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, an important process in scenery terms that we discuss below.
FIG 15. Boulder clay or till, West Runton, north Norfolk.
The present-day Arctic has much to tell us about conditions and processes in Southern England during the cold episodes of the Ice Age. Much of the present-day Arctic is ice-sheet-free, but is often characterised by permanently frozen ground (permafrost). When the ground becomes frozen all the cracks and spaces in the surface-blanket materials and uppermost bedrock become filled by ice, so that normal surface drainage cannot occur. In the summer, ice in the very uppermost material may melt and the landscape surface is likely to be wet and swampy. Ice expands on freezing, and so the continuous change between freezing and thawing conditions, both daily and seasonally, can cause the expansion of cracks and the movement of material, with corresponding movements in the surface of these landscapes. This movement can cause many problems in the present-day Arctic by disturbing the foundations of buildings and other structures.
FIG 16. Artist’s impression of Southern England, south of the ice sheet, during the Ice Age. (Copyright Norfolk Museums and Archaeology Service & Nick Arber)
Remarkable polygonal patterns, ranging from centimetres to tens of metres across, are distinctive features of flat Arctic landscapes, resulting from volume changes in the surface blanket on freezing and thawing (Fig. 17). In cross-section the polygon cracks and ridges correspond to downward-narrowing wedges (often visible also in the walls of gravel pits in Southern England). Thaw lakes are also a feature of flat areas under conditions of Arctic frozen ground (Fig. 18). They appear to be linked to the formation of the polygonal features, but can amalgamate to become kilometres across and may periodically discharge their muddy soup of disturbed sediments down even very gentle slopes.
Not only can these frozen ground processes be studied in Arctic areas today, but they have left characteristic traces in many of the landscapes of Southern England. Some examples from Norfolk are illustrated in Chapter 8 (Figs 306 and 307), and these provide specific examples of the result of ancient freeze-thaw processes on a small scale. However, the more we examine the wider features of present-day landscapes across Southern England, the more it becomes clear that most have been considerably modified by the general operation of frozen ground processes during the last 2 million years. These processes are likely to have been responsible for the retreat of significant slopes and even for the lowering of surfaces that have almost no perceptible slope.