Читать книгу Britain’s Structure and Scenery - L. Stamp Dudley - Страница 13

The principal agent in the sculpturing of the land surface in a rainy temperate climate such as that of Britain is undoubtedly running water. No sooner does rain fall than some of it collects to form tiny temporary rivulets which soon join small permanent brooklets and rills. These, reinforced by springs which represent the reappearance at the surface of that portion of the rainwater which had soaked into the ground, unite in due course to form the river system of the country. Except in certain limestone districts where much of the drainage is underground the whole country is covered with a complex surface drainage pattern of rivers and streams.

It is a common generalisation in most books on physical geography that the course of a river may be divided into three parts. The upper or mountain course is that in which swiftly flowing water, especially after rain, is able to move stones of considerable size, to roll them along, to rub them one against the other and so to reduce angular fragments such as those broken off the mountains by frost action into rounded pebbles. The work of such a mountain torrent is well seen in Plate 2. At the same time the river deepens and widens its own valley so that its valley has a typical V-section with unstable banks. The middle course of the river is that over the foothill belt where it has lost some of its velocity but is still moving rapidly enough to carry sand, silt and mud in suspension and to roll pebbles along its bed. Its main work there is transportation ; its valley has a broad open section and has stable sides so that the erosive power of the river is strictly limited. The lower course is that in which the river meanders lazily across a plain ; though sweeping much mud out to sea or into the lake into which it empties it has lost much of its velocity and so much of its power of transportation. It lays down part of its load as shingle beaches or sandbanks but especially builds up large flat plains of deposition by spreading alluvium over a wide flood plain or a delta. Thus its work is largely deposition.

Quite obviously not all rivers conform to such a generalised pattern. In mountainous regions they may tumble direct from the mountains to the sea (as shown on Plate XXVIII)—they are young rivers associated with an early stage in the cycle of erosion. Others, including such large rivers of this country as the Thames, have no mountain course—they are relatively mature and associated with a late stage in the cycle of erosion.

It is clear that there is a very close relationship between the character of a river and the phase of the erosion cycle.

FIG. 12.—Diagram of a Meandering River This diagram shows how the water of even a slowly-moving river in its lower course swings from side to side resulting in erosion on the concave side and deposition of sandbanks on the convex side.

It is the inexorable law of nature that as soon as mountain building movements have erected a land mass and endowed it with mountains, hills and valleys, the forces of sub-aerial denudation combine to reduce that land to…the question is to what? What is the final form of the land if the forces of denudation are allowed to continue unchecked? The answer is not a flat plain but a peneplain or peneplane which, whichever way it is spelt, means almost a flat surface. The whole process is referred to as “sub-aerial peneplanation.” Although sub-aerial peneplanation is in progress all over the world and although large areas may thus have been so reduced that they have reached “base level”, below which removal of material will not take place, it is rarely if ever in nature that the process is allowed to continue to its logical conclusion. Differential uplift of the land, up or down movements relative to sea level (eustatic movements), or even a slight folding movement will upset the equilibrium which has been reached and will cause “rejuvenation” of the river systems. Before considering these complications it will be well to note the various ways in which river systems may develop and thereby to explain many of the features which are associated with British rivers. In passing we may recall that peneplanation of lands in the British Isles has probably been almost reached in past geological epochs.

FIG. 13.—Diagrammatic Section through the Deposits of a Delta and a Lake or the Sea When the river enters a lake or the sea, the velocity of the water is immediately checked and any coarse material is at once dropped, only the finer mud being carried on. In this way a delta is built up and as the flat land of the delta itself is formed, the velocity of the river is checked before reaching the sea and the finer deposits are spread as a surafce layer of alluvium over the flood plain.

The formation of shallow water limestones which requires clear water is taken as evidence that the surrounding lands had been almost reduced to base level and consequently yielded very little sediment.

We notice that there are thus two types of plains—plains of deposition and plains of denudation and that the former tend to be flatter and are more truly plains.

Let us take the simple case of the floor of the sea which is raised up (not folded) by earth movements so that it becomes land. It will be a flattish surface with a gentle slope seawards and rain falling will collect together into streams, roughly parallel, finding the shortest route seawards. These streams are consequent on the slope and hence are known as consequent streams. Tributary streams, arranged somewhat irregularly, will drain into these main ones and the pattern of drainage developed is that known as dendritic.

