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Valleys

Valleys, relative depressions among hills, are of various origin. They may occasionally result from the opening of earthquake-fissures or from the falling-in of the roofs of caverns; but however originating or determined as to the direction, they owe their contrours most entirely to erosion by sub-aerial agents, rivers, rain, frost, etc. Where they coincide with synclinals, as in the symmetrical flexors of the Swiss Jura, such as Ballsthal, they might truly be termed valleys of elevation; but this term is applied, on the contrary, to such cases as the Valley of the Weald or the Vale of Woolhope, which lie between the escarpments on a planed-down anticlinal. Though subterranean drainage may produce such valleys as the chines or bunnies about Bournemouth; or intermittent springs or "bournes," where the plane of saturation only rises temporarily above the surface of the ground, may originate such dry valleys as those in our Chalk area, most valleys are river-valleys. The general direction of these may be determined by lines of fault, as in that of the Jordan and perhaps in the four pairs of transverse valleys in the Weald, to wit, the Wey and the Arun, the Mole and the Adur, the Darent and the Ouse, and the Medway and the Cuckmere respectively. These eight river-valleys and those of the south of Ireland will illustrate the frequently greater antiquity of valleys of hills or other existing surface features. The contours of valleys depend partly on the composition and structure of the rocks and partly upon the varying intensity of the various factors in erosion. Thus, as a rule, the harder the rock the narrower and steeper the valley. Joints facilitate the cutting of ravines and the formation of such buttresses and pinnacles as adorn the dales of Derbyshire. The passage of a river from a harder rock on to the outcrop of a softer one will commonly result in a waterfall (q.v.), which, as it is gradually cut back, will give rise to a chine, gorge, or defile; whilst conversely the passage of the river from a softer to a harder rock may cause its expansion into a lake (q.v.) behind the barrier, as was once the case with the Medway behind the ragstone barrier at Yalding and behind the chalk barrier at Snodland. The steeper the slope of the surface, the straighter the course of a river, the greater its velocity and vertical cutting power. Though interrupted by cascades or leaps over rocky ledges, it will consequently flow mainly in the narrow steep-sided channel. Horizontal stratification may facilitate this type of erosion, whilst a general rising of the land will certainly do so. If the rainfall of an area be deficient, the channel will be not only deep but precipitous, as are the canyons (q.v.) of the Colorado. On the other hand, in softer rocks or where rain is plentiful, broader valleys with sloping sides may result. Where the ground is nearly a plane, the river-course may wind excessively, the valley occupied at one time or another by the stream being of great width, as in the cases of the lower Thames or the Indus. In such windings river-cliffs often occur on the concave bank, but seldom on both banks simultaneously.

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