then deposited in the channel and removed downstream by the river.
The simplest valleys result from down-cutting by a river or stream to yield a V-shaped profile in cross-section. The gradient of the valley sides depends on the strength of the material that the slopes are composed of in the face of erosion. Stronger materials are more difficult to erode and remove, and so can form steeper slopes than weaker materials. In some areas, the river channel is unable to form valley slopes as the material is too weak to form a noticeable gradient. In the Areas we will be investigating, it is clear that some of the slopes are largely the result of a particularly strong layer in the bedrock resisting erosion as the landscape has developed.
FIG 11. Landforms of rivers.
As the valley develops, its profile can become more complex. In some cases, slopes appear to have retreated across a landscape some distance from the position in which they were initially created by river down-cutting. A river with a wide valley floor is one of the most obvious examples of this, in which movements of the channel across the floor have caused the slopes to retreat as the valley floor has become wider. In some cases, slopes appear to have retreated over many kilometres from the original valley as numerous collapses of the slope took place.
Overall, therefore, the valley profile and the channel course reflect variations in the strength of the material being eroded, and in the strength and flood pattern of the river. Climate changes are likely to have a major effect on the strength of the river by altering the volume of water flowing through the channels. Additionally, the lowering or raising of the channel by Earth movement effects (see Chapter 3) can affect the evolution of the landscape by river processes. For example, both climate change and the vertical movement of the river channel can initiate the formation of river terraces. Different examples of all these river geometries will be discussed in greater detail in the Area descriptions in Chapters 4–8.
Over millions of years, river down-cutting, slope erosion and material transport tend to smooth and lower landscapes until they approximate plains, unless they are raised up again (rejuvenated) by large-scale Earth movements (Chapter 3) or are attacked by a new episode of channel erosion, perhaps due to climate or sea-level change. Southern England generally has a smoothed and lowered landscape, representing hundreds of thousands of years of this river and slope activity.
The branching, map-view patterns of river channels and valleys are an obvious feature of all landscapes. An approach to understanding how this forms is illustrated by a computer-based experiment (Fig. 12) in which a flat surface (plateau or plain) is uplifted along one of its edges, so that it has a uniform slope towards the edge that forms the bottom of the rectangle shown. Rain is then applied uniformly across the surface, causing the formation and down-cutting of channels that erode backwards from the downstream edge. As the experiment continues, the channels and their valleys extend into the uniform sloping surface by headward erosion, resulting in longer valleys, more branches and a greater dissection of the surface by those valleys.
FIG 12. Model showing upstream erosion by tree-like (dendritic) river patterns. (Provided by Dimitri Lague from the work of A. Crave and P. Davy)
As we consider the various Regions and Areas of Southern England, we will summarise the present-day river patterns of each by simplifying the main directions of drainage involved. We will also give an impression of the present-day relative size of the more important rivers by quoting their mean flow rates as estimated in the National River Flow Archive, maintained by the Centre for Ecology and Hydrology at Wallingford.
It seems surprising that today’s often sleepy southern English rivers have been the dominant agent in carving the English landscape. However, even today’s rivers can become surprisingly violent in what are often described as hundred-or thousand-year floods. Floods in the past were certainly more violent at times than those of today, particularly towards the ends of cold episodes, when melting of ice and snow frequently produced floods that we would now regard as very exceptional.
THE ICE AGE TIMESCALE AND LANDSCAPE MODIFICATION
The most recent Ice Age began about 2 million years ago, and is still continuing in Arctic areas. At various times during this period ice has thickly covered most of northwest Europe. Recent research, particularly measurements of oxygen isotopes in polar icecaps and oceanic sediment drill cores, has revealed much of the detail of how the climate has changed during the current Ice Age. It has been discovered that long cold periods have alternated with short warm periods in a complex but rather regular rhythm. Looking at the last half-million years, this alternation has occurred about every 100,000 years, and this is now known to have been a response to regular changes in the way the Earth has rotated and moved in its orbit around the sun. A closer look at the last million years (Fig. 13) reveals that for more than 90 per cent of the time conditions have been colder than those of today. Warm (interglacial) periods, like our present one, have been unusual and short-lived, though they have often left distinctive deposits and organisms.
FIG 13. The last million years of global temperature change. *the Oxygen Isotope Stages are an internationally agreed numbering sequence to label the succession of climatic cold (even numbers) and warm (odd numbers) episodes.
One of the most important cold episodes (glacials), just under half a million years ago, resulted in the Anglian ice sheet. This was up to several hundreds of metres thick and extended from the north southwards, well into Southern England, covering much of East Anglia and the north London area (Fig. 14). As the ice spread slowly southwards, it was constricted between the Chalk hills of Lincolnshire and those of Norfolk. A wide valley, later to become the Wash and the Fens, was filled with ice to a depth well below that of present sea level. As the ice spread outwards from this valley it dumped the rock material it was carrying, including blocks and boulders up to hundreds of metres across, giving some idea of the tremendous power of the ice sheet. The direct evidence for the presence of an ice sheet is material in the surface blanket called till, or boulder clay (Fig. 15). This often rather chaotic mixture of fragments of rock of all sizes (large boulders mixed with sand and mud) lacks the sorting of the fragments by size that would have occurred in flowing water, and so must have been deposited from the melting of ice sheets.
FIG 14. The Anglian ice sheet.
Much of the rest of the surface blanket that accumulated during the last 2 million years was deposited by the rivers that were draining the land or any ice sheets present. As ice sheets have advanced and retreated, so have the rivers changed in their size and in their capacity to carry debris and erode the landscape. Rivers have therefore been much larger in the past as melting winter snow and ice produced torrents of meltwater, laden with sediment, which scoured valleys or dumped large amounts of sediment. The gravel pits scattered along the river valleys and river terraces of Southern England, from which material is removed for building and engineering, are remnants of the beds of old fast-flowing rivers which carried gravel during the cold times.
There are no ice sheets present in the landscape of Figure 16. The scene is typical of most of the Ice Age history (the last 2 million years) of Southern England, in that the ice sheets lie further north. It is summer, snow and ice are lingering, and reindeer, wolves and woolly mammoths are roaming the swampy ground. The river is full of sand and gravel banks, dumped by the violent floods caused by springtime snow-melt. The ground shows ridges of gravel pushed up by freeze-thaw activity,
