The majority of- but not all – surface modification processes act to reduce or flatten topography, mainly by eroding the higher features but also by filling in lower ground with sediment. So logically landscapes might always be regarded as tending towards a flat surface. Our understanding of the processes involved suggests that any land area with mountains or hills will be eroded downwards to an increasingly flat surface as time passes, although the rate of erosion will reduce as the topography becomes more and more smooth. Acting against these flattening processes are periodic movements of the ground surface caused by forces within the Earth, producing new mountains and hills, and so creating new landscapes (Fig. 5).
Continuing research into the processes operating within the Earth shows that movements of the Earth’s crust are taking place continuously, even though the rates involved are generally too slow to be noticeable. The discovery that the Earth’s surface consists of a large number of tectonic plates in continuous relative movement was one of the major breakthroughs in the earth sciences, and has fundamentally changed our understanding of the planet. More on this topic will be considered in Chapter 3, but at this point it is important to realise just how slow the rates of movement are: at most a few centimetres per year on average (often compared to the rate at which fingernails grow). Occasionally, movements of centimetres or metres occur within seconds along faults during earthquakes, but the average rate of movement is still rather slow. Most of us living in stable areas are totally unconscious of any movement at all because we are, ourselves, moving slowly with the landscape that we live on. Slow though the movements may be in a particular landscape, so also are the rates of surface modification, and the balance between the two is a delicate one. In much of Southern England modification by surface processes is dominant, but this has not always been the case.
FIG 5. Landscapes are changed by surface modifications (Chapter 2) and solid earth movements (Chapter 3).
Our next chapter deals with the timescales represented in the landscapes of Southern England and the processes that have been modifying them. Chapter 3 deals with the movements from below – from within the Earth’s crust – that are ultimately creating major landscape patterns.
CHAPTER 2 Time, Process Southern England’s Landscapes
BEDROCK AND SURFACE BLANKET
WALK AROUND THE COUNTRY IN SOUTHERN ENGLAND and the ground beneath your feet is very rarely solid rock. You are walking over soil made of weathered mineral grains and organic debris, along with other relatively soft and granular materials that make up the surface blanket. Beneath the surface blanket lies solid rock, the bedrock of the landscape.
Bedrock forms the bones of the land. From the colour of the soil, to the elevation of the hills, to the types of vegetation present, the landscape is profoundly influenced by the bedrock underlying it. For example, in Southern England the Lower Greensand (a distinctive layer of bedrock of Early Cretaceous age, see page 26) produces soil water with acidic chemical properties. The Lower Greensand was originally deposited as sand over a period of a few million years, more than 100 million years ago. This layer represents a different environment of deposition from the older sediments on which it lies, and was followed by another change of environment which produced the deposits that lie on top of it. Both the preceding and the following bedrock deposits have alkaline chemical properties. In certain regions the bedrock layers have now been brought to the surface of the landscape by erosion and movements within the Earth. The Greensand is harder than the layers above and below it (largely mudstones) and so is generally more resistant to weathering. In some areas the Lower Greensand lies just below the surface blanket and has resisted the general landscape erosion to form a distinct Greensand ridge running across the countryside, characterised by special vegetation adapted to the acidity of the soils.
It is only in cliffs or at man-made excavations such as quarries that we can see bedrock at the surface in most low-lying areas. By using those areas where the bedrock does outcrop at the surface, and the results of drillings (e.g. for wells), we can discover the types and arrangements of rock below any landscape.
THREE DIFFERENT TIMESCALES
More recent past events tend to be better known and of greater interest than distant past events. Figure 6 is plotted on a logarithmic timescale, so that the most recent times are given more space and greater ages are given less and less space.
FIG 6. Three different timescales, plotted to give more space to more recent events.
For the purposes of this book, we can distinguish three overlapping timescales to help us to understand the landscapes of Southern England:
The bedrock timescale (extending from 542 million years ago to about 2 million years ago)
The Ice Age timescale (covering roughly the last 1 million years)
The last 30,000 years timescale
We shall now review each of these, commenting on the sorts of episodes in each that are important in our exploration of Southern England.
THE BEDROCK TIMESCALE
Figure 7 is a simplified version of a generally accepted geological timescale relevant to the landscapes of Southern England. The names of the divisions are universally accepted in the geological world and, unlike the previous diagram, the passage of time is represented on a uniform (linear) timescale. The divisions have been selected, and sometimes grouped, to help in our analysis of the situation in Southern England, and these have been colour-coded for use in the rest of the book.
FIG 7. Bedrock timescale for Southern England.
The rocks at, or just below, the surface of Southern England range in age over hundreds of millions of years, and most of them were formed long before the present scenery began to appear. At the time of their origin, these rocks were deposited in a variety of different environments, mostly when mud or sand materials were transported into and/or around the seas that existed where England is now. Most of the bedrock of Southern England was formed in this way and is said to be of sedimentary origin. The depositional conditions varied from time to time: the climate varied, the geographical pattern of rising and sinking land movements changed, and the supply of mud and sand brought downstream by rivers changed also. Despite these fluctuations, it is possible to generalise the way that sediment has accumulated over an area the size of Southern England, and to offer a succession of layers of different composition, age and average thickness that can provide a general guide. This is shown in Figure 8.
For each of the Regions (and some of the Areas) discussed in Chapters 4 to 8, a rock