CORBASION BY GRAVITY STREAMS. 209 



profiles which had been developed by water action were 

 productive of channel forms similarly shaped and similarly 

 situated to those previously supposed to have been peculiar 

 to valleys which had been but recently deglaciated. Hence 

 arose the idea that comparative stream studies might 

 suggest the explanation of the topography of recently 

 deglaciated valleys ; not indeed was it thus thought that 

 ice, water, viscous and rock masses would be ascertained 

 to move and act in the same way, but rather that some 

 important principles might be found common to all ; prin- 

 ciples possibly quite familiar to everyone in connection 

 with certain stream types but not suspected in connection 

 with other and less mobile types. 



The following papers are illustrative of principles so 

 found, and descriptive of the land profiles formed which 

 depend on the action of such principles. Thus it was 

 ascertained 1 for water channels that trenches with straight 

 sides, hanging valleys, basined floors and valley heads of 

 cirque-like appearance were rapidly developed as the result 

 of heavy water action, such as may be seen in arid or other 

 countries during thunderstorms, or even rainy seasons, and 

 that decrease of the stream velocity was attended by 

 relative incompetence of 2 the stream with: — 



1. Aggradation of the channel basins. 



2. Readjustment of the channel grades. 



3. Excavation in the stream debris of miniatures of 



the profiles formed during the flood stages. 



With these may be compared the forms formerly supposed 

 to be peculiar to valleys recently deglaciated. Thus the 

 fiord and Alpine lake basins, the facetted and much 

 demolished spurs, the cliff-feet or cliff-bases rectilinearly 

 disposed, the hanging valleys and the large cirque forms 

 both along Alpine Valley bases and heads, are all apparently 



1 Andrews (b). 2 First announced scientifically by Gilbert. 



