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however slightly, above the general surface. We may suppose, for example, that the outcrops of the limestones, (e e) would form low ridges, rising, it might be, only a few feet or yards. Such slight inequalities would suffice, however, to divert the waters to right or left. The rivers and streams being
Fig. 16. Diagram Map Of Plateau Of Erosion.
t r, low ridges formed by outcrops of limestone, which are seen in section at the side.
turned in this manner out of their direct course would be compelled to flow along the outcrops until depressions in the ridges allowed them to resume their original direction.
After such a drainage-system had been well established, and the whole surface of the land had been subjected to the action of the various epigene agents of change for some protracted period of time, the inequalities of surface would become greatly accentuated. The regions occupied by "softer" rocks would be generally lowered, so that the outcrops of the harder beds would stand up more and more prominently. These, however, would not remain unchanged. On the contrary, each bed of hard rock, constantly undermined by the wearing away of the softer underlying strata, would continue to recede at its outcrop. This retreat would be most marked in places where the rivers flowed along the base of the escarpments. But even where rivers were absent the escarpments
Fig. 17. Section Across Reduced Plateau Of Erosion.
The upper dotted line represents original surface of plateau as shown in Fig. 16.
would still mark the outcrops of the harder beds. These, no doubt, might not be so prominent as the others, and would not retreat so rapidly, but they would nevertheless come to form striking features in the landscape. In a word, the region would eventually be traversed from left to right by pronounced lines of escarpment rising to many hundreds of feet above the low grounds at their base, and falling away in a long gentle slope in the direction of the dip. When these land-forms were fully developed a section across the reduced plateau would show the structure seen in Fig. 17.
In the case we have been considering the surface of the plateau of erosion is inclined in the same direction as the dip of the strata. Consequently all the escarpments face the water-parting of the region, and all the dip-slopes sink towards the sea. But the surface of such a plateau may be inclined against the direction of the dip; the outcrops, instead of facing the water-parting, may look seawards. Nevertheless, should hard beds be intercalated amongst more yielding strata, escarpments are certain to make their appearance under the influence of denudation, and
F1g. 18. Long1tud1nal Sect1on Of R1ver-course.
River flowing from a to b; h, outcrop of hard stratum ; Jj», shales; wl, position of waterfall when river-bed has been eroded to the level ll ; w1, position of waterfall when river-bed has been eroded to the level /V
may become quite as prominent as in the case we have just been considering. Nor will the character of river-valleys excavated in the direction of the "strike " of the strata differ; cliffs will tend as before to be developed on one side, and gentle slopes on the other. But in the river-courses that traverse the strike more or less at right angles we shall meet with certain marked contrasts. In regions where the rivers flow in the same direction as the dip of gently inclined strata, waterfalls are not readily formed. When the outcrop of a relatively hard bed is encountered the overlying softer rocks may be rapidly
washed away, and the surface of the underlying hard bed be exposed. At most, however, this simply gives rise to a rapid, which can only approach the character of a waterfall when the strata are inclined at a high angle. But when the strata dip up-stream the conditions are reversed. The outcrop of every hard ledge then gives rise to a cascade, and should the hard rocks attain a considerable thickness a notable waterfall may be produced. In the diagram annexed (Fig. 18) the upper line shows the course of a river (a-b) flowing across a series of strata inclined at a low angle upstream. At h we see the outcrop of a bed of hard sandstone or other relatively durable stratum, underlaid and overlaid by soft shales. It is obvious that the river cannot lower the surface of the overlying soft shales (f-) much below the outcrop of the hard stratum. So long as that endures the beds at s* are safe. It is otherwise, however, with the underlying shales (s). These are more or less rapidly eroded, and in the process of their removal the superjacent hard stratum is undermined, and from time to time gives way along its joint-planes. In this manner the waterfall (wl) gradually retreats further up the valley (iv1), and a gorge comes into existence.
Thus in the river-courses of a plateau of erosion, composed of gently inclined strata with an up-stream dip, waterfalls tend to be developed at the outcrops of intercalated hard beds. But, as erosion proceeds, these waterfalls retreat up the valley, and so are gradually replaced by gorges.
Now it may be said generally that in all regions composed of gently inclined strata, amongst which relatively hard beds are intercalated, escarpments and dip-slopes are developed by denudation. When the dip of the strata is persistent over a wide extent of country, we shall have more or less prominent escarpments traversing such a region continuously for miles. The escarpments will obviously vary in character with the angle of dip and the nature and thickness of the rocks. If the hard bed or beds be of no great thickness and the dip high, the resulting escarpment and slope will constitute a somewhat narrow ridge; but if the thickness of the hard beds be very considerable and the dip gentle, the escarpment may assume the form of a belt of plateau or a range of high ground, having a more or less diversified surface. England supplies some excellent examples of the kind. The general inclination of the strata between the borders of Wales and the North Sea is easterly, at a low angle; consequently, as we walk in that direction we cross the outcrops of several great geological systems. These are built up of sedimentary rocks, some of which are relatively soft and yielding, such as clay and shale, while others are harder and generally more porous, such as limestone, chalk, etc. Hence in time the latter have come to form a series of more or less prominent escarpments or belts of high ground, separated by broad tracts of gently undulating low ground. Starting from the foot of the Malvern Hills, and proceeding in an easterly