Modern Geography. Marion I. Newbigin
over the Continental Shelf, giving a markedly irregular appearance to the ocean floor. The British Islands lie upon a Continental Shelf of this kind, and this is one of our reasons for knowing that they are really only a part of the continent of Europe, separated from it by a slight depression.
The Continental Shelf slopes away from the land gently, and is widest where it fringes low continents, and narrowest where mountains approach the coast. Over it is spread the waste of the land, the coarser lying near the shore-line, the finer extending outwards to the steep seaward slope. This rapid slope leads down to the more or less uniform ocean plateau, whose surface is broken by the great ocean abysses, the greatest of which has a depth of about six miles. Relative but not absolute uniformity thus characterises all that part of the ocean floor which lies below about 100 fathoms.
Again, though the ocean floor is doubtless being slowly raised by the deposition upon it of the oceanic oozes, yet it is also true that as compared with the land surface it displays great constancy. While the land surface is constantly changing owing to the varying forces which act upon it, the floor of the ocean can vary but little from age to age, unless it is acted upon by the internal forces of the earth.
Turn now to the land. We note at once the two characters of marked irregularity of surface, and of changeableness. The changeableness is due to the forces of erosion which act upon the surface, and of these forces the most important to the geographer is running water. It is running water, aided by other agents, which carves the land into hill and valley, which produces gorge and lake, only ultimately to fill up the lake and plane away the gorge. It is running water which spreads out on the lower ground the waste of the higher, and thus prepares the way for the operations of man.
The result of the long-continued action of the varied forces of erosion must necessarily be to reduce the surface to an almost level condition. The denuding agents first produce irregularities and then finally remove these, until the whole surface is once again almost level. The whole globe would thus be reduced to the condition of a plain were it not for the intervention of the internal forces which raise up the surface anew into folds, or which produce volcanoes and outbursts of molten rock.
This constantly repeated series of changes may be said to be chiefly the concern of the geologist, especially as it is a series which has repeated itself in all time. But it is to be noted that at various parts of the surface of the globe at the present time every stage in the process occurs, and everywhere the question whether a particular land area has been exposed for a relatively long or for a relatively short period to the forces of erosion, has a profound influence upon life. It is therefore important for the geographer to be able to recognise the different stages. This he cannot hope to do without some detailed knowledge of the effects of erosion.
Theoretically every land surface elevated above sea-level should pass through what has been called a cycle of erosion. There should be a period when the active forces are working upon a surface as yet but little modified; this is the period called by analogy youth. At a later stage the drainage has been well established, and the rivers run in broad valleys, from which lakes and waterfalls have largely disappeared. To this condition the term mature has been applied. At a still later stage the land surface has been so worn by the eroding forces that the whole process of erosion is slackened, and an uplift must occur before the erosive forces regain their lost strength. This is the so-called “cycle of normal erosion,” but it is constantly liable to variations due to local crust movements, to changes in climate, and to local conditions, though at the same time the distinction of the various stages has value for the geographer because of their varying effects upon human life. It is necessary for us, therefore, to consider how the different stages may be recognized, and how the forces of erosion act.
Let us begin our study of erosion by a general survey of the striking features of the earth’s surface at the present day. We know that at various parts of the surface there rise lofty mountain chains, whose summits are often permanently snow-clad, and which, from the sharpness of their forms and from the masses of rock rubbish which are accumulating round them, have obviously only been exposed for a geologically short period to the action of the atmosphere and of running water. When examined such mountain chains are all found to have the same peculiarities of internal form, the rocks composing them being elaborately folded and fractured. Careful investigation has convinced geologists that all the existing great chains owe their origin to a series of earth movements which occurred in the period called Tertiary, that is, in the third of the great geological periods, the one immediately preceding that in which we live.
These lofty mountain chains of Tertiary origin are most familiar in the great series of folds which appear at the surface to form the Pyrenees, the Alps, the Caucasus and the Himalayas, but the Atlas Mountains belong to the same series, as does also that great mountain chain which, under various names, runs down the western coast of the American continent.
As already indicated, these areas are recognised not only by the fact that there appear at the surface a great number of peaks forming a mountain chain, but also by the internal structure, the characteristically complex folding of the rocks. Now outside of these recently elevated areas in, for example, the continent of Europe, we find two conditions. On the one hand, there are regions of upland type but with rounded and smoothed forms, which are sometimes almost reduced to the condition of a plain. Such regions occur in Ireland, in the west of Great Britain generally, in Brittany, in the central plateau of France, in the Ardennes, in Bohemia, in the central plateau of Spain, in Scandinavia, and so forth. Between these relatively elevated areas we have plains and low-lying river basins, such as the London basin, the Paris basin, and so on. When the rocks are examined in both cases it is found that in the basins and plains the rocks, as a general rule, are only slightly inclined, while in the uplands and plateaux there are obvious remnants of folding, and the rocks are of ancient types, not relatively modern like those of the Alps, Himalayas, etc. (see fig. 1).
If, then, the existing mountain chains show complex folding in their constituent rocks, and though geologically but of yesterday have been already deeply affected by the denuding agents, must we not suppose that the folded and contorted uplands of Europe and elsewhere are the last remnants of very ancient mountain chains? It is they which form the framework of the continents, and by their wear and tear the low grounds have been formed, owing to the filling in of the great gulfs which ran between the old mountain chains.
We may elaborate a little further this very interesting subject. Let us first note that the geologists group the rocks composing the earth’s crust into three great divisions. We have, first, the Primary rocks, which are the oldest, and include as their most generally interesting member the Carboniferous rocks, with their coal-bearing beds, so important in the modern industrial world. Second, we have the Secondary beds, the most interesting members of which is the Chalk, so well-developed in parts of England and France. Finally, the Tertiary series includes the rocks of the period immediately preceding that in which the first undoubted remains of man occur.
Each of these periods was of enormous length, and the labours of successive generations of geologists have brought to light, at least in broad outline, the general appearance of the globe in so far as affected by the distribution of land and water, and the main earth movements, in each separate period. Thus we know that during that long period of time which is included in the Primary epoch, very extensive earth movements, resulting in extensive folding and mountain formation, took place. The geologists distinguish no less than three separate periods of folding in Primary times. It is not necessary for us to consider these in detail; their total result was to produce the mountain regions whose worn-down stumps now