The sizes and shapes of the Areas have been adjusted to fit the shape of the land areas concerned: these Areas range from 50 to 100 km wide (from west to east) and 70 to 130 km high (from south to north), covering the shape and form of the mainland and islands of Scotland. On average each Area is about 100 × 100 km. All Areas are defined by National Grid south to north and west to east lines, and except for a few oblique view maps, all our maps use the same boundary orientation so that Grid North is parallel to the up-and-down margins.
Shaded, colour-coded maps are used to convey the height and approximate shape of the land surface in each Area. These maps have been produced from data collected by the space shuttle Endeavour in February 2000 as part of NASA’s Shuttle Radar Topographic Mission (SRTM). This 11-day mission used stereo pairs of radar images to build up a Digital Elevation Model (DEM) covering nearly 80 per cent of the Earth’s surface. The original dataset used in this book is publicly available via the SRTM website (www2.jpl.nasa.gov/srtm) and consists of pixels, approximately 90 × 90 m, each of which has an associated elevation value. The absolute vertical accuracy of these data is estimated to be ± 16 m, whilst the absolute horizontal accuracy is ± 20 m.
Data on roads, railways, coastlines, town boundaries, rivers etc., suitable for reproduction at a scale of 1 : 200,000, have been made available by the Collins Bartholomew mapping agency. For further detail it is recommended that the Ordnance Survey Landranger (1 : 50,000) maps are used.
We have used ESRI ARC Geographic Information System (GIS) software in the processing and manipulating of the map data. This software makes it possible to present maps with artificial hill-shading, so that topography becomes easier to visualise. Maps presenting the directions and slope angles of sloping features are also very useful in some situations.
Many of the maps make use of a standard colour scheme, ranging from greens for the lowest ground through yellows to browns and greys for the highest ground. In general, the full range of colours has been used for each map, no matter what numerical range of heights is involved. This makes it possible to convey the fine detail of slopes and other features, whether the map covers flat ground or valleys and high peaks. To make it possible to compare between maps using this colour sequence, we have quoted the maximum elevation reached in each Area.
CHAPTER 2
Surface Modifications
THE LANDSCAPE CYCLE
IN CHAPTER 1, WE ILLUSTRATED our use of the word landscape to indicate an area a few kilometres to many kilometres across that is distinctive in appearance and origin. We have also found the word landform useful for smaller features of landscapes formed by distinctive surface processes during the modification of the landscape surface. In this chapter, we shall be examining further some aspects of certain of these landforms.
Our developing understanding of the larger workings of the Earth has shown us that although surface modifications are almost always apparent, the Earth’s crust and its surface have been subject to continual movements generated within the Earth. Earthquakes and volcanoes are obvious signs of these internal movements. Any landscape is the result of the interplay between these contrasting internal and external systems, as illustrated using the cycle diagrams (Figs 4, 5). These illustrate the two systems in usefully different ways.
DIGITAL MAPS, SLOPES AND DOWNSLOPE MOVEMENT
We explained in Chapter 1 that our primary information about the shapes and patterns of Scottish landscapes comes from the use of the digital elevation datasets that are now available. Most people are familiar with the representation of elevation information on maps, using colour shading, or contours representing lines of specified elevation on the surfaces. The GIS software that we have used is a powerful tool for presenting topography in these ways. The same software makes it possible to represent topography using a hill-shade approach, which portrays topography using a shadowing effect, as estimated by an artificial light source with a specific orientation and elevation angle. The effects can appear similar to those produced by hachuring, as used in early Ordnance Survey maps, although hachure shading owed much to the eye of the individual draughtsman.
FIG 4. Diagram illustrating the processes of movement occurring within the outer layers of the Earth’s crust, and how these relate to the processes and features of the Earth’s surface and atmosphere.
As outlined in Chapter 1, our maps of Scotland are based on digital elevation data where areas are divided into large numbers of small square unit areas (pixels), arranged in a rectangular grid. The elevation above sea level of each of the pixels is recorded in the database, and much of our data are based on a pixel size of 90 × 90 m. Although this resolution is adequate to provide information on larger landforms, we have to accept that many smaller landforms will be invisible if the pixel size is similar in area to, or larger than, the landform.
Digital elevation data can be directly represented on a map using colour shading or contours. It is also possible to define slopes by measuring changes of elevation within clusters of neighbouring pixels, allowing each pixel to be assigned a local slope value and converting the simple grid of elevation measurements into a grid of differences in elevation, or slopes. These maps are sometimes referred to as ‘first derivative’ maps of the topography, because they represent changes of topography (local slopes) rather than the elevations themselves. Whatever the limitations of scale, there is no question that examining patterns of slope variation is a powerful way of studying the shapes of landscapes, and the Area chapters that follow make frequent use of maps of this sort.
FIG 5. Landscapes are changed by surface modifications (Chapter 2) and solid Earth movements (Chapter 3).
We now consider the sorts of processes that are likely to give rise to various different features and patterns of slopes through time.
Slopes are likely to have a direct and profound influence on the way topography evolves through time, because any slope surface has the potential for downslope movement under gravity (Fig. 6). Movement will often require triggering, for example by earthquakes, freeze–thaw ice changes or even heavy rainfall.
Even more important in determining the amount of movement and the angle of slope that can occur is the nature of the materials making the slope. Bedrock of igneous or metamorphic origin, consisting mainly of coarse crystals of interlocking minerals, formed at high temperatures in the Earth, such as quartz, feldspar and other silicate minerals, is likely to produce a strong material in terms of its surface weathering behaviour. At the other extreme, certain sedimentary rocks, consisting of small particles of clay minerals separated from their neighbours by films of water, will be weak and strongly liable to downslope mass flow, tending to produce a distinctly lower slope angle.
RIVER CATCHMENTS AND VALLEY PROCESSES
Looking back at the landscapes from Skye (Fig. 1), and excluding coastline considerations for the moment, the first step in analysing landscape shape or morphology, as just discussed, is to realise that most of the detailed features visible can be considered as combinations of different scales and combinations of slopes. Our survey of Scotland has confirmed for us that, under present-day climate conditions, rivers and streams are the fundamental agents forming and changing valleys and slopes. This is why we have designed our computer-based maps to display clearly the locations and