2IO THE EVOLUTION OF DERBYSHIRE SCENERY. 



Large roughly rectangular blocks detached from the roof occupy 



the floor, and thus the level of a cave is sometimes slowly raised. 



This is also illustrated by Rains Cave, and by the underground 



watercourse previously referred to as occurring in the Bagshawe 



Cavern. It will thus be seen, without quoting further examples, 



that the limestone is perforated with numerous conduits, 



opening now and again into large cavernous spaces, due to the 



solvent action of natural waters. In studying the scenery of 



limestone districts, the presence of these must be taken into 



account, for at some time or other these underground streams 



will be converted into overground streams. The general erosion 



taking place at the surface will at last convert these spaces into 



river gorges, brought about partly by the recession of the mouth 



of the cavern — as is well seen in the ravine extending from the 



mouth of the Peak Cavern — and partly by the collapse of the 



roof, when this has become too thin to keep in position. This 



will take place gradually, and the fallen debris will gradually 



be removed by the water. This will most probably account for 



the phenomena presented by some transverse valleys, as may be 



seen by reference to Fig. 4. An interesting example occurs in 



Wensley Dale, where a steep-sided ravine carries the drainage 



through the middle of a hill of smooth contour into Wensley 



Dale. The rounded contour must probably be referred to the 



action of ice during that period known to geologists as the Glacial 



Epoch. The ravine referred to abruptly interrupts the contour of 



this hill. Now there is nothing in the geological structure of the 



ground which affords any other explanation of a stream cutting 



its way straight through an opposing hill, than that it first made 



its way underground into Wensley Dale, and that at some period 



since the Glacial Epoch the roof collapsed. There are cases 



where a river suddenly passes from a broad alluvial valley into a 



wall-like cliff of limestone rising right in its path. We may note, 



for example, the case of the Derwent, which passes from the 



broad shale valley of Darley Dale to cut its way through the 



limestone which rises abruptly right across its path, on to 



Cromford. Here, it might be thought, is surely a case where the 



