J. G. Goodchild — Glacial Erosion. 325 



it is of the ordinary Mountain Limestone type — a more or less com- 

 pact, grey rock, occurring in "posts" from a few inches to several 

 feet in thickness. Each bed of limestone is almost invariably over- 

 lain by more or less shale, and nearly as often it lies directly upon: 

 sandstone ; so that the order of the beds from the top of the series 

 to the lowest beds seen is, soft shale upon limestone, which, in its 

 turn, lies upon a still harder bed of sandstone. 



Under the action of the weather each kind of rock behaves differ- 

 ently. Where the outcrop is of shale, and forms a steep bank along- 

 side a stream, the numerous divisional planes help to make the rock 

 go to pieces in a very short time ; so that, in such a case, whatever 

 the overlying beds may be like, the bank is not long in being cut 

 back. But where the outcrop forms a gentle slope that is out of the 

 way of constantly running water, shale that is not more than usually 

 sandy decomposes into a tough elay, much of whieh remains at the 

 surface, and thereby greatly helps to lessen 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- 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 are kept in the condition most favourable for their rapid de- 

 composition, 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 

 that has been caused by the gradual recession of the waterfall, so 

 little has subaerial denudation accomplished, notwithstanding that 

 a rapidly flowing stream is at hand, that the difference between the 

 rate of recession of the fall and that of the sides of the ravine is 

 occasionally as 40 to- 3. In other words, while the waterfall is cut- 

 ting back forty feet each cliff it has left recedes- only eighteen inches. 

 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 

 that overlies the shale, this is usually cut back much faster, because 

 the surface water finds an easy passage through the joints of the 

 harder bed. It will be interesting to compare the figures given 

 above with those obtained in similar instances elsewhere. In all 

 such cases the difference between the width of the outer end of the 

 ravine and the width close to the fall, compared with the distance 

 between the two points thus measured, will give very nearly the 

 ratio between the rate of denudation, on auy given rock, by stream 

 action, and that of ordinary weathering. 



If then, so little 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 ought to find the 

 rate of denudation so slow as to produce results that are hardly per- 

 ceptible. Accordingly, it is not uncommon to find glacial strias within 

 a few feet below the outcrop of a bed of shale ; in which case the 

 horizontal distance between the ice-markings and the base-line of 



