Modern Geography. Marion I. Newbigin
uplands which we have described in Europe. But they do not occur in Europe alone. That vast and relatively infertile area in Eastern Canada which geologists call the Canadian Shield is a region of very old rocks, once folded into a mountain region, but long since worn down to an upland. In the eastern United States that long, but interrupted, range of hills, which, under various names, runs from the mouth of the St. Lawrence to Alabama and Georgia, and partially shuts the seaboard off from the prairies and plains beyond, is a region where the folding is still well marked, in spite of long denudation.
The Secondary period seems to have been one in which comparatively little folding took place, while, as already indicated, the Tertiary was one in which there was enormous folding in almost all parts of the globe, the result being the appearance at the surface of the great mountain chains of the present day. The structure of these chains makes them relatively unstable, and the forces of erosion are now acting upon them with extraordinary activity, beginning that process of wearing down which has reduced their prototypes of the Primary period to mere remnants of their former greatness.
Extensive as the Tertiary folding was, however, it left great areas unaffected, or but slightly affected, and such areas form plains or basins, where the rocks are but slightly tilted, or show a very simple form of folding. In Europe such slightly modified rocks occur, e.g. in the Paris basin, and in the fertile plains of south-eastern England.
In the United States beds of a similar character occur right over the great plains, filling what seems once to have been a great gulf between the old highlands to the east and the towering modern mountain chains of the west.
It must be realised that this is only a very summary and partial account of a difficult and complicated problem; but from the standpoint of pure geography it seems desirable to distinguish between those remnants of ancient mountains which form the backbone of the continents, the recently elevated mountain chains where enormously rapid erosion is taking place, and the largely unmodified rocks which often form fertile plains.
Let us next proceed to consider how the eroding agents act upon the surface of the land as soon as it is exposed. We may begin with the effect of running water upon a recently exposed surface, e.g. upon land slowly emerging above sea-level, or even with the effect of heavy rain upon sloping ground unprotected by a covering of vegetation. Alike in the one case and in the other the first effect is the formation of a number of shallow rills, which at first run parallel to one another. Sooner or later, however, these parallel channels tend to converge, and a torrent is formed such as may be seen in any mountain region.
Such a torrent consists of three often well-defined parts. First we have the numerous tiny rills which collect together to form what the French physiographers call a receiving basin (bassin de réception); then there is the stream proper forming a canal which drains the basin, while below, where the torrent debouches on the low ground, we find that it spreads out fanwise and throws down its load of débris to form a cone (cône de déjection). The torrent therefore already imitates a full-grown river, with its threefold division into mountain track, valley track, and plain track. It further illustrates the twofold work of the river, that of erosion and deposition.
Observation on an unprotected surface after a heavy rainfall will illustrate another point which is of much interest in connection with the work of rivers. This is that the water has most excavating power, not, as might be supposed, in the collecting basin, but in the valley region, where the slope is still great, where the volume of the water is at its maximum, and where it has acquired a load of débris by means of which it carves out its bed. The excavation of the bed therefore proceeds from below upwards towards the collecting basin. The result is that the slope of the valley floor diminishes as we pass from the upper region to the lower, owing to the levelling effect of erosion. The process of levelling down cannot be carried beyond a certain point, the so-called base level of erosion, which in a lateral stream is determined by the point of junction with the main stream, and in a main stream by the point which marks sea or lake level, for obviously no point in the river valley can be worn down much below its mouth.
When the work of a river is completed, the line which marks the profile of its bed should have a gentle and continuous slope downwards to base level. The existence of irregularities, of breaks in the smoothness of the slope, means that the work of excavation has not proceeded far, that the river is young. But it is not necessary to proceed to the laborious drawing of a profile in order to determine the extent to which the process of excavation has been carried. The existence of rapids, of waterfalls, the alternation of swift and slow-flowing reaches are all proofs that it has not been carried far. In short, if a river is navigable, the navigable reach at least is mature; if it is capable of furnishing power, that region at least is youthful. If, as sometimes happens, the middle course is navigable and slow-flowing, and the lower course broken by rapids and falls, then the probability is that earth movements have occurred, so that the two regions are of different age. This is a condition which occurs relatively often in the case of large rivers.
One other point is worth notice, because it illustrates another way in which the analogy of youth and maturity holds good. The youthful river, with its interrupted slope, its lakes and falls, does not permit the water to flow off with the same regularity as the mature river with its smoothed outlines. The mature river is thus a more perfect instrument of drainage.
It is not necessary for our purpose to consider in detail the characteristic forms of river erosion. It may be sufficient to notice that rapids and waterfalls are due to the varying hardness of the rocks forming the bed of the river, and that the normal course of events is the transition from waterfall to rapid, and from rapid to stream flowing quietly at the bottom of a rocky gorge. Long gorges or canyons tend to occur in regions where river erosion is not greatly assisted by the other eroding agents. As a general rule, as the river cuts its way down, the other agents cut back the walls so exposed, so that a wide valley is formed.
But a river does not only eat out its bed in its valley track. A necessary consequence of this erosion is that it is also able to eat back the slope on which it is rising, as a result of the smoothing out of the curves of its bed, so that its source retreats further and further into the mountain. In regions of abundant rainfall every slope is abundantly supplied with streams, and therefore those streams which cut back their region of origin most rapidly will necessarily encroach upon their neighbours’ territory. They therefore tend to tap some of the tributaries of the other streams, a phenomenon which has sometimes considerable human importance, and has been extensively studied of late years under the name of river-capture.
Some examples may serve to make the phenomenon clear. Every one who has travelled up the Rhone valley in Switzerland has noted the enormous number of lateral streams, of all sizes, which tumble down the mountain sides into the Rhone. These streams on, e.g., the south side, are, roughly speaking, parallel to each other, and to a large extent enter the main stream independently. That is, for the most part they are very youthful streams. In some cases, however, e.g. in the case of the Dranse and the Visp, the drainage is of a more advanced character, and we find a large stream with tributaries of considerable size as distinct from mere torrents. A glance at any great river system on the map, e.g. the Mississippi, the Amazon, etc., will show that the condition of a great stream with many tributaries is normal in a district where the drainage is of the developed type. How are the two conditions, that of numerous parallel mountain torrents and that of a great river system, related to one another? There is no doubt that capture, the encroachment of one stream upon the territory of another, has played an important part in the process.