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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.