The Hebrides

Tertiary basalt pavement showing hexagonal jointing on Heisgeir (Oigh-sgeir) off Canna (Photo J. M. Boyd)

Fig. 5 The tectonic provinces of the North Atlantic prior to continental drift (Smith & Fettes, 1979)

The MT runs from the west of Shetland, entering the Scottish mainland at Loch Eribol and traversing the north-west Highlands roughly parallel to the coast, through Kylerhea and the Sleat peninsula of Skye and possibly through the Sound of Iona. To the west are the northern Inner Hebrides where the gneiss basement is evident in Tiree, Coll and Iona and is interrupted in Skye, Small Isles and St Kilda by massive emplacements of Tertiary lava, granite and gabbro. The Moine and associated thrusts occur from Loch na Dal to the Point of Sleat, and as far west as Broadford and Beinn an Dubhaich. The MT may just clip Rum at Welshman’s rock. To the east there is gneiss; to the west there is Torridonian sandstone and Durness limestone. Under the Sea of the Hebrides and the Minches, there are trenches in the gneiss basement filled with much younger sedimentary rocks. These have been derived from bygone mountains and are akin to the New Red Sandstones around Broad Bay in Lewis and to sedimentary strata of the wider shelf to the west of the Hebrides and around Orkney and Shetland, which may hold oil and gas. The C-SF, running from the Loch Scavaig in Skye through the Rum and Tiree Passages to the Skerryvore, is the western limit of a Mesozoic basin extending southward from Strathaird under Eigg and Muck to Mull.

Lastly, the OHT runs from the North Minch to beyond Barra Head along the east coast of the Outer Hebrides. It defines the main mountain chain of Barra and the Uists but northwards, in Lewis, it splits into a number of discontinuous planes before finally reaching the sea just north of Tolsta Head. To the east there are the sedimentary rocks in the submarine trench, while to the west is the Lewisian platform, interrupted in Harris and West Lewis by massive blocks of granite of Lewisian age.

Pre-Cambrian and Palaeozoic Rocks

These are mainly the Lewisian gneisses and granites, most of which were in existence 3,000 million years ago. In this vast span of time they have been changed. The granites, found mainly in Harris and west Lewis, are locally sheared and reduced to mylonite. South Harris is banded south-east to north-west with all the major rocks of the Lewisian series: gneiss, granite, gneiss veined with granite (all of acid character), metamorphic intermediate and basic igneous rocks, metasediments and anorthosite at Rodel. Metasediments are formed by the recrystalisation of sedimentary rocks, and occur at the north tip of Lewis, the south tip of Harris, and in the Uists and Benbecula. Substantial bands of mylonite (a slaty rock formed from crushed material along the OHT) occur in south-west Lewis and on the east coast of South Uist.

There were two distinct periods of metamorphic change, named after the districts of Sutherland where the original studies were done. The Scourian, 3,000 to 2,500 million years old, was followed by the Laxfordian, 2,500 to 1,400 million years old, and were separated by a period of crustal tension forming fissures into which a swarm of dykes were intruded. These are the Scourie Dykes which serve as distinct time-markers, separating Scourian from Laxfordian events. The Laxfordian period is marked by large-scale folding of the rocks. It concluded with the injection of the granites and pegmatites, 1,750 million years old, in Harris and Lewis, and the OHT, which was reactivated at the time of the Caledonian orogeny about 1,200 million years later, i.e. 400 million years ago (Smith and Fettes, 1979). None of the Torridonian, Moine, or Cambro-Ordovician rocks, which are well represented in the Inner Hebrides and the West Highland mainland, are present in the Outer Hebrides. The only sedimentary rocks are sandstones and conglomerates of Permian or Triassic age, 225 million years old, around Broad Bay in Lewis.

