THE EVOLUTION OF DERBYSHIRE SCENERY. 207 



boulders carried by the torrent was particularly noticeable 

 especally m widening out the swallow hole. Fig. 3 illustrates the' 

 above; part of the water of a higher valley is diverted by a short 

 underground passage to a valley on a lower level. 



Fig. 3. 



We have several most interesting local examples of this action 

 one of the best known being that of the Peak Cavern at 

 Castleton. We must regard this magnificent cavern as having 

 been produced by the erosive action of the water derived from 

 the district lying to the west and south-west of Castleton The 

 surface water finds its way into the limestone mainly along the 

 junction of the Yoredale Shale from Windy Knoll to at least as 

 far as Perry Foot. The trough-shaped valley mainly constituting 

 this area was previously drained by a stream, the bed of which 

 may now be seen, dry and deserted, running in the direction of 

 the Mersey, in whose watershed the area in question lies. Along 

 the bed of the valley are a number of "swallow holes," into which 

 the water disappears, finding its way into the Derwent through 

 Its subterranean channel. The outlet was previously through 

 Peak Cavern entirely, and it is to the erosive action of this water 

 that the cavern owes its existence. At the present time, however 

 but little water finds its way through the mouth of the cavern 

 except in flood-time, a lower course having been cut finding 

 Its outlet at the spring near the mouth of the cavern, known as 

 Russet Well. A section of the cavern, roughly indicating the 

 course of the underground water, is shewn in Figure 4 As 

 Illustrating the manner in which river gorges are sometimes 



