Britain’s Structure and Scenery. L. Stamp Dudley
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