Geology of Skye
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The geology of the
Isle of Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated ...
in
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
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 Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...
, metamorphic and
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
, ranging in age from the Archaean through to the
Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
.


Precambrian

The oldest rocks found on Skye are
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
es of the
Lewisian complex 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 Paleoprote ...
that were formed about 2,800 million years ago during the Archaean. These gneisses outcrop on the southeastern coast of the
Sleat Sleat is a peninsula and civil parish on the island of Skye in the Highland council area of Scotland, known as "the garden of Skye". It is the home of the clan ''MacDonald of Sleat''. The name comes from the Scottish Gaelic , which in turn comes ...
peninsula and were originally granitic igneous rocks. Near Tarskavaig, Neoproterozoic metasediments of the Moine Supergroup are found above strongly deformed Lewisian rocks. The Lewisian and Moine sequences are
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
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 In geology, the term Torridonian is the informal name for the Torridonian Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. The s ...
and the younger
Torridon Group In geology, the term Torridonian is the informal name for the Torridonian Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. The s ...
. Both groups consist dominantly of sandstones and were deposited mainly by alluvial fans and rivers.


Lower Palaeozoic

A sequence of
Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
to Lower Ordovician sediments of the
Eriboll Eriboll (Scottish Gaelic: Earabol) is a village in Sutherland, Scotland. The village is situated on the south eastern shore of Loch Eriboll, in the northern part of the former county of Sutherland. The main A838 coast road connects the villages ...
and Durness Groups lie above the Torridonian with an angular
unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...
. The Lower Cambrian Eriboll Group comprises a basal
quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...
, locally with a basal
conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...
, followed by the distinctive ''Pipe Rock Member'', a
quartz arenite A quartz arenite or quartzarenite is a sandstone composed of greater than 90% detrital quartz. Quartz arenites are the most mature sedimentary rocks possible, and are often referred to as ultra- or super-mature, and are usually cemented by sili ...
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
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
s 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
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
s with
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
. The two main exposures of Cambro-Ordovician sediments are the "Ord
Window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent materia ...
" (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 The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s in the northern outcrop, locally forming
marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or Dolomite (mineral), dolomite. Marble is typically not Foliation (geology), foliated (layered), although there are exceptions. In geology, the ...
, such as at
Torrin Torrin ( gd, Na Torrain) is a settlement on the island of Skye in Scotland. Geography The crofting and fishing village of Torrin lies on the eastern shore of Loch Slapin, southwest of Broadford (''An t-Àth Leathann''), on the road to Elgol ...
.


