DEVELOPMENT BY PEOPLE
My concern in this book is primarily with natural landscapes, and I will tend to comment on the development by people since the Bronze Age only where this relates to the natural features in an interesting way. However, in reviewing the appearance of the whole of Southern England, I have been struck by an intriguing distinction made by some landscape historians: the distinction between ancient and planned countryside (Figs 26–28). I have based my approach on the discussions offered by Oliver Rackham, ecologist and landscape historian, and these are summarised below.
Ancient countryside (Fig. 26) consists of many hamlets, small towns, ancient farms and hedges (of mixed varieties of shrubs and trees), along with roads that are not straight, numerous footpaths and many antiquities.
Planned countryside (Fig. 27) has distinct villages, much larger than the hamlets, along with larger eighteenth- and nineteenth-century farms, hedges of hawthorn and straight roads. Footpaths are less common and the few antiquities that are present are generally prehistoric.
I have re-examined the same areas used by Oliver Rackham as examples of these two countryside types, and compared the early Ordnance Survey maps with maps of the same area generated by me using the data and methods used in this book (see Chapter 1). The shading and ‘hachured’ patterning used in the earlier maps represents the hills and slopes rather clearly – better than the contour representation used in the present-day Ordnance Survey Landranger maps, although these show man-made features much more clearly. My map representation is a compromise in that it represents elevations and slopes using colours and hill-shading, but also allows the patterns of roads and settlements to be seen.
Oliver Rackham’s conclusion is that many of the distinctive features of planned countryside were created by the general parliamentary enclosure of land during the eighteenth and nineteenth centuries. This involved the wholesale conversion of commonly held land with open fields into enclosed fields awarded to individuals and institutions. Many landscape historians have claimed earlier origins for the difference between ancient and planned countryside, believing that historical and cultural differences in the people who settled and developed the two areas played an important role. Variations in the bedrock geology also seem to be important here. For example, the ancient countryside shown in Figure 26 is underlain by strongly deformed Variscan bedrock that has been eroded into small hills and valleys (see Chapter 4).
FIG 26. Example of ancient countryside at the Devon-Somerset border, near Tiverton, with 1809 and recent mapping compared. (Upper part from Cassini Old Series map 181, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 27. Example of planned countryside at the Berkshire-Oxfordshire border, around Didcot, with 1830s and recent mapping compared. (Upper part taken from Cassini Old Series maps 164 and 174, copyright Cassini Publishing 2007/www.cassinimaps.co.uk)
FIG 28. Generalised map distinguishing ancient and planned countryside across Southern England.
In contrast, the planned countryside covered by Figure 27 consists of only gently tilted Mesozoic bedrock that has formed a much flatter and more open landscape.
CHAPTER 3 Movement of the Earth’s Surface from Within
WIDESPREAD MOVEMENTS OF THE EARTH’S SURFACE
TO UNDERSTAND THE CHANGES and movements affecting the appearance of the landscape on large scales we need to review some geological systems, especially plate tectonics. Many of the large changes that have created landscapes over long periods of time can now be understood using this discovery.
Knowledge of the processes causing the movement of large (10–1,000 km length-scale) areas of the Earth’s surface has been revolutionised by scientific advances made over the last 40 years. During this time, scientists have become convinced that the whole of the Earth’s surface consists of a pattern of interlocking tectonic plates (Fig. 29). The word ‘tectonic’ refers to processes that have built features of the Earth’s crust (Greek: tektōn, a builder). The worldwide plate pattern is confusing – particularly when seen on a flat map – and it is easier to visualise the plates in terms of an interlocking arrangement of panels on the Earth’s spherical surface, broadly like the panels forming the skin of a football.
Tectonic plates are features of the lithosphere, the name given to the ≈125 km thick outer shell of the Earth, distinguished from the material below by the strength of its materials (Greek: lithos, stone). The strength depends upon the composition of the material and also upon its temperature and pressure, both of which tend to increase with depth below the Earth’s surface. In contrast to the mechanically strong lithosphere, the underlying material is weaker and known as the asthenosphere (Greek: asthenos, no-strength). Note that on figure 30 the crustal and outer mantle layers are shown with exaggerated thickness, so that they are visible.
FIG 29. World map showing the present pattern of the largest lithosphere plates.
Most of the strength difference between the lithosphere and the asthenosphere depends on the temperature difference between them. The lithosphere plates are cooler than the underlying material, so they behave in a more rigid way when subjected to the forces generated within the Earth. The asthenosphere is hotter and behaves in a more plastic way, capable of deforming without fracturing and, to some extent, of ‘flowing’. Because of this difference in mechanical properties and the complex internal forces present, the lithosphere plates can move relative to the material below. To visualise the motion of the plates, we can use the idea of lithospheric plates floating on top of the asthenosphere.
Looking at the surface of the Earth (Fig. 29), the largest plates show up as relatively rigid areas of the lithosphere, with interiors that do not experience as much disturbance as their edges. Plates move relative to each other along plate boundaries, in various ways that will be described below. The plate patterns have been worked out by investigating distinctive markers within the plates and at their edges, allowing the relative rates of movement between neighbouring plates to be calculated. These rates are very slow, rarely exceeding a few centimetres per year, but over the millions of years of geological time they can account for thousands of kilometres of relative movement.
It has proved to be much easier to measure plate movements than to work out what has been causing them. However, the general belief today is that the plates move in response to a number of different forces. Heat-driven circulation (convection) occurs within the mantle, but other forces are also at play. Where plates diverge, warm, new material is formed that is elevated above the rest of the plate, providing a pushing force to move the plate laterally, around the surface of the Earth. At convergent boundaries, cold, older material ‘sinks’ into the asthenosphere, providing a pulling force which drags the rest of the plate along behind it. Deep within the Earth, the sinking material melts and is ultimately recycled and brought back to the surface