122 E. C. ANDREWS. 



only of the debris in its channel is not attacking the 

 channel structures, but on the other hand the debris in 

 such a case actually protects its channel profiles. Stated 

 in terms of mechanics, then such a stream is not doing 

 "work" on its channel structures. Thus a glacier which 

 overrides its ground moraine, or an ordinary stream which 

 enters a deep hole in its course without stirring the 

 pebbles and boulders at the bottom, or a torrent which forms 

 a great alluvial fan on the floor of a senkungsfeld are all 

 magnificent examples of streams which at such points are 

 not corrading but are aggrading their channel structures. 



In periods of great stream volumes in mountainous areas 

 the channel debris overlying any particular point is carried 

 over that point as a whole, while during a period of "fresh" 

 or ordinary "flood" the channel debris is only moved as a 

 whole in a few places. In times of such moderate stream 

 volume the debris in the deeper holes and on the edges of 

 the larger cutting curves is not moved as a whole, and the 

 stream accomplishes but little work on the channel struc- 

 tures. The word flood may then be defined in terms of 

 mechanics for mountain tracks ; in the case of the rock 

 channel it is that stream volume which accomplishes 

 "work" on its channel sides and base by moving the channel 

 debris as a whole over any point of such channel structures. 

 In this case the curve of corrasion is concave to the sky» 

 The time factor is here not taken into consideration. 



A stream again in its course of development may have 

 been affected by the development of a deep senkungsfeld 

 across its path. In this case it will attempt to construct 

 a bridge across the senkungsfeld along which it may trans- 

 port its load. Thus the senkungsfeld base becomes heavily 

 aggraded. In this case, even during periods of heaviest 

 flood, corrasion is only accomplished on the upfaulted block 

 while the downthrown block is actually protected. During 