A quarry face in South Uist showing a section of Lewisian gneiss with characteristic banding of the minerals (Photo British Geological Surrey)

Among the Inner Hebrides the Pre-Cambrian rocks are widespread. The Lewisian complex occurs in Islay, Coll, Tiree, Skye, Raasay, and South Rona. Research on Rona has played a part in the elucidation of the Lewisian complex, and has revealed the oldest rocks in the British Isles—gneiss containing zircons older than 3,200 million years (Bowes et al., 1976). As in the Outer Hebrides, there are metasediments among the predominant gneisses in Coll, Tiree, and Iona. These include garnet and graphite gneisses and marbles. Torridonian sandstones occur in Handa, Summer Isles, Raasay, Scalpay, Skye, Soay, Rum, Iona, Colonsay, and Islay. These sandstones and shales lie unconformably upon the Lewisian gneiss to the west of the Moine Thrust, have their greatest development in Wester Ross, and continue in a band some 150km long and 15km broad south-east from Skye, under the sea, to the west of Coll and Tiree. There are several different groups of Torridonian characterised by their colour, grain-size and degree of deformation—‘Sleat’, Skye (3.5km thick), ‘Torridon’, Raasay (7km), ‘Colonsay’ (4km), ‘Bowmore’ Islay (4km), and ‘Iona’ (500m). Moine schists of similar age occur in Skye and Mull. Late Cambrian rocks, c. 550 million years old, are restricted to outcrops of pipe rock (quartzite with ‘pipes’ of worm burrows), serpulite grit and limestone in south-east Skye. Dalradian schists, which dominate the West Highland mainland south of the GGF, appear in the southern Hebrides. Quartzites, schists, limestones and slates occur in Islay, Gigha, Jura, Scarba, Garvellachs, Lismore, Luing and Seil. So far, no rocks of Silurian age have been found. Only small outcrops of Devonian (ORS) occur in Kerrera (130m thick) and Seil (5m) with contemporaneous lavas at Loch Don, Mull. The sole possible representatives of the Carboniferous period are lavas and sediments on Glas Eilein, Jura. Similarly, the only possible representative of the Permian period in the Inner Hebrides is a small pocket of boulder sediment on the Oa, Islay. New Red Sandstones in Skye, Raasay, Rum, and Mull are thought to be Triassic, but these continue under the sea and may include rocks of Permian age. Summary accounts of both the Pre-Cambrian and Palaeozoic rocks of the Hebrides are given in studies by Smith and Fettes (1979) and Anderton and Bowes (1983).

Torridonian sandstone eroded by the sea into freakish, dinosaur-like shapes on the north-west coast of Rum (Photo J. M. Boyd)

Mesozoic Rocks

The Triassic, Jurassic and Cretaceous periods are well represented in Raasay, Skye, Eigg and Mull with lesser outcrops in Rum, Scalpay, Pabay, Soay and the Shiants, which, though geographically part of the Outer Hebrides, are geologically part of the Inner Hebrides (See here). These Mesozoic rocks are, however, the exception rather than the rule in the Hebrides, which, paradoxically, adds to their importance for two reasons: firstly, they are a vital link in the geological history of the islands, joining the distant Palaeozoic period and the much more recent Tertiary era; secondly, they are predominantly lime-rich rocks which have a marked effect on the ecology of the islands in which they occur. The outcrops above sea-level generally are set unconformably upon the Pre-Cambrian-Paleozoic basement, and are overlain by the Tertiary volcanic rocks. Under the North Minch and the Sea of the Hebrides, these mesozoic rocks now fill deep basins, and are much more extensive than on the islands, which may hold only thin fringe outcrops on the margins of the submarine basins. Much of the research data from these potential oil-bearing basins is unpublished. However, the labelling of the Stornoway Beds as Permo-Triassic leads to the conclusion that these basins probably hold a New Red Sandstone series.

It is impossible to say whether or not the well-separated Mesozoic rocks on the various islands are the surviving parts of one continuous basin, or of separate basins, though the lateral continuity of the Great Estuarine rocks from Muck in the south through Eigg, Strathaird, and Raasay to Duntulm on the north point of Skye, is highly sustained. The most complete succession from the Triassic up through the Lower, Middle and Upper Jurassic to the Cretaceous occurs in Raasay and Trotternish (Fig. 6).

