Laterization does not necessarily render a soil useless and many relatively laterized soils are cultivated, although some soils resembling lateritic soils, for example in parts of the eastern United States, are not truly laterized. Indeed, there are doubts about the extent to which laterization is occurring at present. Lateritic soils in the West Indies, Indonesia, Australia, India, and China may well be of ancient origin (HUNT, 1972, p. 193).
Living organisms contribute to weathering. By moving through soil they assist the penetration of air and water, and the decomposition of organic material releases acids and carbon dioxide, some of which dissolves into the soil water. Biological activity contributes greatly to the formation of soil.
Physical weathering is also important in soil formation, especially in its initial stages, but it can also degrade soils through erosion. Thermal weathering, which is the expansion and contraction due to repeated heating and cooling, causes rocks to flake, especially if water is held within small crevices.
Small particles detached from the rock may then be carried by the wind and if they strike other rocks more particles may be chipped from them. Depending on their size, the particles may be carried well clear of the ground or may roll and bounce along the surface; the process is called ‘saltation’. Most serious erosion is due to water, however. All water flowing across the land surface carries soil particles with it. This can lead to the formation of rills and gullies into which more particles are washed and then transported, or where water flows as sheets whole surface layers can be removed. In addition to this, all rivers erode their banks, and waves erode the shores of lakes and the sea (HUDSON, 1971, pp. 33–46).
These processes are entirely natural and part of the cycles by which originally igneous rocks are converted into sediments and landforms are made and age, but human activities can accelerate them.
The UN estimates that in the world as a whole, some 1.093 billion hectares (ha) of land have been degraded by water erosion, 920 million ha by sheet and surface erosion and a further 173 million ha by the development of rills and gullies. Of the total area subject to serious degradation by water, 43 per cent is attributed to the removal of natural vegetation and deforestation, 29 per cent to over-grazing, 24 per cent to poor farming practices, such as the use of machinery that is too heavy for the soil structure to support and the cultivation of steep slopes, and 4 per cent to the over-exploitation of vegetation (TOLBA AND EL-KHODY, 1992, pp. 149–150). There is, however, some evidence that modern farming techniques can reduce soil erosion substantially. A study of a site in Wisconsin found that erosion in the period 1975–93 was only 6 per cent of the rate in the 1930s (TRIMBLE, 1999). This may be due to higher yields from the best land, combined with methods of tillage designed to minimize erosion (AVERY, 1995).
Weathering is the general name given to a variety of natural processes by which rock is recycled and soil and landscapes created. It creates and alters environments, but human activities can accelerate it on vulnerable land, degrading natural habitats and reducing agricultural productivity.
In winter the water expanded as it froze, widening the crevices, and in summer the water shrank as it melted, releasing flakes of rock and also large boulders. For those few weeks in summer when the weather was warm enough to thaw the surface layers of the permafrost, turning soil locked solid by ice into wet mud, the mud, together with large boulders embedded in it, slid downhill, only to be brought to a halt when the temperature dropped and the mud froze again. Today, although there is no permafrost, the scattering of boulders around the tors remains as a record of the climate more than 10000 years ago. Similar periglacial processes acting on the weak, jointed chalk of southern England caused slopes to retreat through the loss of material from their faces and produced large deposits of the angular debris comprising fragments of varying sizes called ‘coombe rock’ or sometimes ‘head’
(other definitions confine ‘head’ to deposits other than chalk). There are similar periglacial relics in North America and elsewhere in Europe.
The Lynmouth flood
On Exmoor, in south-west England, in the summer of 1952, almost 230 mm of rain fell in 24 hours on to land that was already waterlogged. The water drained northward, carried in two rivers, the East and West Lyn, which enter the sea together at the small village of Lynmouth, falling some 300 m in rather more than 1 km. Unable to carry the volume of water, during the night of 15 and 16 August both rivers flooded and the overflow from the West Lyn cut a new channel that took it through Lynmouth, rejoining the original course at the mouth.
