250 E. C. ANDREWS. 



being equal, the more variable is the local corrasion ; hence, 

 as a corollary, the more marked are the interruptions of the 

 channel base. Now with a weaker stream flowing along 

 the same channel, there must be less marked work of cor- 

 rasion locally, hence those very spots which indicated the 

 intense action of the flood or heavy volume stage, must 

 now be selected for purposes of channel readjustment by 

 the weaker stream. The first care then of the reduced 

 stream will be both the partial filling of the great basins, 

 and aggradation of the flattish floors, until the bridge of 

 debris is raised sufficiently for transportation purposes; 

 heavy deposition will also take place along the old cutting 

 curves with partial filling of the basins there, and much 

 aggradation action will ensue working progressively from 

 the embayed wall toward the centre of the valley. 



Let us see if this point may not be made clearer as its 

 apprehension is vital to the proper understanding of present 

 and recent glacial action. In Fig. 10 (a) let A O B represent 

 a basin excavated below the old channel slope A Z B. 

 Formerly the slope AB appears to have represented a 

 stream channel base; then came either increased volume 

 of a similar stream type or a much stronger stream of 

 different type. This increase of stream strength, how- 

 ever produced, became gradually expressed by the growth 

 of the basin A O B excavated below the older grade 

 A A'B. 1 O represents the greatest depth of the upper 

 portion of the basin, and Z O the depth is the amount of 

 vertical cutting permissible to the stream until the point 

 B shall have been lowered. The basin grew by headward 

 recession and Z O progressively receded upstream. Z O is 

 also a variable ; it may have been greater or less, when at 

 some point nearer to B, than at present. It marks the 



1 We take the case of the basin preferably as showing the extreme 

 variability of stream corrasion locally. The case of a very flat floor pro- 

 duced by heavy stream volume would answer as well. 



