But if caves have been thus excavated, it is obvious that ravines and valleys in limestone districts are due to the operation of the same causes. If, for instance, we refer to [Figures 1] and [6], we shall see that the open valley passes insensibly into a ravine, and that into a cave. The ravine is merely a cave which has lost its roof, and the valley is merely the result of the weathering of the sides of the ravine. There can be no manner of doubt but that, in both these cases, the ravine is gradually encroaching on the cave, and the valley on the ravine; and if the strata be exposed to atmospheric agencies long enough, the valley of the Axe will extend as far as Priddy ([Fig. 1]), and that of Dalebeck to the watershed above the Gatekirk cave ([Fig. 6]).
In the same manner the lofty precipice of Malham Cove, near Settle, in Yorkshire ([Fig. 8]), is slowly falling away and uncovering the subterranean course of the Aire. Eventually the ravine thus formed will extend as far as Malham Tarn, and the Aire flow exposed to the light of day from its source to the sea.[34]
Fig. 8.—Diagram of Source of the Aire at Malham.
This view is applicable to many if not to all ravines and valleys in calcareous rocks, such as the Pass at Cheddar, or the gorge of the Avon at Clifton, and those of Derbyshire, Yorkshire, and Wales. And since the agents by which the work is done are universal, and calcareous rock for the most part of the same chemical composition, the results are the same, and the calcareous scenery everywhere of the same type. In the lapse of past time, so enormous as to be incapable of being grasped by the human intellect, these agents are fully capable of producing the deepest ravines, the widest valleys, and the largest caves.
This view of the relation of caves to ravines was so strongly held by M. Desnoyers, that he terms the latter “cavernes à ciel ouvert.” I arrived independently at the same conclusion after the study of the scenery of limestone for many years.
In many cases, however, in northern latitudes and in high altitudes, the ravine or valley so formed has been subsequently widened and deepened by glacial action. That, for instance, of Chapel-en-le-Dale bears unmistakeable evidence of the former flow of a glacier, in the roches moutonnées and travelled blocks that it contains. To this is due the flowing contour and even slope of its lower portion.
The pot-holes and “cirques” in calcareous rocks with no outlet at the surface, may also be accounted for by the operation of the same causes as those which have produced caves. Each represents the weak point towards which the rainfall has converged, caused very generally by the intersection of the joints. This has gradually been widened out, because the upper portions of the rock would be the first to seize the atoms of carbonic acid, and thus be dissolved more quickly than the lower portions. Hence the funnel shape which they generally assume, and which can be studied equally in the compact limestone or in the soft upper chalk. They are to be seen on a small scale also in all limestone “pavements.” Sometimes, however, the first chance which the upper portions of the funnels have of being eroded by the acidulated water, is more than counter-balanced by the increased quantity converging at the bottom, and the funnel ends in a vertical shaft. If the area in the rock thus excavated be sufficiently large to allow of the development of a current of water, the mechanical action of the fragments swept along its course will have an important share in the work, as we have seen to be the case in Helln Pot.