310 



PROF. W. MORRIS DAVIS ON 



[Aug. 1909, 



grade, and therefore no deepening can again take place (except 

 that which slowly accompanies the wearing down of the whole 

 mountain-mass in old age) unless the stream is revived by uplift. 

 Such an uplift is particularly needed here as a cause of valley- 

 deepening, because observation has abundantly shown that the 

 extension of a glacier down its valley is usually accompanied by 

 the aggradation of the valley-floor down-stream from the glacier 

 with waste brought by the glacial stream ; hence all the more 

 must uplift be assumed, if glacial extension is associated with 

 valley-deepening. 



Assuming that an uplift takes place, let the deepening and 

 widening of the chief valley go on until the form shown in fig. 12 

 is produced. This change will, as before, be associated with an 

 appropriate deepening and widening of the side valleys and with 

 the development of accordant junctions between smaller and larger 

 streams ; except in special cases where a side-glacier just reaches a 

 side-valley mouth, as in the left front of fig. 12 (below). Additional 



Pig. 11. — Diagram showing an 

 extension of the glaciers of 

 fig. 9 (p. 808). 



Pig. 12. — Diagram showing the 

 effects of normal erosion on 

 fig. 11. 



repetitions of similar changes may take place, and finally the ice 

 may be supposed to extend far beyond the limits of the figure 

 during the climax of the glacial climate, protecting and preserving 

 all the valley forms that had been produced during its successive 

 advances. 



Let the retreat of the glaciers be now considered, and let a long 

 pause occur when they have shrunk to moderate dimensions. 

 If the land still holds the altitude that it had gained by previous 

 uplifts, it would follow that the production of a new rock-step at 

 the end of the shrunken glacier will be accompanied by the more 

 or less complete obliteration of all the rock-steps previously formed 

 farther down the valley, and by the reduction to accordant junc- 

 tions of all the hanging lateral valleys from the mouths of which 



