Britain’s Structure and Scenery. L. Stamp Dudley
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