FIG. 14

FIG. 15

FIG. 16

Figs. 14 to 16 illustrate three stages in the development of the drainage of the Weald. In each the line of dots represents the main axis of the Wealden uplift. When the dome was first uplifted (Fig. 14) chalk covered the whole and water drained naturally and consequently down the northern and southern slopes forming consequent streams. In the next stage (Fig. 15) the chalk has been removed by denudation over the central area and some streams have become stronger than others and subsequent streams, running in strike valleys, have developed. Fig. 16 shows the developments at the present day. The three divisions shown are the Tertiary, the Chalk and the pre-Chalk beds. It is clear that such a river as the Darent has had its headwaters captured by the Medway.

Perhaps even more common in nature is the initiation of a drainage system by uplift accompanied by folding. If the rocks are raised up to form a broad arch or anticline, consequent streams flow down either side following the general dip of the rocks. Thus consequent streams follow the dip of the rocks. Very soon two things will happen. Some streams will become stronger than others—it may be through some slight differences in the relative softness or hardness of the beds over which they are flowing. They deepen their beds more rapidly than their neighbours, they cut back at their heads (headward erosion) more rapidly. Water which might have gone into neighbouring streams drains into them by laterals which, because they thus develop subsequently to the main consequent are known as subsequents. The subsequents flow at right angles to the dip of the rocks—that is along the strike—and so are flowing in strike valleys. In due course some of the more vigorous subsequents capture the waters and drain the valleys of the weaker consequents and so, by this process of river capture, a complex system develops. It may even happen that the flow of water in part of a former consequent valley is reversed so that it becomes an obsequent stream feeding the conquering subsequent.

Such a river system as that just described may cut down on to older rocks which lie underneath the sheets of strata which gave it its birth. Indeed all traces of those later rocks may be completely removed. On to the older rocks there is implanted a river system which seems to have simply no relationship to the structure. This is a common feature in many parts of the British Isles and gives us the reason for the passage of the Bristol Avon through Clifton Gorge when there apparently were so many other easier courses. Such a system is called a superimposed drainage. But in its further development the members of the system will find out the weaker rocks, the lines of faulting and crushing, and it is along such lines that the major excavation will take place.

What happens when earthquakes and folding movements take place in an area with a well developed river system? There may be reversals of drainage and many examples of river capture can only be explained by postulating differential earth movements. But if folding movements take place slowly existing rivers in their downcutting may keep pace with the growth of folds and one gets thus examples of antecedent drainage which may be defined as drainage developed in its early stages before the present surface features. For the supreme examples of this one must look to the mighty rivers of the Himalayas which cut right through the greatest chain of mountains on the face of the earth.

The sequence of development of consequents, subsequents and obsequents was worked out by W. M. Davis in the river systems of the Weald which is thus classical ground. One may picture the Wealden dome, in structure resembling an overturned boat, rising by slow stages from the latter part of the Cretaceous period downwards. At first the uplift formed a low dome scarcely above sea level but sufficiently near the sea surface for wave action to get to work wearing away the chalk and rolling the angular flint nodules into pebbles. By Middle Eocene times the ridge was sufficiently high to be partly covered by shingle beds (Blackheath Pebble Beds) and the crest probably formed an island. As soon as an island appeared above the surface consequent streams flowing from the east-west crest to the north and to the south developed. By a combination of marine erosion and then of sub-aerial denudation the chalk was entirely eroded from the central area and revealed below the varied succession of beds which make up the lower Cretaceous. Some of the beds are weak and easily eroded, others are relatively resistant. Subsequent streams found out the weaker rocks and eroded valleys at right angles to the consequent streams, some of which cut down through the chalk and to-day are seen flowing in steep-sided valleys through the chalk rim of the North Downs and the South Downs. The weaker consequents were beheaded by the capture of their head streams and many failed to cut down through the chalk. The Weald thus illustrates extremely well the association of subsequent streams with valleys in the weaker rocks which are parallel to the strike of the rocks (strike valleys) whereas the consequents have valleys parallel to the dip of the rocks (dip valleys). The later history of the Weald has been complicated by the submersion of much of the area under the Pliocene sea, then its subjection to tundra conditions during the Great Ice Age and by the complications caused by the breaching of the eastern end of the fold when Britain became separated from the continent, but the main pattern of the drainage has remained as it was developed by the gradual uprise of the Weald. The phenomena of subsequent streams occupying well-defined strike valleys is repeated all over the lowland of Britain.

The form of a river valley is able to yield much information both with regard to the age of the valley itself and the history of the river system.

