"steps," "treads," and cirques on a steep thalweg 

 (declivity). (a) Preglacial thalweg, (b) AA^A" Youthful 

 glacial thalweg. N W N" Matured glacial thalweg. 



Now the velocity of the preglacial stream either had a 

 uniform velocity or it had not. If the former, then the 

 channel base was uninterrupted when all other things were 

 equal, but if the latter, then the channel base was inter- 

 rupted and the interruptions were proportioned to the 

 strength of the stream. But channel constrictions imply 

 increased velocity, therefore reduced grades (or channel 

 base interruptions) will occur at the points B, C, D and E. 

 This is of course on the assumption that homogeneous 

 structures are being acted upon. In nature rocks are never 

 quite homogeneous, and the differential strengths of the 

 structures acted upon causes the incising stream to exca- 

 vate a basin at one place and occupy a declivity at another 

 place. In nature the interruption of grade would occur a 

 little down stream of the constriction, that is, at the point 

 where the softer rock bar was most opposed to the direc- 

 tion of the stream. 



