PHYSICAL GEOGRAPHY. 3§ 



In this investigation we have considered merely the effect of 

 Unequal amount of denudation on a single section. In nature this 

 denudation. j s n0 £ uniformly distributed, but has greater 



effect at certain places than at others, owing to variations in the 

 vigour of the attack, caused by the concentration of drainage in 

 greater or smaller stream channels, and to variations in the power 

 of resistance of the rocks, caused by variations in the degree of 

 induration, and more especially of the number of joints by which it 

 is cut. As a result of this the amount of the hard bed removed in 

 a given time will vary at different points, and the effect will be that 

 where the dip is horizontal we will find great irregularity in the 

 outline of the edge of the plateau, but none in its height ; where the 

 dip is vertical we will find no deviations from a straight line but 

 great variations in the vertical profile of the ridge, while at inter- 

 mediate angles the variations of either kind will be less, one or the 

 other preponderating according to whether the dip is under or over 45 . 

 From this it will be seen that the best marked ridges, that is 

 to say, those which vary comparatively least from a straight line, and 

 an even crest are formed not by vertical beds but by those inclined 

 at an angle of about 45 or more, and the effect is emphasized in 

 nature by the fact that in these circumstances the whole of the 

 denudation would not be from the end of the hard bed, but in part 

 from its upper surface too, as indicated in Fig. 4. Here the removal 

 of material from the dip surface throws the crest towards its original 

 position and, if the total amount removed is the same, lessens the 

 diminution of height. 



Fi 2' 4- ( 39 ) 



