of how such movements are measured.
For present-day applications, it seems natural to regard sea level as a datum against which vertical landscape movements can be measured, as long as we remember to allow for tidal and storm variations. However, much work has demonstrated that global sea level has changed rapidly and frequently through time, due to climate fluctuations affecting the size of the polar icecaps and changing the total amount of liquid water present in the oceans and seas (see Chapter 5). It has also been shown that plate tectonic movements can have an important effect on global sea level by changing the size and shape of ocean basins.
Attempts have been made to develop charts showing how sea level, generalised for the whole world, has varied through time. However, it has proved very difficult to distinguish a worldwide signal from local variations, and the dating of the changes is often too uncertain to allow confident correlation between areas.
In sedimentary basins, estimates of vertical movements have been made using the thicknesses of sediment layers accumulating over different time intervals in different depths of water. In areas of mountain building, amounts of vertical uplift have been estimated using certain indicator minerals that show the rates of cooling that rocks have experienced as they were brought up to the surface. However, both these approaches are only really possible in areas that have been subjected to movements of the Earth’s crust that are large and continuous enough to dominate completely other possible sources of error.
Local movements are also difficult to estimate, although fold and/or fault patterns may allow a simple measure in some cases. Over short present-day periods of time it has proved to be possible to detect vertical movement patterns using satellite imagery. Movement of sediment across the Earth’s surface by rivers or sea currents can be estimated if mineral grains in the sediment can be tracked back to the areas from which they have come. In the detailed consideration of landscapes in this book, we have to rely on using the widest possible range of types of evidence, carefully distinguishing the times and scales involved. Even then, we are often left with probable movement suggestions rather than certainties.
CHAPTER 4
Episodes in the Bedrock History of Scotland
CHAPTERS 2 AND 3 HAVE INTRODUCED the idea that natural landscapes are the results of combinations of surface modification (Chapter 2) and internal movements (Chapter 3). The responses of the Earth’s surface to these combinations have depended on the bedrock present locally on the surface at each stage. It is now time, therefore, to turn specifically to Scotland, to summarise the distribution and history of its bedrock.
The mapping of the bedrock of Scotland has been a heroic task that started over 200 years ago. Most of the systematic work has been carried out by the British Geological Survey, and is now available on different scales, forming a monument to the efforts of many remarkable people and the Survey itself (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
