The Hebrides. J. M. Boyd

now occupying Scandinavia, Scotland and eastern USA. Moine rocks were heated and altered again, while Cambrian strata became the schists of the Dalradian Supergroup. Into the folded and refolded rocks, huge masses of molten crust were emplaced as granite, now widespread in the Highlands and represented in the Hebrides in the Ross of Mull. Along the western seaboard, however, rocks of the metamorphic mountains were thrust upwards and outwards in a dislocation of up to 80km. This is known as the Moine Thrust which runs on the land surface from Loch Eribol to the Point of Sleat in Skye. To the west of the Thrust, the Lewisian, Torridonian and Cambro-Ordovician rocks are in unmoved (and unaltered) sequence; to the east of the Thrust, within the Caledonian mountain belt, lie the Moines of Sleat and western Mull and the Dalradian of eastern Mull, Jura and Islay.

      Fig. 4 The main geological faults of the Hebrides and West Highlands (Craig (ed) 1983)

      The mountains formed from this orogeny were subsequently eroded to form the Old Red Sandstone (ORS) c. 350 million years ago, a vast continental fluviatile and lacustrine deposit. Orkney is composed almost entirely of ORS but only small outcrops occur in the Hebrides—sediments in Kerrera and Seil, and lavas at Loch Don in Mull.

      The third upheaval was the rift of the European and Greenland continental plates which created the British Isles, the continental shelf and the Hebrides, but not as we know them today. This rifting, which began 70 million years ago and still continues today, was accompanied by much volcanic activity, the thrusting up of masses of gabbro and granite, the outblasting of vast quantities of dust, ash and cinder and the outpouring of basalt lavas. These are the Tertiary volcanic complexes of Arran, Mull, Ardnamurchan, Rum Cuillin, Skye Cuillin and St Kilda, with associated plateau lavas in North Skye, Canna, Eigg, Muck, West Mull and Morven. They are related to other such centres in Ireland (Giant’s Causeway), Faeroe Islands and Iceland, where the volcanic activity still continues. The islands as we know them today have been evolved through a northward drift of the crustal plate(s) of the planet from which the British Isles were formed, from a latitude of 30°S to the present latitude of 55°N. Throughout the drift, the palaeogeography was also continuously transformed by mountain building of the type described above, erosion, sedimentation, and volcanic activity. The genesis of the British Isles throughout geological time has been described simply by J.P.B. Lovell (1977).

Geological Framework

      The solid geology is shown in Fig. 3 and Table 1.1. The Hebrides lie at the south-eastern margin of a crustal plate which included much of the material which forms Greenland and eastern Canada (Fig. 5). This plate broke and the parts drifted away from each other, ‘floating’ for tens of millions of years on the plastic sub-crust. The great trough between the parts now holds the Atlantic Ocean. This common basement between the Old and New Worlds 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	[phone]	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