and Marlborough Railways. 



187 



To correct any erroneous impression, which the exaggeration of 

 ; the vertical scale compared with the horizontal scale, may give rise 

 to, it may be mentioned that the angle with the horizon of the 

 steepest part of the section towards the valley is 25°, and towards 

 the coomb 20°. Both these angles are considerably greater than 

 the average slope of the sides of the valley. 



It is impossible to say how much of the hollowing out of these 

 ! and similar coombs is due to marine denudation, acting at the sea 

 level as the land rose ; and how much to atmospheric causes, such 

 as the rain, and the surface drainage running off down the slopes, 

 and away from the valley ; which latter causes have been in opera- 

 tion as long as the land has been above the level of the sea. It is 

 probable that faults or lines of fracture in the chalk, may have 

 given the first direction to both these agencies, and so far have 

 been the primary cause of some of the coombs. 



As the great anticlinal fold of chalk rose above the waters, there 

 would be a wearing down and excavation going on on the flanks, 

 at the same time that the valley was being eroded among the fis- 

 sures along the axis. The denudation on the flanks would however 

 be at work on the backs of the beds, owing to their outward dip, 

 and would in general produce less marked results than in the 

 central valley, where the same outward dip would give much 

 greater undermining power to the action of the sea. 



The direction, and the amount of dip in rocks now exposed to the 

 waves, both exercise a great influence on the rate at which the 

 strata are undermined and brought down to the sea level, and it is 

 important to bear this in mind when considering the effects of 

 former marine denudation. 



As a result of the different rate of denudation, the upper ends of 

 the coombs would sooner or later be left above the sea level, 

 and exposed to the action of atmospheric causes only, while the 

 sides of the valley were still being undermined and eaten back by 

 the sea ; so that the " col " between the valley and the head of the 

 coomb would be continually lowered and narrowed. 



A little consideration will shew that the result would be the 

 same, however great a share in the hollowing out of the coombs we 

 allot to atmospheric influences. 