Calcareous concretions like cannon balls exposed by marine erosion of the Jurassic sandstones at Bay of Laig, Eigg (Photo British Geological Survey)

Fig. 6 Map showing the distribution of the Mesozoic rocks in the Hebrides (Hudson 1983)

The Triassic rocks are mainly sandstone and conglomerates derived from riverine and lake-bed deposits—fossil soils indicate periodic inundations in a semi-arid climate. The Jurassic rocks are by far the most extensive of the Mesozoic series above sea level. They are mostly sandstones, shales and limestones in that order of importance. However, the sandstones and shales often contain carbonate concretions which are eroded out like great cannonballs from the softer sandstone. The beds are generally fossil-bearing allowing precise correlation between strata at different sites using ammonites. The lower levels are marine and fine-grained, the middle is riverine, estuarine and deltaic in character, and the upper is again marine and fine-grained. These are the finest fossil beds in the Hebrides, and have attracted geologists since Hugh Miller so graphically described his collecting foray in Eigg and Skye in The Cruise of the Betsey (1858) (See here). Fossil sharks’ teeth and the bones of Pleisosauri and crocodiles have been found on Eigg; there are specimens of these and other Hebridean fossils in the Royal Museum of Scotland in Edinburgh. Lagoon beds were covered with mussels (Praemytilus) and other indicator species include brachiopods, oysters, and ostracods. Ammonites and belemnites abound in the shales and sandstones of Skye and Raasay. The Mesozoic series is capped locally with thin beds of upper Cretaceous sandstone in Skye, Raasay, Scaplay, Soay and Eigg. These have a maximum thickness of 25m but are usually less than 10m. They represent deposits in a shallow sea during a short interlude of erosion between the end of the Jurassic and the onset of the Tertiary volcanic epoch, when the Mesozoic rocks were buried under several hundred metres of lava and ash beds and cut by sills and dykes. A summary account of the Mesozoic in the Hebrides has been given by Hudson (1983), and the Jurassic and Cretaceous sediments in Scotland is reviewed by Hallam (1983).

Tertiary Rocks

During the Eocene commencing 60–65 million years ago, Europe and Greenland began moving apart. This crustal movement created the wide rift now filled by the North Atlantic, and was accompanied by much volcanic activity along the line of parting, from the south of England through Wales, Ireland, western Scotland, the Faeroes, and Iceland to Greenland. The whole segment of the earth’s crust is known as the North Atlantic or Thulean Igneous Province, of which the Hebridean Province, stretching from Ailsa Craig to St Kilda, is a part. The main centres of volcanic activity were in Arran, Mull, Ardnamurchan, Rum, Skye, St Kilda, and the submarine Blackstones Bank, all of which hold the magma chambers of large volcanoes (See here). Most of these and other islands—Treshnish, Staffa, Muck, Eigg, Canna and the Shiants—contain fragments of the laval flows and ash falls from these Eocene volcanoes. The sources of these outpourings are uncertain. One view is that they emanated from the above centres; another is that they issued from long fissures in the earth’s crust above, and stretching away from, the centres, much as occurs in the ‘shield’ volcanoes of Iceland today. The country rocks of many islands are also intruded by dykes and sills. These trend south-east to north-west and consist of both basalt and its coarser-grained relative, dolerite (See here). C.H. Emeleus (1983) has reviewed the geology of the Tertiary in Scotland and C.H. Donaldson (1983) has provided a useful summary account of the Tertiary in the Hebrides.

The Sgurr of Eigg (387m) is a pile of Tertiary rocks. A layer of pitchstone on the summit ridge lies unconformably on basalt lavas (Photo British Geological Survey)