Mesozoic

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 Trotternish or Tròndairnis (Scottish Gaelic) is the northernmost peninsula of the Isle of Skye, in Scotland. Its most northerly point, Rubha Hùinis, is the most northerly point of Skye. One of the peninsula's better-known features is the Trot ...
peninsula, on the Strathaird peninsula and between the Red Hills and Sleat. 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 Pabay is a islands of Scotland, Scottish island just off the coast of Skye. The name Pabay is derived from an old Norse word meaning "priest's isle" and there are the remains of a 13th-century chapel. Geography Pabay is an island in the Inner So ...
, the
Scalpay Sandstone Formation There is more than one island named Scalpay ( gd, Sgalpaigh): *Scalpay, Inner Hebrides (near Skye) *Scalpay, Outer Hebrides Scalpay (; gd, Sgalpaigh or ''Sgalpaigh na Hearadh''; i.e. "Scalpay of Harris" to distinguish it from Scalpay off Skye ...
, the Portree Shale Formation and the Raasay Ironstone Formation. The sequence continues with the Lower to Middle Jurassic
Bearreraig Sandstone Formation The Bearreraig Sandstone Formation is a geological formation in Scotland. It preserves fossils dating back to the lower to middle parts of the Jurassic period (Toarcian–Bajocian). The remains of the proximal portion of a right ulna and radius o ...
followed by Middle Jurassic
Great Estuarine Group The Great Estuarine Group is a sequence of rocks which outcrop around the coast of the West Highlands of Scotland. Laid down in the Hebrides Basin during the middle Jurassic, they are the rough time equivalent of the Inferior and Great Oolite G ...
, comprising the
Cullaidh Shale Formation The Cullaidh Shale Formation is a geological formation in Scotland, and the lowermost unit of the Great Estuarine Group. It is Bajocian in age. The lithology largely consists of organic rich mudstone and shales, including oil shale Oil shale ...
,
Elgol Sandstone Formation The Elgol Sandstone Formation is a geological formation in Scotland, part of the Great Estuarine Group. It spans the transition between the Bajocian and Bathonian stages of the Middle Jurassic The Middle Jurassic is the second epoch of the Juras ...
, Lealt Shale Formation,
Valtos Sandstone Formation The Valtos Sandstone Formation is a Middle Jurassic (Bathonian) formation found in the Inner Hebrides of Scotland. It is the thickest member of the Great Estuarine Group. The lithology consists of sets of approximately 6 metre thick cross bedde ...
,
Duntulm Formation The Duntulm Formation is a geologic formation in Scotland. It preserves fossils dating back to the Bathonian stage of the Middle Jurassic. It forms part of the Great Estuarine Group. The lithology consists of interbedded fissile mudstone and mon ...
,
Kilmaluag Formation The Kilmaluag Formation is a Middle Jurassic geologic formation in Scotland. It was formerly known as the Ostracod Limestone for the abundance of fossil freshwater ostracods within it. The Kilmaluag Formation is very fossiliferous, with ostracods ...
and the
Skudiburgh Formation The Skudiburgh Formation is a geological formation in Scotland and the uppermost unit of the Great Estuarine Group, it is late Bathonian In the geologic timescale the Bathonian is an age and stage of the Middle Jurassic. It lasted from approxi ...
. The Upper Jurassic is represented by the
Staffin Shale Formation Staffin ( gd, Stafain) is a district with the Gaelic name ''An Taobh Sear'', which translates as "the East Side", on the northeast coast of the Trotternish peninsula of the island of Skye. It is located on the A855 road about north of Portre ...
. The only Cretaceous unit exposed on Skye is the Strathaird Limestone Formation, thought to be either
Turonian The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stage in the Upper Cretaceous Series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded by t ...
or Campanian in age, which lies unconformably on the Jurassic and is overlain unconformably by Palaeocene lavas.


Paleogene

During the
Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
to Early
Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''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. The dominant lava type is
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
, with subsidiary hawaiite and
mugearite Mugearite () is a type of oligoclase-bearing basalt, comprising olivine, apatite, and opaque oxides. The main feldspar in mugearite is oligoclase. Mugearite is a sodium-rich member of the alkaline magma series. In the TAS classification of volc ...
derived from silica-poor magma and minor amounts of
trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...
from a silica-rich magma. Part of the magma chambers for the volcanoes are exposed at the surface as major intrusions of
gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is che ...
and granite. These coarse-grained igneous rocks are relatively resistant to erosion and now form the
Cuillin The Cuillin ( gd, An Cuiltheann) 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 Sliga ...
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. 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 Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and alk ...
. 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 Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as Joint (geology), joints, result in the formation of a regular array of polygonal Prism (geometry), prisms, or columns. Columnar joint ...
, such as in the upper part of the Kilt Rock at
Staffin Staffin ( gd, Stafain) is a district with the Gaelic name ''An Taobh Sear'', which translates as "the East Side", on the northeast coast of the Trotternish peninsula of the island of Skye. It is located on the A855 road about north of Portre ...
.


Quaternary

During this period the island was affected by the
Quaternary glaciation The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma (million years ago) and is ongoing. Although geologists describe ...
, with the development of an
ice cap In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description Ice caps are not constrained by topographical features ...
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.


Economic geology

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 Glen Brittle (''Gleann Breadail'' in Scottish Gaelic) is a large glen in the south of the Isle of Skye, in Scotland. It runs roughly south to north, along the River Brittle, which has its mouth at Loch Brittle (a sea loch). The glen is bounded on ...
. 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 Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
and
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...
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 The Great Estuarine Group is a sequence of rocks which outcrop around the coast of the West Highlands of Scotland. Laid down in the Hebrides Basin during the middle Jurassic, they are the rough time equivalent of the Inferior and Great Oolite G ...
in the north.
Peat Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficien ...
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.


References

{{Commons category, Geology of the Isle of Skye
Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated b ...
Isle of Skye Paleogene volcanism