THE EVOLUTION OF DERBYSHIRE SCENERY. 205 



would be readily accounted for if streams ran at their bases, but 

 this is not the case. 'I'heir origin must be referred to two causes, 

 (i) The rapid disintegration and undercutting of the under- 

 lying shales, and (2) the regular recurrence of joints in the 

 grit by which it is divided into more or less rectangular blocks, 

 having little or no cohesion. As the siiale is worn away the 

 blocks of Millstone grit slide down as shown in Figure 2. The 



dislodged blocks, having no stream to remove them, strew the 

 slope for a considerable distance below the edge, as may be seen 

 along the Derwent Valley north of Baslow, and near the railway 

 cuttings at Padley. 



Reverting to the denudation of limestone areas, it has been 

 pomted out that the shape, and possibly in many cases the 

 direction of valleys in limestone strata, depend upon the natural 

 divisional planes, viz., the planes of jointing and stratification. 

 In many cases these joints give rise to other phenomena than the 

 production of valleys. Surface waters finding their way into joints 

 and crevasses gradually enlarge them by dissolving away the 

 rock, percolating further and further, until they at last find, perhaps, 

 an outlet at a lower level, often at a considerable distance from 

 the point of entrance. By-and-bye this subterranean channel 

 becomes widened by the constant trickling of the solvent 

 water, and one of the many springs with which our limestone 

 district abounds, is formed. Undoubtedly large areas in Derby- 

 shire must be perfectly honeycombed in this way, large under- 

 ground cavities being full to overflowing with the waters derived 

 from the surface. A consideration of the phenomena of some 

 of the better known springs would serve to make this clear. 



