Britain’s Structure and Scenery. L. Stamp Dudley
follow the successive belts of the Scarplands (see Chapter 18), sweeping across England from north-east to south-west—brown for the Liassic clays, yellow for the Oolitic sequence, dark green for the lower Cretaceous rocks, and light blue-green for the Chalk follow one another in sequence of ever decreasing age as one goes towards the south-east. It is in the south-east that the orderly sequence is interrupted and this is a reminder that the south and south-east of Britain lay on the fringes of the great Alpine storm and that some of the rocks there were folded by the Alpine movements. The uplift of the Weald, roughly east and west in its main axis, separates the two main Tertiary basins of London and Hampshire ; the sharpest folds of Alpine date are those in the extreme south—across the Isle of Wight and the Isle of Purbeck. The location of the main stretch of Pliocene rocks in the coastal parts of Norfolk and Suffolk is suggestive, and rightly so, that by that period the geography of Britain had acquired something of its present form—only certain parts, on the whole near the present coasts, came under Pliocene seas.
There the geological story shown by the general map we have been discussing ceases. It tells us nothing of the stupendous events of the Great Ice Age. For that we must turn to the detailed drift maps and from them try to piece together what is one of the most fascinating and important, and yet, despite its recent occurrence in terms of geological time, one of the most difficult episodes to reconstruct in the geological story of the building of Britain. Before we deal with this, however, it is essential to consider how many of the major surface features of Britain have evolved.
LAND FORMS AND SCENERY THE WORK OF RIVERS
ALTHOUGH it is clear enough that the form of the surface—the relief of the land—is in the main determined by the underlying geological structure, the relationship between land forms and structures is by no means a simple one. It is only within the past few decades that the specialist study of land forms, the science of geomorphology, by developing its own technique, has demonstrated that it is possible to reconstruct a long and often complex history by detailed investigation of the form of the ground and that the details of land relief may bear surprisingly little relationship to the structural geology. For the most part geologists have paid little attention to this natural development of their studies. Apart from a few outstanding works by geologists such as J. E. Marr’s Scientific Study of Scenery first published in 1920 and more general works such as Lord Avebury’s Scenery of England and Wales, the foundation of detailed work was laid by such physical geographers as the American W. M. Davis, whose famous studies of the evolution of rivers was nevertheless carried out in our own Wealden country, and the Frenchman Emmanuel de Martonne whose Traite de Géographie Physique contains many British examples. Much recent work has emanated from America and other detailed work from Germany. In this country some of the younger geographers, headed by J. A. Steers and W. V. Lewis, have concerned themselves especially with coastal phenomena, others such as Professor D. L. Linton and Professor A. A. Miller with river evolution whilst a leader amongst those devoted to general geomorphological studies is Professor S. W. Wooldridge, whose Physical Basis of Geography: an Outline of Geomorphology, was first published in 1937.
Briefly, it may be said that land-forms depend first on the nature of the rocks and their disposition (that is, in other words, on lithology and structure), secondly on the climatic conditions, with resulting soil mantle and vegetation cover, under which the sculpturing of the land surface has been and is taking place, and thirdly on the phase or stage within the erosion cycle.
However erroneous, it is common to find references to “hard” rocks and “soft” rocks which are regarded as respectively resistant to and less resistant to weathering. Since most of the older rocks are “hard” in this sense the common distinction is drawn between the old hard rocks and the young soft rocks characteristic respectively of Highland and Lowland Britain. Although in any given area it is broadly true that the positive features of the relief, the mountains, hills and plateaus, are coincident with the outcrop of resistant rocks and the negative features, the valleys and plains, to that of “weak” rocks, resistance to weathering is not a matter of actual hardness. Chalk could not be described as a hard rock, yet it gives rise to the main hill ridges of south-eastern England. Under certain circumstances even a bed of gravel is sufficiently “hard” to form a capping and preserve a hill from denudation as in the case of Shooter’s Hill to the south-east of London. Both with chalk and gravel this is largely due to the fact that rain water soaks into the rock so readily that it does not have time to collect in rivulets on the surface and wash away the surface soil. When reached in deep excavations such as wells even clay is quite hard but when at the surface it has absorbed a certain amount of water it is impervious to more. When rain falls on the surface it is then easily eroded—as muddy streams bear witness—and so outcrops of clay are marked by valleys and lowlands.
In the British Isles we are concerned with the land-forms which develop in a moist, temperate climate. We are not, for example, directly concerned with land-forms which develop in hot deserts or in the rainy tropics except in so far as such conditions once prevailed in distant geological epochs and have bequeathed to us fragments of “fossil” landscapes in the sun-shattered rocks which peep from beneath a cover of later strata in the Wrekin or the ridges of Charnwood Forest to remind us of the deserts of Triassic days. We are, however, concerned with land forms which develop under conditions of extreme cold under great ice-sheets or valley glaciers or on the margins of ice-covered seas, for much of the surface of this country was profoundly modified during the Great Ice Age. This is geologically so recent that not a few of our lakes and swamps are the last remains of those left behind by the retreating ice.
Over large parts of this country the relief seems to be completely unrelated to the underlying structure. Plains are developed quite independently of either the hardness or dip of the underlying rocks: rivers seem to go out of their way (as does the Bristol Avon) to pass through the highest hill ranges they can find instead of following an easy passage on low ground and it is here that we realise the importance of the erosion cycle. It is in the interpretation of such apparent anomalies that the geomorphologist has made his major contribution. In the following pages we shall examine in detail a number of examples from Britain.
THE WORK OF RIVERS
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.