Scotland. Peter Friend
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rel="nofollow" href="#ulink_14158c9a-c802-5caf-a8ea-fcd8d794c594">Fig. 19). For the generalising approach of this book much of this work has had to be simplified.
Scotland’s geological history is unusually long and varied for a country of its size. One reason for this is that present-day Scotland is the result of the convergence or movement together of at least five different areas of crust, often referred to as terranes (Fig. 20). These terranes are fragments of continental crust that have been carried together by plate tectonic movements that resulted eventually in the construction of crustal Scotland, as we find it now.
Although most of the surface modifications and internal movements have overlapped in time and space, it helps to pick out discrete episodes in summarising aspects of Scotland’s history. The first nine of these episodes are represented in the bedrock record and are outlined in the rest of this chapter. Episodes 10–12 are mainly represented in the record of recent surface modifications, and they are described in Chapter 5. All 12 episodes have been placed in chronological order in Figure 21 (where Episode 1 is the oldest and 12 the most recent) using the International Stratigraphy Chart 2009, which provides an accepted standard for the names used in dividing and describing geological time (see www.stratigraphy.org).
FIG 19. Simplified geological map of Scotland. (Redrawn from British Geological Survey 1 : 625,000 map)
We provide a ‘Timeline’ as part of the description of the geology of each of our Areas. These timelines are designed to summarise the time sequence of events that is represented in or near each Area, using the standard International Stratigraphic divisions. Standard colours are used for the divisions and ages. If part of the stratigraphic record is absent, the division is not coloured.
The bedrock episodes can be grouped as follows:
(1) Pre-Caledonian Greenland-margin episodes (Episodes 1–3)
(2) Caledonian mountain-building episodes (Episodes 4–6)
(3) Post-Caledonian episodes (Episodes 7–9)
The distribution of these groups of rocks is shown in Figure 22, and the episodes involved in their formation are described below.
PRE-CALEDONIAN GREENLAND-MARGIN EPISODES
Episode 1: formation of the Lewisian Complex
Rocks of the Lewisian Complex are very largely restricted to the Hebridean terrane, where they make up almost all of the bedrock of the Outer Hebrides, and much of the bedrock of the mainland. They also occur occasionally in the neighbouring part of the Northern Highland terrane, where they became involved in the much younger Caledonian movement history. The Lewisian Complex takes its name from the largest and northernmost of the islands of the Outer Hebrides.
The Lewisian Complex consists of metamorphic rocks (typically coarsely crystalline gneisses) that formed by alteration of earlier rocks when high temperatures and/or pressures peaked during movements at deep levels within the Earth’s crust. The great interest of these metamorphic rocks is that they can provide information about the conditions deep within the crust when these movements were taking place. Unlike igneous rocks that formed by crystallisation from completely melted rock material, metamorphic rocks have involved changes in rocks that were at least partly solid, so they preserve information about features present before, as well as conditions during, the metamorphism. In most cases, the minerals now present are stable at present-day surface temperatures and pressures, are large in crystal size, and interlock with neighbouring crystals, so the rocks are resistant to surface weathering compared with many other rock types.
FIG 20. The five terranes of Scotland. (After Trewin 2002)
FIG 21. Episodes in Scotland’s geological history. The age scale is not linear and has been deliberately chosen so that younger episodes are given greater space than older ones, because they are usually known in greater detail. The chart indicates the ages covered by the 12 episodes, and the dominant processes represented by them. (Redrawn from International Stratigraphy Chart 2009, www.stratigraphy.org)
One of the most important research tools applied to the Lewisian rocks has been the dating of the various mineral components, using the fact that some of them contain radioactive materials that have been steadily changing since they were first trapped when the minerals formed. The amount of change gives a measure of the time over which it has been taking place. New analytical methods have led to increasingly accurate and reliable figures. As this work has continued, it has become clear that the Lewisian is truly a ‘complex’, made up of many distinct volumes of crust, each preserving certain episodes of movement and rock alteration. Many of the folds or fractures mapped in the Lewisian have a northwest/southeast trend, almost at right angles to the Moine Thrust Zone and the associated folds and fractures that form the margin of the Hebridean terrane. However, mapping of these structures has shown that the movement and alteration of the Lewisian occurred in a number of phases with different compression and shearing directions. The evidence is too fragmentary to allow identification of the boundaries of tectonic plates similar to those that can be identified in younger bedrock areas. This is hardly surprising, because these are some of the earliest movement events recognised anywhere on the surface of the whole Earth, representing glimpses of early crustal activity that has escaped reworking or obliteration in more recent episodes.
Important phases of activity and mineral alteration have been recognised in the Lewisian Complex, some in the Archaean (3.2 and 2.8 billion years ago), generally named Scourian and Inverian. Other rocks were formed and/or altered in the Proterozoic (2.4, 1.7 and 1.1 billion years ago) and are named Laxfordian. The Archaean phases are older than any other for which there is evidence in Britain. Most of the rocks altered in these phases were originally igneous but some were sedimentary, and all had actually been formed as rocks even earlier. It is clear from the minerals present that some phases involved crust being moved downwards to considerable depths – several tens of kilometres below the surface – although before the next episode (described below) the rocks had been moved back upwards and were exposed at the surface.
Surface modification of the Lewisian during the Tertiary and the Ice Age has carved it into typical ‘knock-and-lochan’ topography, in which the land surface consists of hillocks of exposed rock tens to hundreds of metres across (called knocks, from the Gaelic cnoc, a small, rocky hill), separated by water and bog-filled hollows (lochans) which often pick out folds and linear fractures in the bedrock (Fig. 23). This wild knock-and-lochan landscape was once thought to represent the first formed surface of the Earth, but it is now realised that the surface shapes of the landscape are very much younger, and that the metamorphic alterations and movements, although very old, were preceded by even earlier episodes.
FIG 22. The distribution of the Pre-, Syn- and Post-Caledonian rocks in Scotland.
FIG 23. Aerial oblique view of Suilven (731 m), carved from Neoproterozoic Torridonian Sandstones resting unconformably on the knock-and-lochan topography of the Lewisian Complex. (© Adrian Warren/lastrefuge.co.uk)
Episode 2: formation of the Torridonian Sandstones
Mountains and slopes made of uniform but well-layered successions of Torridonian Sandstones, often tens to hundreds of metres thick, provide some highly characteristic