15^ 



the waste of the beds beneath. Some good examples of the 

 different rate of weathering of the same bed of shale where exposed 

 to the direct action of running water, and where affected only by- 

 weathering, are found about the waterfalls or "fosses" in the Dale 

 District. Under the waterfall the shales that there underlie the 

 sandstone, as this does the limestone of the foss, are kept in the 

 condition most favourable for their rapid disintegration, so they 

 are quickly cut back beneath the harder beds that form the edge 

 of the fall. But at the outer end of the ravine caused by the 

 gradual recession of the waterfall, subaerial denudation has accom- 

 plished so little, notwithstanding that a rapidly-flowing stream is at 

 hand, that the difference between the rate of recession of the foss 

 and that of the sides of its ravine in one case is about as forty to 

 three. In other words, while that waterfall has cut back forty feet 

 each cliff it has left has receded only eighteen inches. 





Fig. 9. 



Diagram to illustrate the ratio between the rate of formation of a ravine under 

 the action of a waterfall, and that of the widening of its sides, where these are 

 not exposed to the direct action of falHng water. The figures given in the 

 diagram are, in round numbers, those taken at Moasdale Force in Wensleydale. 



The particular instance here referred to is, doubtless, an extreme 

 case where the beds overlying the shale are more than usually 

 durable ; but it serves to prove that even where there is a rapid 

 stream flowing, the denudation of shale does not go on very 

 rapidly unless the stream actually flows close to the outcrop. 

 Where limestone is the rock directly overlying the shale, this last 

 is usually cut back much faster, because the surface water finds an 

 easy passage into it through the weathered joints of the harder bed 

 above. In such cases the difference between the width of the 

 outer end of the ravine and the width close to the fall, compared 



