited by rivers. These beds are overlain by the lower Jurassic Lias Group with the Broadford Beds at the base, passing up into the Pabay Shale Formation, the Scalpay Sandstone Formation, the Portree Shale Formation and the Raasay Ironstone Formation. The sequence continues with the Lower to Middle Jurassic Bearreraig Sandstone Formation followed by Middle Jurassic Great Estuarine Group, comprising the Cullaidh Shale Formation, Elgol Sandstone Formation, Lealt Shale Formation, Valtos Sandstone Formation, Duntulm Formation, Kilmaluag Formation and the Skudiburgh Formation. The Upper Jurassic is represented by the Staffin Shale Formation. The only Cretaceous unit exposed on Skye is the Strathaird Limestone Formation, thought to be either Turonian or Campanian in age, which lies unconformably on the Jurassic and is overlain unconformably by Palaeocene lavas. [2]
During the Paleocene to Early Eocene Skye formed one of the main volcanic centres of the North Atlantic Igneous Province. Gently dipping lavas from the volcanoes cover most of northern Skye, giving a stepped trap type landscape. [1] The dominant lava type is basalt, with subsidiary hawaiite and mugearite derived from silica-poor magma and minor amounts of trachyte from a silica-rich magma. Part of the magma chambers for the volcanoes are exposed at the surface as major intrusions of gabbro and granite. These coarse-grained igneous rocks are relatively resistant to erosion and now form the Cuillin hills. The Black Cuillin are formed of gabbro, which erodes to form the characteristically jagged outlines, although this is in large part due to the many minor intrusions, such as dykes and cone sheets that cut the gabbro. [1] The Red Hills are formed of granite and have a more rounded topography. All pre-Quaternary rock types on the island are affected by a major swarm of dykes, which forms part of the North Britain Palaeogene Dyke Suite. Most of the dykes are basaltic in composition but a minority are trachytic. The dominant trend of the dykes is northwest–southeast although they are locally in part radial near the old volcanic centre. On the Trotternish peninsula, mafic magma was intruded along the bedding planes of the Jurassic sedimentary rocks beneath the lavas to form sills that are up to 90m thick. They commonly display columnar jointing, such as in the upper part of the Kilt Rock at Staffin. [1]
During this period the island was affected by the Quaternary glaciation, with the development of an ice cap centred on the Cuillin and Red Hills. The main ice sheet that flowed westwards from the Scottish mainland was diverted around this upland area. The island is covered by large areas of glacial till, left behind when the ice melted. [1]
Lower Jurassic rocks near Broadford have provided building stone for local use whilst aggregate for road construction is sourced in a Torridonian sandstone quarry near Sconser. Hornfelsed lava has been worked near Sligachan and dolerite quarried from a sill near Invertote for a similar purpose. Sand and gravel have been extracted from the raised beach deposits west of Kyleakin with local use made of gravels from the mouth of the river in Glen Brittle. The Skye Marble Company works the Cambro-Ordovician limestones at Torrin, metamorphosed through contact with the adjacent granite and gabbro intrusions. A former quarry at Strath was linked by tramroad to Broadford where the marble was exported, prior to the quarry's abandonment.
Graphite and coal are also present in small quantities, near Portree and at Loch Sligachan respectively, but neither has been economical to work. Exploratory drilling for oil has taken place within the strata of the Great Estuarine Group in the north. Peat has been worked extensively in the past for domestic fuel and indeed continues to be so in the north on a smaller scale even today. Diatomite was worked at Loch Cuithir prior to 1914, the works being connected by tramway to the coast at Invertote. Its end use was in dynamite manufacture and later as a filter and insulator. [3]
The Cuillin is a range of mostly jagged rocky mountains on the Isle of Skye in Scotland. The main Cuillin ridge is also called the Black Cuillin to distinguish it from the Red Cuillin, which lie to the east of Glen Sligachan.
The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 kilometres (120 mi) south-west to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thrust faults that branch off the Moine Thrust itself. Topographically, the belt marks a change from rugged, terraced mountains with steep sides sculptured from weathered igneous, sedimentary and metamorphic rocks in the west to an extensive landscape of rolling hills over a metamorphic rock base to the east. Mountains within the belt display complexly folded and faulted layers and the width of the main part of the zone varies up to 10 kilometres (6.2 mi), although it is significantly wider on Skye.
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.
In geology, the term Torridonian is the informal name for the Torridon Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. The strata of the Torridonian Group are particularly well exposed in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks by James Nicol. Stratigraphically, they lie unconformably on gneisses of the Lewisian complex and their outcrop extent is restricted to the Hebridean Terrane.
The geology of Jersey is characterised by the Late Proterozoic Brioverian volcanics, the Cadomian Orogeny, and only small signs of later deposits from the Cambrian and Quaternary periods. The kind of rocks go from conglomerate to shale, volcanic, intrusive and plutonic igneous rocks of many compositions, and metamorphic rocks as well, thus including most major types.
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 Stoer Group, Wester Ross Supergroup and probably the Loch Ness 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.
The Hebridean Terrane is one of the terranes that form part of the Caledonian orogenic belt in northwest Scotland. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt. The basement is formed by Archaean and Paleoproterozoic gneisses of the Lewisian complex, unconformably overlain by the Neoproterozoic Torridonian sediments, which in turn are unconformably overlain by a sequence of Cambro–Ordovician sediments. It formed part of the Laurentian foreland during the Caledonian continental collision.
