Fig. 5.—Sketch-map of northern England, to show the position of the Tyne and Aire Gaps, and the peculiar character of the rivers of Yorkshire. The black areas are heights above 600 feet.
One other important point in connection with river-capture has been already suggested in the account given of the Feshie. In the little sketch-map we see clearly the shift of the watershed to the east. The ultimate cause of this shift is doubtless the fact that in Great Britain the rainfall diminishes to the east, so that, generally speaking, the westerly streams have more erosive power than the easterly. But the special interest of the case is simply that it may serve to suggest a fact not at first sight obvious, which is that water-partings are excessively unstable features. One set of streams is continually striving to encroach upon the others, and by capturing their headwaters to reduce their erosive power. A very striking example of capture on the large scale is seen in southern Patagonia, where the water-parting does not lie near the summit of the chain of the Andes, as might be expected, but considerably to the east, the western streams (or glaciers) having captured all the headwaters of the eastern streams, which lie in a region of much lower rainfall.
The net result is that running water not only scours valleys in the sides of mountain chains, but also, sooner or later, wears away the crest itself, and with the assistance of the other agents of denudation tends to reduce the mountains to plains—or at least “peneplains.” The deduction is, of course, old enough, but the recent emphasis placed upon river-capture helps us to realise it, showing us the actual “shift of the divide,” or, in other words, the wearing down of the summit levels.
This is a theoretical matter, but there is another point which has practical significance. Referring once again to the sketch-map on [p. 43], we note that just at the sharp bend in the Feshie, that is, at the elbow of capture, there is a narrow region, crossed by the boundary line, which was once traversed by the headwaters of the Geldie, but is now a dry valley. Such “gaps,” as they are called, are present where recent capture has occurred, and where they occur in hilly country they sometimes form useful passes, permitting the construction of an easy road across the hills. A good example is the Aire Gap (see [fig. 5]) in the Pennine range of Great Britain, apparently connected with the fact that the Ribble has captured the headwaters of the Aire. Another interesting example is the so-called Tyne Gap, that breach in the Pennines which occurs near the present head of the South Tyne; it was traversed by the Roman wall, and is now crossed by the road and the railway from Newcastle to Carlisle.
As we shall see, ice appears to have this power of cutting passes through mountain chains to a much greater extent than running water; but here, as in many other respects, there does not appear to be a sharp breach between the action of the two.