278 E. C. ANDREWS. 



mainly to the energy determined by the topographic con- 

 figuration immediately upstream, and there is apparently 

 no necessity to call in land subsidence or faulting to account 

 for their depths when a more simple explanation of the 

 phenomena (such as that of stream formation) is at hand. 



(b) Hanging valleys. — All other things being equal, the 

 channel of the main stream is cut down to a lower point 

 than that of the channel base of any tributary stream. 

 This arises from the fact that stream corrasion increases 

 in some high geometrical ratio with the stream velocity 

 (function of volume). The more senile the topography the 

 less pronounced this relative over deepening of the main 

 channel, while the stage of the youthful dissection of 

 plateaus is attended by more pronounced over deepening 

 of the main channel. The mighty " Hanging Valley " of 

 Alpine regions is simply a corollary to the main proposition 

 of the great over deepening by ice of the main canon. The 

 mere phenomenon, however, of the ''Hanging Valley" is to 

 be seen along any stream channel. 



The greater the volume of the stream and the greater 

 its velocity gained either by motion down declivities or 

 through valley constrictions, so much more emphasized 

 will be the over deepening of the main valley as compared 

 with that of the tributary valley. Therefore, under a con- 

 dition of small stream volume we have the base of the 

 tributary channel hung above that of the main channel, but 

 upon the increase of stream volume along the same channel 

 the tributary channel base is still further "hungup" with 

 respect to that of the main channel. 



Only in profound and constricted canons is the great 

 velocity obtainable which is necessary for the formation 

 of the great hanging valleys common to all Alpine regions. 

 It is the canon walls which allow of the great depth and 

 confinementof theice stream which then responds by greatly 