The Okavango Dyke Swarm is a giant dyke swarm of the Karoo Large Igneous Province in northeast Botswana, southern Africa. It consists of a group of Proterozoic and Jurassic dykes, trending east-southeast across Botswana, spanning a region nearly 2,000 kilometres (1,200 mi) long and 110 kilometres (68 mi) wide. The Jurassic dykes were formed approximately 179 million years ago, composed of mainly tholeiitic mafic rocks. The formation is related to the magmatism at the Karoo triple junction, induced by the plate tectonic break up of the Gondwana supercontinent in the early Jurassic.
The geology of the Isle of Man consists primarily of a thick pile of sedimentary rocks dating from the Ordovician period, together with smaller areas of later sedimentary and extrusive igneous strata. The older strata was folded and faulted during the Caledonian and Acadian orogenies The bedrock is overlain by a range of glacial and post-glacial deposits. Igneous intrusions in the form of dykes and plutons are common, some associated with mineralisation which spawned a minor metal mining industry.
The geology of England's Lake District is dominated by sedimentary and volcanic rocks of mainly Ordovician age underpinned by large granitic intrusions. Younger sedimentary sequences outcrop on the edges of the Lake District area, with Silurian to the south, Carboniferous to the north, east, and west and Permo-Triassic to the west and east. The entire area was covered by a Mesozoic sequence that was eroded off during the Paleogene uplift related to the opening of the North Atlantic. During the Quaternary the area was affected by repeated glaciations, which sculpted the current mountainous landscape.
The geology of Mauritania is built on more than two billion year old Archean crystalline basement rock in the Reguibat Shield of the West African Craton, a section of ancient and stable continental crust. Mobile belts and the large Taoudeni Basin formed and filled with sediments in the connection with the Pan-African orogeny mountain building event 600 million years ago and a subsequent orogeny created the Mauritanide Belt. In the last 251 million years, Mauritania has accumulated additional sedimentary rocks during periods of marine transgression and sea level retreat. The arid country is 50% covered in sand dunes and has extensive mineral resources, although iron plays the most important role in the economy.
The geological history of Zambia begins in the Proterozoic eon of the Precambrian. The igneous and metamorphic basement rocks tend to be highly metamorphosed and may have formed earlier in the Archean, but heat and pressure has destroyed evidence of earlier conditions. Major sedimentary and metamorphic groups formed in the mid-Proterozoic, followed by a series of glaciations in the Neoproterozoic and much of the Paleozoic which deposited glacial conglomerate as well as other sediments to form the Katanga Supergroup and rift-related Karoo Supergroup. Basalt eruptions blanketed the Karoo Supergroup in the Mesozoic and Zambia shifted to coal and sandstone formation. Geologically recent windblown sands from the Kalahari Desert and alluvial deposits near rivers play an important role in the modern surficial geology of Zambia. The country has extensive natural resources, particularly copper, but also cobalt, emeralds, other gemstones, uranium and coal.
The geology of Nigeria formed beginning in the Archean and Proterozoic eons of the Precambrian. The country forms the Nigerian Province and more than half of its surface is igneous and metamorphic crystalline basement rock from the Precambrian. Between 2.9 billion and 500 million years ago, Nigeria was affected by three major orogeny mountain-building events and related igneous intrusions. Following the Pan-African orogeny, in the Cambrian at the time that multi-cellular life proliferated, Nigeria began to experience regional sedimentation and witnessed new igneous intrusions. By the Cretaceous period of the late Mesozoic, massive sedimentation was underway in different basins, due to a large marine transgression. By the Eocene, in the Cenozoic, the region returned to terrestrial conditions.
The geology of Missouri includes deep Precambrian basement rocks formed within the last two billion years and overlain by thick sequences of marine sedimentary rocks, interspersed with igneous rocks by periods of volcanic activity. Missouri is a leading producer of lead from minerals formed in Paleozoic dolomite.
The geology of Alaska includes Precambrian igneous and metamorphic rocks formed in offshore terranes and added to the western margin of North America from the Paleozoic through modern times. The region was submerged for much of the Paleozoic and Mesozoic and formed extensive oil and gas reserves due to tectonic activity in the Arctic Ocean. Alaska was largely ice free during the Pleistocene, allowing humans to migrate into the Americas.
The geology of Montana includes thick sequences of Paleozoic, Mesozoic and Cenozoic sedimentary rocks overlying ancient Archean and Proterozoic crystalline basement rock. Eastern Montana has considerable oil and gas resources, while the uplifted Rocky Mountains in the west, which resulted from the Laramide orogeny and other tectonic events have locations with metal ore.
The geology of Uzbekistan consists of two microcontinents and the remnants of oceanic crust, which fused together into a tectonically complex but resource rich land mass during the Paleozoic, before becoming draped in thick, primarily marine sedimentary units.
The geology of Israel includes igneous and metamorphic crystalline basement rocks from the Precambrian overlain by a lengthy sequence of sedimentary rocks extending up to the Pleistocene and overlain with alluvium, sand dunes and playa deposits.
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.
The Wester Ross Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks in the Scottish Highlands. It lies unconformably on medium to high-grade metamorphic rocks and associated igneous rocks of the Archaean and Paleoproterozoic age Lewisian complex or locally over the Mesoproterozoic sedimentary rocks of the Stoer Group. The contact between the Wester Ross Supergroup and the next youngest of the Neoproterozoic sequences in the Scottish Highlands, the Loch Ness Supergroup, is everywhere a tectonic one.
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