169 



with the distance between the two points thus measured, gives 

 very nearly the ratio between the rate of denudation of any given 

 rock on the one hand by the direct action of streams, and on the 

 other by that of ordinary weathering. 



If then, so Httle denudation of a rock as easily worn as shale has 

 been accomplished in Post-Glacial times by the rapid streams of 

 the Dale District, where these streams are absent we should be 

 prepared to find that the rate of denudation is so slow as to 

 produce results that are hardly perceptible. Amongst many facts 

 bearing upon this same point may be mentioned the occurrence of 

 strise dating from the climax of the glacial period, which are yet 

 found in many places in this district within a few feet of the 

 outcrop of a bed of shale, in the relative position shewn by B, in 

 Figure lo. It is obvious in such a case that the horizontal distance 

 between the ice markings and the base line of the shale marks the 

 greatest distance that this can have been cut back in Post-Glacial 

 times. 



The thinner kinds of sandstone, especially where these are much 

 split up by beds of shale, seem to go to pieces very readily ; but 

 upon the more compact, blocky, and little-jointed kinds, ordinary 

 weathering seems able to produce very little effect. A very good 

 example of this is to be found at Moasdale Foss in Wensleydale, 

 Fig. 9, a waterfall caused by the superposition of a hard and blocky 

 sandstone upon a bed of soft and thinly-laminated shale. The 

 length of the ravine leading to the fall is nearly eight hundred feet, 

 measured from its outer end to the fall itself; while the difference 

 in the width of the ravine at the two points measured is only sixty 

 feet. Yet in this instance a rapid stream flows within a few yards 

 of the foot of the scars, which have thus receded only thirty feet 

 on each side since the ravine was formed. There is good reason 

 for thinking that the entire excavation has taken place in Post- 

 Glacial times. The above remarks relative to the nearness of 

 glacial strise to the outcrop of higher beds of shale apply equally 

 to the accompanying sandstones in similar positions ; so that we 

 get direct evidence that some of the sandstone scars have not been 

 much altered in form since the close of the Glacial Period. 



