314 E. C. ANDREWS. 



A characteristic of the scenery is the intimate associa- 

 tion of these deep and strongly glaciated canons with 

 surrounding mountainous country which has not been 

 modified to any great extent by glaciation. An examin- 

 ation of the country reveals the fact that an ice sheet 

 moved over the district and accomplished but relatively 

 slight corrasion of the upland surfaces, while during a later 

 and minor development of valley glaciers the canons appear 

 to have suffered great modification of their profiles. 



Assuming that glaciers are capable of great rock corras- 

 ion and that the cirques, hanging valleys and long spurless 

 valley walls in the Blair Athol District are due to glacial 

 action, it remains to be seen how an ice cap could appar- 

 ently do such minor work only and yet a series of mere valley 

 glaciers could accomplish such great work as is here 

 evidenced. 



From a consideration of the mechanics of streams it is 

 evident that a stream may pass over an area of well 

 matured or senile topography, (however small or great) 

 without the production of any very pronounced corrasion, 

 at the same time that a younger channel which has been 

 already incised in the older surface may suffer so much 

 from the action of corrasion as to call for comment on the 

 part of even casual observers. 



For the stream, all other things being equal, becomes 

 more mobile under increased pressure ; the most definite 

 and continuous get-a-way for the stream occurs along the 

 thalweg of the incising gorge rather than along the associ- 

 ated upland surface, hence there is additional reason for 

 increased mobility of that part of the stream which flows 

 along the young channel. This is the analogy of the 

 varying energy between the backwater and the main 

 channel water of an ordinary stream in flood. As in the 

 case of the ordinary stream so in the case of all, the greatest 