Mountain torrents stand rather by themselves: they cut deep notches in the mountain sides (an example is given in Plate 2), usually finding some line of weakness, as for example along a fault where the rocks have been crushed, and the material which is dislodged is swept to lower levels both by the power of the water and by the force of gravity. If dislodged blocks fall by gravity alone they form screes with an angle of rest of about 40°—the angle of the scree shown on Plate 8B is exactly 38°. If the fall is aided by running water the debris is fanned out and has a lower angle of rest—forming what is termed an alluvial fan or alluvial cone (such as the ones shown on Plate 30B) though the word “alluvial” is apt to cause confusion with the much finer material associated with deltas and with the flood plains of the lower courses of rivers.

Where initial slopes are not quite so steep the mountain stream carves out a narrow steep-sided V-shaped valley. Even at this early stage the valley is not straight: the stream swings from side to side so that “interlocking spurs” develop between the meanders and obstruct the view upstream. Nature has provided the swiftly flowing stream with a remarkable mechanism for drilling holes in its bed. A few stones are caught in a whirl of water and swing round and round to drill out the well-known “pot-holes.” This is an active force in deepening the bed of the river and so of its valley. An excellent and large example is shown in Plate VIIIA. Widening of the valley comes gradually with the action of gravity—lateral slipping aided by tributary streams so that, broadly speaking, the older or more mature the valley the wider it is. In these early stages the form of the valley, especially its long section (i.e. the section drawn down the valley—the longitudinal profile for which the not very appropriate German word talweg is often used), is closely related to the character of the rocks over which it passes. Hard bands cause rapids or waterfalls and between these the river may assume the characteristics of maturity. In cross-section the valley sides may exhibit ledges due to the outcrops of hard bands whilst dipping strata may cause a valley with an asymmetric cross section. Even more common is the varying width of the valley—broad and open where it traverses soft rocks, narrow and even gorge-like where it passes through a belt of hard rocks or limestone. Even an old river like the Thames has these features—the beautiful narrow valley at Goring is where it passes through the chalk ridge.

Gradually, however, a river tends to reach a state of equilibrium and its longitudinal profile will form a smooth curve from source to mouth. When it reaches this stage a river is said to be graded and the land around has reached the stage of sub-aerial peneplanation. To achieve the graded curve, which will first be reached near the river’s mouth, the stream must necessarily cut back into the hills from which it takes its source and this involves headward erosion. It is found that

FIG. 17.—Diagrammatic Sections along a Talweg The upper diagram is a longitudinal section following the course of a relatively young river from its source to its mouth. Bands of hard rock cause waterfalls and rapids between which the river tends to assume a graded curve. Diagram II is the graded curve of a more mature river: the whole longitudinal section is evenly graded from source to mouth independently of any hard beds. Diagram III illustrates what happens if a fully graded mature river, such as that shown in II, is subjected to rejuvenation by a general uplift of the land surface relative to sea level. A knickpoint is formed independently of the character of the rocks and gradually works back, i.e. up the course of the river.

many mature rivers rise in a sort of amphitheatre, steep-sided but not nearly so steep-sided as the cirques from which glaciers have their origin.

Over the middle and lower courses of mature rivers, or rivers which have almost reached base-level, there are several characteristic features. The water swings from side to side and long winding meanders are the result (Fig. 12). Once a meander has been initiated there is a natural tendency for the swing of the water to make the curves ever more acute till at last the water breaks through the neck and the cut-off portion forms a stagnant “cut-off” or “ox-bow” lake. This will be clear from the diagram ; but what is not always realised is that the continuance of such a process results in a broad flat-floored valley with a deposit of gravel, sand, silt or alluvium. Such a flat floor is liable to flood when the river is in spate and so one gets a flood plain. Land liable to flood occurs along the lower courses of most British rivers. When the flooding is uncontrolled, a film of mud is spread by each flood and results in the gradual building up of alluvial flats. There is thus deposition closely associated with erosion in the middle and lower courses of a river. When the river reaches its mouth with a load of fine mud in suspension this may be swept seawards, especially if the sea into which the river discharges has a marked tidal movement. This is the case round the British Isles where nearly all our rivers enter into estuaries with a strong tidal movement. Where tides and currents are less strong the sediment is dropped near the mouth of the river and a delta of alluvium is gradually built up, passing seawards almost imperceptibly into very shallow muddy water. Since deltas are not typically formed round Britain it is unnecessary to enter into the details of their formation though there are many good examples where rivers enter lakes such as that shown in the foreground in Plate 31B. It is important to note the leading role played by vegetation in fixing the mud and then acting as a trap to catch more mud. In this way, though not directly associated with river mouths, there is accretion of land in such areas as around the Wash and in Morecambe Bay and advantage is taken of the natural processes in reclaiming land by building dykes or retaining walls to hold sediment. The stages in silting up are well shown in Plate XXV. Inland, artificially controlled flooding has long been practised, using the waters of such rivers as the Trent and Yorkshire Ouse to spread silt over the land after the manner of the Nile in Egypt and so both to build up the level and to spread a fertile layer rich in mineral salts and organic matter and of excellent mechanical texture. This controlled flooding is known as warping and the mud deposited as warp.