Houses, roads, and bridges were destroyed, an estimated 40000 tons of trees, soil, boulders, and rubble and masonry from collapsed structures piled up in the village, and 31 people were killed. The disaster was caused by nothing more than rain. Lynmouth was subsequently restored and is now a popular and attractive holiday resort.
Present permafrost regions occur in latitudes much higher than Britain. In Canada and Alaska within the Arctic Circle in places the permafrost is 400 m thick and in parts of Siberia it is 700 m thick. In Resolute Bay, in the Canadian Arctic, it extends to a depth of about 1000 m. Overall, nearly 20 per cent of the land area within the Arctic Circle is permafrost, and has remained in this state since the retreat of the ice sheets that once covered it.
Ice sheets are major sculptors of landscapes. As they move, they scour away all soil and other loose material, pushing it ahead and to the sides of them, where it may form moraines. They smooth angular rocks and the weight of the ice depresses the ground beneath. During a major glaciation ice sheets may grow to a thickness of more than 2500 m and depress the underlying surface by 600 m, which may take it to below sea level. As the ice retreats, the surface rises again, but it is a slow process, at least as measured on a human scale. Northern Canada, where shore-lines rose several tens of metres in less than 1000 years, and Scandinavia are still rising to compensate for the loss of their ice sheets around 10000 years ago; in Scandinavia the surface was depressed by about 1000 m and has subsequently risen by 520 m. This ‘glacioisostasy’ demonstrates the slight flexibility of the Earth’s crust.
Because there is a lag between the disappearance of the ice sheets and the recovery of the original surface elevation, bowls may remain where the ice was thickest. Depending on their location, these may be flooded by the sea or fill with fresh water. The North American Great Lakes and the Baltic Sea were made in this way. On a much smaller scale, so were the lakes of the English Lake District.
Ice accumulating in a pre-existing hollow will erode the sides to the open-sided, approximately circular shape of a cirque (also known as a ‘corrie’ or ‘cwm’). Where a relatively narrow glacier flows into the sea the trough it excavates may later form a fjord, known in Scotland as a ‘sea loch’.
Some fjords are more than 1200 m deep. In latitudes higher than about 50°, ice has been the major geomorphological (‘landscape-forming’) agency.
Soil will tend to move slowly downslope by ‘soil creep’, caused by the expansion and contraction of material due to repeated wetting and drying, or ‘solifluction’, where the soil is lubricated by rain water (formerly the term ‘solifluction’ was applied only to periglacial environments where the ground is frozen for part of the year, but it is now used more widely and is recognized as an important process in some tropical areas). The rate of soil creep has been measured in the English Pennines as between 0.5 and 2.0 mm at the surface and 0.25 to 1.0 mm in the uppermost 10 cm (SMALL, 1970, p. 224). If the soil is deep and the underlying rock extensively weathered, large masses may slip suddenly and move rapidly as ‘earth flows’. The collapse of coal tips at the Welsh village of Aberfan in 1966 was of this type (in this case known strictly as a ‘flowslide’).
The tips had been built over springs. Tip material absorbed the water, greatly increasing its weight but simultaneously lubricating it until it lost its inertia catastrophically (SMALL, 1970, p. 29–34). Earthquakes can break the bonds holding soil particles together, resulting in earth flows of dry material.
There are several ways in which masses of rock and earth can move downslope (HOLMES, 1965, p.
481). All such movements alter the shape of slopes, generally smoothing and reducing them. Figure 2.6 shows the stages by which this happens:
(1) material from the free face is detached and falls to form a scree which buries a convex lower slope; (2) further falls cause the free face to retreat until it disappears altogether, leaving a slope that grades smoothly to the level of the higher ground;
(3) the slope itself then erodes further. It can also happen that accumulated water increases the weight of a mass of weathered material until it shears away and slides down a concavely curved shear plane between it and the adjacent material. Because of the curved slope, the sliding layers are tilted backwards as they descend, so the toe of the slide is tipped upwards, forming a barrier behind which further debris will be held. This is a
‘rotational slide’, examples of which can be seen in several places along the south coast of England and in the Isle of Wight. Most failures are quite complex and involve more than one mechanism.
