Geological map of SkyeBasalquartzite of the Eriboll Group from the Ord WindowDurness Groupdolomites forming alimestone pavement on Bheinn ShuardailCross-bedded Jurassic sandstones of the Bearreraig Sandstone Formation near Glasnakille,StrathairdMacLeod's Tables, flat-topped hills and stepped topography from erosion of Paleocene lavas,Duirinish peninsulaMaficdyke near Broadford cutting Durness GroupdolomiteThe Kilt Rock with a columnar jointeddoleritesill above Jurassic sandstones of theValtos Sandstone Formation
Thegeology of theIsle of Skye inScotland 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 andigneous, ranging in age from theArchaean through to theQuaternary.
The oldest rocks found on Skye aregneisses of theLewisian complex that were formed about 2,800 million years ago during the Archaean.[1] These gneisses outcrop on the southeastern coast of theSleat peninsula and were originallygranitic igneous rocks. NearTarskavaig,Neoproterozoic metasediments of theTarskavaig Group are found above strongly deformed Lewisian rocks. The Lewisian and Tarskavaig sequences arethrust over another sequence of unmetamorphosed Neoproterozoic sediments, theTorridonian, along theMoine Thrust Zone. The Torridonian on Skye comprises two conformable sequences, the olderSleat Group and the youngerTorridon Group. Both groups consist dominantly of sandstones and were deposited mainly byalluvial fans and rivers.
A sequence ofCambrian to LowerOrdovician sediments of theEriboll andDurness Groups lie above the Torridonian with an angularunconformity. The Lower Cambrian Eriboll Group comprises a basalquartzite, locally with a basalconglomerate, followed by the distinctivePipe Rock Member, aquartz arenite with white weatheringskolithostrace fossils. The Pipe Rock is overlain by theSalterella Grit, a coarse sandstone, and theFucoid Beds, a sequence of calcareous sandstone and siltstone. The overlying Durness Group comprises a series ofdolomites of Upper Cambrian to Middle Ordovician in age. The lowest unit is theGhrudaidh Formation, followed by theEliean Dubh Formation, theSailmhor Formation and theSangomore Formation, all consisting ofdolomites withchert. The two main exposures of Cambro-Ordovician sediments are the "OrdWindow" (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 betweenBroadford and Loch Slapin. These beds are affected by thrusting in both areas and bycontact metamorphism fromPalaeogenegraniteintrusions in the northern outcrop, locally formingmarble, such as atTorrin.[1]
During thePaleocene to EarlyEocene Skye formed one of the main volcanic centres of theNorth Atlantic Igneous Province. Gently dipping lavas from the volcanoes cover most of northern Skye, giving a steppedtrap type landscape.[1] The dominant lava type isbasalt, with subsidiaryhawaiite andmugearite derived from silica-poor magma and minor amounts oftrachyte from a silica-rich magma. Part of themagma chambers for the volcanoes are exposed at the surface as major intrusions ofgabbro and granite. These coarse-grained igneous rocks are relatively resistant to erosion and now form theCuillin 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 andcone 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 majorswarm of dykes, which forms part of the North Britain Palaeogene Dyke Suite. Most of the dykes are basaltic in composition but a minority aretrachytic. 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 formsills that are up to 90m thick. They commonly displaycolumnar jointing, such as in the upper part of the Kilt Rock atStaffin.[1]
During this period the island was affected by theQuaternary glaciation, with the development of anice cap centred on the Cuillin and Red Hills. The mainice sheet that flowed westwards from the Scottish mainland was diverted around this upland area. The island is covered by large areas of glacialtill, 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 inGlen 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 andcoal 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 theGreat 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]