There are two distinct types of igneous rocks—the fine-grained which have cooled quickly on or close to the earth’s surface, and the coarse-grained which have cooled slowly at depth in the crust. Basalts, rhyolites and pitchstones are of the former; gabbros, dolerites and granites are of the latter. In the main centres, i.e. the magma chambers of the great volcanoes, granites and gabbros predominate. The granites are rich in silica and feldspar and are grey or pink; the gabbros are dark, rich in iron, magnesium and calcium and poor in silica. Of the fine-grained volcanic rocks, basalt is by far the most important. It is a dark, fine-to-medium-grained analogue of the gabbro. Basalt is the stuff of the plateau lavas of north Skye and west Mull, and the terraced tablelands and columnar scarps of Durinish (Macleod’s Tables), Canna, Rum (Bloodstone), Eigg, Muck, Treshnish Isles, Staffa (Fingal’s Cave), and The Burg (MacCulloch’s Tree) in Mull. The Sgurr of Eigg is a layer of pitchstone overlying a conglomerate-filled valley in the basalt flows. The rock called Heisgeir off Canna has a hexagonal pitchstone pavement—a miniature Giant’s Causeway (See here). Probably basalt of over a kilometre thick was extruded over the crustal area now occupied by the Inner Hebrides. Much of this has since been removed by erosion, but the thickness of the existing basalt plateau reaches 1800m in central Mull and 600m in Skye and the Small Isles. The maximum thickness of individual flows is about 30m with the average about 8m, and the most extensive known flow stretches over 22km. An impression of the depth and extent of the Hebridean lava plateau in the late Tertiary (15 million years ago) can be obtained today in the Faeroe Islands, where a magnificent basalt plateau persists throughout most of the archipelago.

Fig. 7 Map showing the Tertiary volcanic centres and swarms of dykes in the Hebrides (Donaldson 1983)

One of the salient features of the main Tertiary centres, particularly well-displayed in central Mull, is the formation of calderas caused by crustal ring fractures. Within these fractures—which were superimposed upon each other over millions of years—the solid rock sank, and the space was filled by upwelling magma. This has caused the fairly sharp juxtaposition of great masses of granite many cubic kilometres in extent with even greater masses of gabbro. The plane of fracture is a shatter zone in which fragments of the dark gabbro became embedded in the upwelling granite, or vice-versa, to create a breccia. This is well seen in the sheer cliff of Mullach Mor at St Kilda, where the eucrite (gabbro) to the west marches with the granophyre to the east, and is deeply veined by it. (A more detailed description of the magma-chambers of Rum and St Kilda is given in Chapters 15 and 16, respectively.)

The enormous explosive forces of these volcanoes resulted in the cracking and fissuring of the crust. The geological map shows Skye, Mull and Arran like the points of strike of bullets on a pane of glass, with the systems of cracks running roughly north-west to south-east. The cracks range from a hair’s breath to over 30m thick and they have been filled with dark dolerite and basalt dykes (vertical) and sills (inclined). The swarms of Tertiary dykes from Skye, Mull and Arran traverse the crust from Lewis to Loch Linnhe, from Coll to Yorkshire and from Colonsay to Ayrshire respectively. The Inner Hebrides are riven with basalt and dolerite dykes which are usually 1–5m thick. These often become hard ridges among softer rocks like those in the Jurassic limestones on the shores and cliffs of Eigg, and trenches in harder rocks such as the gneiss on the shores of Tiree and Coll. The Camus Mor dyke on Muck is a striking example, which slices through the limestones and lavas and has a broad vertical exposure on the sea-cliff. Sills often occur in the bedding planes between lava-flows and sedimentary rocks and, like the lava, have columnar jointing. There are good examples of sill complexes to be seen in northern Skye and the Shiant Islands and, being composed of a hard coarse-grained dolerite, they often form a break of slope or escarpment edge in the basalt country. They play an important part in the composition of the landscapes of Raasay, Skye, and Mull.

The earthquakes which accompanied these episodes of cracking and fissuring must have been enormous, certainly greater than any that happen in the world today. Another type of circular Assuring occurs as a result of great crustal explosions within the roots of the volcanoes, which split the existing igneous complex in a nest of conical cracks several kilometres in diameter at the present land surface. Sometimes they are several metres wide and are filled with basalts and dolerites—these are called cone-sheets. All the Tertiary centres possess them, and they are exceptionally well displayed on the Oiseval and Conachair cliffs at St Kilda and at Gribum in Mull.