Geomorphology, the study of landforms and the processes by which they are produced and change, began with the work of an American geologist, William Morris Davis (1850–
1934), of Harvard University. He proposed that landscapes evolve through a ‘cycle of
erosion’ (the ‘Davisian cycle’). This begins Figure 2.6 Slope development
when land is raised by tectonic movements. In a young landscape hills slope steeply and the slope of river beds is irregular. As the landscape matures, hill slopes become gentle and river beds slope smoothly.
Eventually, the old landscape has eroded to a gently rolling peneplain (‘almost a plain’), a word Davis coined. His idea was (posthumously) challenged, in 1924, by the German geologist Walther Penck (1888–1923), who argued that once a slope has settled at an angle which is mechanically stable for the material of which it is composed, it will maintain that angle. Erosion will wear away its face, but will not make it more shallow, so the face will retreat but the angle will remain fairly constant, and the steeper the slope the faster it will erode, because the slow-moving weathered material on a shallow slope will protect the underlying surface. Thus, if a slope is steeper near the bottom than it is higher up, the lower slope will erode faster than the upper slope and the structure will collapse. As the argument developed, geomorphologists came to realize that a true understanding of ‘the slope problem’ can best be gained from studies of low-latitude landscapes that have not been formed mainly by glacial action, as were those on which the theories of Davis and Penck were largely based (SMALL, 1970, pp. 194–
224). Interest in the topic is not purely academic, for an understanding of how rock and soil behaves on sloping ground is necessary for engineers calculating the risks of landslides, erosion, and flooding, and devising schemes to minimize them. The matter is of major environmental importance.
Rivers provide the principal means by which particles eroded from surface rocks are transported from the uplands to the lowlands and eventually to the sea. Rivers are also major landscape features in their own right and, by cutting channels across the surface, important agents in the evolution of landscapes.
It is not only mineral particles they transport, of course. Water draining into a river from adjacent land also contains organic matter and dissolved plant nutrients, and rivers also carry those substances we discharge into them as an apparently convenient method of waste disposal. They are also a major source of water supplied for domestic and industrial use.
Water drains from higher to lower ground, moving slowly as ground water between the freely draining soil and an impermeable layer of rock or clay, eventually emerging at the surface as a spring, seeping from the ground, or feeding directly into a river. The ‘water table’ is the upper limit of the ground water, below which the soil is fully saturated. These terms have the same meanings in British and North American usage, but confusion can arise over ‘watershed’, which has two different meanings. A drainage system removes water from a particular area, and one such area is separated from adjacent areas.
In Britain, the area from which water is removed by a particular drainage system is called a ‘catchment’ and in North America it is called a ‘watershed’. One catchment is separated from another by a ‘divide’, which in Britain is sometimes known as a
‘watershed’, and within a catchment the drainage system forms a pattern. Figure 2.7 Figure 2.7 Drainage patterns. A, subparallel;
B, dendritic; C, semi-denritic; D, trellised;
E, rectangular; F, radial
illustrates six of the commonest patterns, but others are possible and real patterns are seldom so clearly defined as the pictures may suggest. Climate, the type of rock, and the extent of erosion all play a part in determining the type of pattern that will develop. Dendritic patterns, for example, usually form on gently sloping land of fairly uniform geologic character. Radial patterns occur around domed hills and batholiths, and trellis patterns where rivers cross, more or less at right angles, alternating bands of relatively hard and soft rocks.
As they flow, rivers can be approximately classified into zones, mainly on a biological basis. The headstream, or highland brook, is small, usually torrential (which means its water flows at more than 90 cms-1), and the water temperature varies widely. Few aquatic animals can survive in it. A little lower, trout can survive in what is still a fast-flowing stream, the troutbeck. Silt and mud begin to collect at the bottom of the minnow reach or grayling zone, some plants can survive, and the animal life becomes a little more diverse. In the lowland reach or bream zone the water flows slowly, the river is often meandering, and animal life is diverse. In this final zone the river flows across the coastal plain into the estuary.