contains some of the oldest rocks known to science, c. 3,000 million years old, from which younger rocks such as the Torridonian sandstone have been derived, and upon which the sandstones and other younger rocks are placed. In the Outer Hebrides, Tiree, Coll, Iona and Sleat in Skye the gneiss forms the present-day land surface—all the younger rocks have been removed by epochs of erosion. Elsewhere, the basement is covered by an array of younger rocks, or has been penetrated or pushed aside by great intrusions of magma and covered by extrusions of lava.
| Era | Period | Age (m.y.) | Rocks | Islands |
| Pre-Cambian | +3000–600 | |||
| Lewisian | +2800–1200 | acid & basic gneisses, granites, limestones | N. Rona, Lewis, Harris, Uists, Barra, Coli, Tiree, Skye, Raasay, S. Rona, lona, Islay | |
| Torridonian | 1000–800 | sandstones | Handa, Summer Isles, Raasay, Scalpay, Skye, Soay, Rum, lona, Colonsay, lslay | |
| Rocks east of the Moine Thrust affected by the Grenville Orogeny, c. 1000m.y. | ||||
| Moine | 1000–700 | schists, | Skye, Mull | |
| Supergroup | granulites | |||
| Palaeozoic | 600–230 | |||
| Cambro-Ordovician | 600–500 | piperock, serp. Skye grit, Durness limestone | ||
| Rocks east of the Moine Thrust affected by the Caledonian Orogeny, 500–400m.y. | ||||
| Dalradian Supergroup | +600–500 | quartzites schists | Lismore, Kerrera, Seil, Garvellachs | |
| limestones, slates | Luing Scarba, Jura, Islay, Gigha | |||
| Silurian | 440–400 | none | none | |
| Devonian | 400–350 | conglomerate | Kerrera, Seil | |
| Carboniferous | 350–270 | lava, sediments | Jura | |
| Permian | 270–225 | sandstones, conglomerate | Lewis, Raasay, Mull | |
| Mesozoic | 230–65 | |||
| Triassic | 225–180 | sandstones, conglomerate | Lewis, Raasay, Skye, Rum, Mull | |
| Jurassic | 180–135 | sandstones, limestones | Shiants, Skye, Raasay, Eigg | |
| Cretaceous | 135–70 | sandstone | Skye, Mull, Eigg, Raasay, Scalpay, Soay | |
| Cainozoic | 70–0 | |||
| Tertiary | 70–1 | |||
| Eocene | 70–40 | basalts, granites, syenites, gabbros, dolerites, rhyolites | Shiants, Skye, Raasay, Rum, Eigg, Canna, Muck, Mull, Treshnish Is., Staffa, St Kilda, Oighsgeir | |
| Oligocene | 40–45 | erosion pdts | widespread | |
| Miocene | 25–11 | erosion pdts | widespread | |
| Pliocene | 11–1 | erosion pdts | widespread | |
| Pliocene | 11–1 | erosion pdts | widespread | |
| Quaternary | 1–Present | |||
| Pliestocene | 0.6–0.013 | erosion pdts | widespread | |
| Holocene | 0.013–0 | erosion pdts | widespread | |
| shell sand | widespread | |||
Table 1.1 The distribution and age in millions of years (m.y.) of the rocks of the Hebrides.
The major faults in northern Britain run from south-west to north-east (Fig. 4). The Southern Uplands Fault and the Highland Boundary Fault do not affect the Hebridean shelf; the Great Glen Fault (GGF), the Moine Thrust (MT), the Camasunary–Skerryvore Fault (C–SF) and the Outer Hebrides Thrust (OHT) all have an important bearing on the Hebrides. The GGF runs from Shetland to north Ireland, passes between Lismore and Kingairloch, through south-east Mull and just to the north of Colonsay; to the east there are the Caledonian granites with the Dalradian schists, slates and quartzites; to the west there is the Moine Supergroup of schists, bounded in the west by the Moine Thrust and interrupted in the south by the Tertiary complexes of Mull and Ardnamurchan. The only terrestrial sections of the GGF in the Hebrides are from Duart Bay to Loch Buie in Mull, which is an area of great interest with faulted Liassic sediments folded in Tertiary times around the Mull volcanic centre.
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
