THE EVOLUTION OF DERBYSHIRE SCENERY. 211 



river_found its way in the first place through the limestone, and that 

 the collapse of the roof converted the underground watercourse 

 into the Matlock Valley. There is not the slightest necessity for 

 such an assumption. The river course was determined before 

 the broad valley of Darley Dale had been formed, and the 

 cutting out of the valley in the shale and in the limestone must 

 of necessity have proceeded pari passu, for it is obvious that 

 material worn from the former must be removed through the 

 valley cut in the latter, and hence the rate of vertical erosion in 

 the shale would be controlled by the rate of erosion in the 

 limestone. At the same time, owing to the widely different 

 physical nature of the shale and the limestone, the valley cut in 

 the former is broad, while that in the latter has that peculiar 

 shape described and explained in my paper in last year's Journal. 

 In the case of the ravine cut through the hill on the south-west 

 of VVensley Dale, however, the case is quite otherwise, for the 

 rock is hmestone on both sides of the hill, which was obviously 

 rounded before the ravine had any existence, and so its origin 

 must be referred to the causes cited above. 



I have already mentioned landslips, which are really so 

 numerous locally as to form an important factor in the evolution 

 of scenery. We have the well-known cases of the slips of lime- 

 stone which have taken place at Crich, and Hob's House in 

 Moiisal Dale. A phenomenon of a somewhat similar nature 

 but more difficult of explanation, is to be seen near Abney, and 

 another at Alport. These are rock-movements of a somewhat 

 different nature to landslips of the Crich type. Let us consider 

 the example near Abney. A hasty survey of the district of the 

 Highlow Brook shows that the valley is cut through 

 a plateau of thinly-bedded shales (Shale Grit) resting upon 

 the black Yoredale shales previously described. The sides of 

 the valley are normally rather gentle and grass clad ; but in the 

 section of the valley (about a mile long) where the slips have 

 occurred, the southern side is usually precipitous, naked, and 

 separated from the brook, which here flows at the foot of the 

 northern side, by a gradually descending shelf, or " undercliff," 



