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LAND-FORMS IN REGIONS OF GENTL Y INCLINED
ESCARPMENTS AND DIP-SLOPES—DIP-VALLEYS AND STRIKE-VALLEYS—FORMS ASSUMED BY A PLATEAU OF EROSION—VARIOUS DIRECTIONS OF ESCARPMENTS—SYNCLINAL HILLS AND ANTICLINAL HOLLOWS—ANTICLINAL HILLS.
THE most characteristic land-forms met with in regions where the strata are inclined in some general direction are escarpments and dip-slopes, the former coinciding with the outcrops, and the latter with the inclination or dip of the strata. In such regions some streams and rivers not infrequently flow in the direction of dip, and thus cut across the escarpments, while others may traverse the land along the base of the escarpments.
The origin of these phenomena is not hard to trace. Let us suppose that some wide tract of horizontal strata has been elevated along an axis so as to form a considerable island. If the movement of elevation were slowly effected the sea would doubtless modify the land-surface as it arose, but for simplicity■s sake we shall ignore such action, and suppose that the new-born land exists as an elongated island, the surface sloping away at a low angle on either side of a somewhat flattened axis. (Fig. 12.) At first, then, the surface coincides with the underground structure —a dome-shaped land formed of dome-shaped strata. (Fig. 13.) It is obvious that the drainage will be in
Fig. 12. Map Of As Island Composed Of Dome-shaped Strata.
The strata an inclined is the direction of the arrows.
the direction of the dip of the strata—all the main rivers will take the quickest route to the sea. But as we cannot suppose that the surface of the new-made land would be without some irregularities, the streams and rivers would not actually follow straight courses.
Fig. 13. Section Through The Island Shown In Fig. 12.
Slopes of surface coincide with arrangement of strata.
On the contrary, it could not but happen that one stream would eventually join another, and in this way many might become tributaries of one or more large rivers. Thus we should have certain courses cut in the general direction of the dip, while others joining these would in some places go with the inclination of the strata, and in other places would traverse that at various angles. The strata consist, we shall suppose, of "hard" and "soft" rocks—limestones, sandstones, shales, etc., and they are well jointed at right angles to the planes of bedding. Thus, while the strata dip seaward, one set of joints is inclined at a high angle in the opposite direction—the other set cutting the strata in the direction of the dip. Now so long as the streams follow the dip it is obvious that they will tend to form trench-like valleys—the rocks will be undermined and give way along vertical joint-planes.
Fig. 14. Section Of River-valley.
The valley coincides in direction with the " strike " of the strata, i. *., it trends at right angles to the dip or inclination : <x\ cliff determined by joint; s j, springs; r, river.
We need not for the present consider the modifications arising from the varying character of the rocks. It is enough to remember that since they yield along the joint-planes, they tend to produce vertical or steeply inclined walls in the same manner as if they were horizontally bedded. But when the course of a stream is more or less at right angles to the dip of the strata, the valley it forms will not have the same trench-like aspect. On one side of such a valley the strata dip away from the stream, and when undermined they yield along the joints which incline inland. A cliff thus determined is not so liable to be broken down by the action of springs and frost. Underground water tends to move away down the dipplanes, so that no springs come out on the face of the cliff d (Fig. 14), which is only renewed from time to time by the undermining action of the river and the consequent collapse of the rock along a steeply inclined joint. On the opposite side of the valley the conditions are different. There the dip is towards the river—a weak structure, for the strata are easily undermined and sapped by springs, coming out along the planes of bedding (s, s). Hence they readily give way, their ddbris sliding and rolling towards the river. Thus valleys that coincide in direction with the outcrop of the strata will usually show a somewhat precipitous cliff on one side and a more or less gentle slope on the other.
We shall not follow the subsequent history of the erosion of our island in any detail. It is obvious, however, that it must pass through the same stages of erosion as any similar area of horizontally bedded rocks. The rivers and their multitudinous feeders will deepen and widen their valleys until the ground is cut up into a more or less numerous series of segments or blocks. But these will differ in form from those which are carved out of horizontal strata. Instead of flat-topped mesas and buttes and pyramidal-shaped hills, we shall have a series of heights presenting escarpments towards the watershed and long slopes in the opposite direction. (Fig. 15.) Eventually these will largely disappear, and the whole region will be resolved into a gently undulating plain of erosion.
Now let us suppose that this plain is upheaved and converted into a plateau, the surface of which has a very gentle inclination in the same general direction as the dip. (See Fig. 16, p. 78.) The section at the side of the map shows the geological structure. Here obviously the surface-slope is not so great as the
Fig. 15. Enlarged Section Of A Portion Of The Island Shown In Fig. 12.
Upper dotted line shows original surface; t ?, outcrops of " hard " beds forming escarpments.
inclination of the underlying strata; the plateau is therefore a plateau of erosion.
The map represents the course of a main stream with its tributaries. The trend of the drainage will naturally be in the same direction as the dip, and the rivers must therefore traverse the outcrops of the strata. Were the surface of the plateau quite even the waters would, of course, descend by a direct route to the sea. For various reasons, however, it is very unlikely that such should be the case. The strata had no doubt been planed down to a base-level, but some inequalities would still exist-—the outcrops of the most durable rocks would here and there project,