Quaternary Features

In the last 18,000 years Scotland has endured a glaciation, and a period of emergence from the ice and the recovery of life. In the Devensian period between 18,000 and 11,000 years BP, the ice-age gradually declined leaving an arctic habitat with receding valley glaciers and seasonally exposed land and sea surfaces. The effect of the ice on the land was enormous, gouging and planing the uplands and depositing the detritus on an array of downstream surfaces ranging from large boulders to fine muds. About 13,000 BP, the summer temperatures must have been about 15°C, judging from the insect remains recovered from contemporary sediments, which, in Skye, also contain the pollen of birch, hazel, grasses, sedges, clubmosses, sorrel and others (Birks and Williams, 1983). By 12,000 BP, however, the insect evidence suggests a drop in summer temperature to 3°C, and there was a re-establishment of glaciation between 11,000 and 10,000 years BP. This is thought to have been caused by a sweep of polar water southward along the west coast, based on evidence of arctic Foraminifera and dinoflagellate remains in contemporary marine sediments off Colonsay. The main ice accumulation was in the West Highlands from Wester Ross to Loch Lomond and the episode is known as the Loch Lomond Readvance (LLR). It had a limited effect in the Hebrides, creating scree slopes on the mountains of Mull, Rum and Skye.

The disintegration of the ice sheets and the disappearance of the valley glaciers in the islands brought to light a great number of glacial and fluvio-glacial features; landslips, raised beaches, and accumulations of shell sand, dolomite, and peat. The end of the permafrost brought with it the collapse of many escarpments and cliffs and the shattering of rock-faces, resulting in a range of postglacial sheets of scree and stoneshoots, in which the islands abound and which are particularly well developed in the Cuillins of Skye. Landslips on a vast scale occurred at the Storr and Quirang in Skye, and below the northern ridge of Eigg. Fields of giant boulders were created—some as large as a house, with a cap of soil and vegetation, as in upper Guirdil in Rum. Solifluction terracettes and stone polygons related to the LLR are present on the summits of Mull and Rum.

The relationship of the wave-cut benches along the coasts and raised beaches to the glacial structures is not fully understood. The changes in sea level which accompanied the disappearance of the ice during the Quaternary period, resulted from two related factors—the melting of a great part of the polar ice caps which served to raise the sea level, and the isostatic raising of the land released from the superincumbent load of the ice sheet. The interplay of the two factors and the reworking of coastal and marine deposits by the sea at different levels is highly complex. The heights above present sea level of the raised beaches in the Hebrides are grouped around 8m and 30m, and on the west coast of Jura these two levels are well developed in the same system. Enormous drifts of even-sized quartzite pebbles are placed in steps above the waters of Loch Tarbert. J.B. Sissons (1983) has reviewed the Quaternary in Scotland and J.D. Peacock (1983) has given a useful summary account of it in the Inner Hebrides. Recent (Holocene) sediments and sedimentation, which include the dominant formations of shell sand and peat mentioned in later chapters of this book, are summarised by G.E. Farrow (1983).

The rocks of the Hebrides have therefore a dramatic story to tell. Those who have an eye for country can read the geology of the islands from their architecture, often at a great distance. The shape of the granite and quartzite hills is distinct from the gabbro, and both are distinct from the basalt. In Skye the granites of the Red Cuillin are cheek-by-jowl with the gabbros of the Black Cuillin; the former are smooth paps and the latter are a jumble of serrated peaks and ridges. The same is seen at St Kilda; viewed from North Uist on a clear day, the smooth granite cones of Conachair and Oiseval are flanked by the peaked gabbro of Dun, Mullach Bi and Boreray. The basalt islands, of which Canna, western Mull and northern Skye are typical, have stepped landscapes with beetling, horizontal scarps, terraces, tablelands, and galleried sea-cliffs several hundred metres high—the eroded basalt gives the Treshnish Isles the look of a fleet of dreadnoughts. The ecological effects of the country rocks is usually masked by wind-blown sand, a blanket of peat, or by agricultural improvement. However, the greenery of hill and wood in Raasay, Strath and Ord in Skye, Gribun and Loch Don in Mull, Lismore and around Ballygrant in Islay strongly suggests the presence of limestone.

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