The well-graded meandering river with its broad valley floored with alluvium is a familiar feature in the British landscape. But even in geologically recent times, certainly since the Ice Age, there have been several changes in the relative level of land and sea, slight it may be but significant. What happens to such a mature river system when the land is lowered or raised relative to sea-level? First, if the land sinks, the lower valley is invaded by an arm of the sea and one gets the familiar feature of a drowned valley or ria. The best example of a coastline of drowned valleys or ria coast is the south-west of Ireland. Soundings show that the floor of the ria, the old river talweg, slopes steadily seawards and there is no “lip” as there is in the case of a glaciated valley with a rocky or morainic bar at the entrance (as in many of the Scottish fiords). Drowned valleys give rise to the picturesque winding creeks of south Devon and Cornwall—the estuary of the Fal and Tamar for example (Plate 26). It is clear that the branching tidal creeks shown in Plate 26 could not have been excavated by the action of the sea which now occupies them.

If, on the other hand, the level of the land is raised relative to the sea, the river undergoes rejuvenation; it is given new erosive powers and immediately begins lowering its bed. But such a rejuvenated river exhibits certain special features. It was, before the new uplift, a meandering mature river and the effect of the uplift is for it to follow its same meanders but to cut them deeply and so one gets the interesting and picturesque feature of incised meanders with a river winding in a gorge, it may be of considerable depth. If in such a case a meander is cut off one gets between the abandoned course and the new course a “meander core.” Incised meanders tend to develop where the rocks are relatively hard. Where a broad valley is excavated in relatively soft rocks the rejuvenated river develops for itself a new alluvial covered flood plain at a lower level than the old one and so fragments of the old one are left as gravel-covered or alluvium-covered terraces. Successive uplifts produce successive terraces at several levels. Those of the Findhorn in Scotland are well shown in Plate XXXI. The terraces of the Thames are not only well known but have been and are very important economically—for the dry sites they offer for settlement, for the water supplies once afforded by the gravels, for the excellent well-drained soils to which they give rise, for the brickearth they formerly supplied for brick making, and latterly for the supplies of gravel which, alas, is being excavated regardless of the future use of the devastated land. In the case of the Thames near London it is possible to distinguish one gravel-covered terrace at about 100 to 120 feet above present sea-level, though naturally varying in height with distance from the sea. This is the Boyn Hill Terrace and is very clearly marked in several areas. There is another terrace, of wide extent, at about 50 feet above sea level known as the Taplow or Middle Terrace. A third one is the Low or Flood Plain Terrace at some 10 or 15 feet above sea level. Then there followed a time when the Thames was lower than at present—or rather when the sea-level was lower and the river excavated what is now a buried channel so that to this extent the estuary of the Thames is a drowned valley. Actually the history of the Thames is much more complex than this, and such a complex history is typical of British rivers. Each change has some corresponding effect on tributaries. In the lower courses of a well graded river the effects of hard bands which may cross the valley have been eliminated and an interesting feature is found when the course of a rejuvenated river is followed upstream. There is found to be a point where there is a break in the longitudinal profile of the river. This is where it is still cutting back as a result of the change in level. Such a break of slope is known as a “knick point” and its development is to a large extent independent of any differences in the rocks of the river bed.

It must be remembered that the British Isles had a well developed river-system before the oneoming of the ice sheets of the Great Ice Age and that the effect of glaciation was to modify rather than to change completely the existing valleys and land forms.

A number of the plates in this book illustrate a few of the extraordinary complex character of British rivers. A drowned estuary such as that shown in Plate 26 may become silted up and a marshy plain may result—well seen in the estuary of the Glaslyn in Plate 20. Rejuvenation may result in gorges even in the middle courses of rivers—as shown in Plate 2. A mature landscape with well-rounded hills affected by rejuvenation is often more apparent from the air than on the ground and an example from the Southern Uplands is well shown in Plate XXII. The interesting case of “drowning” exhibited by the Norfolk Broads is shown in Plate XXIII.