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.
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
