296 E. C. ANDREWS. 



intervening "treads," 1 such "treads" generally possessing 

 a flattened or even basined floor. Afterwards came reces- 

 sion of the ice and then apparently two later but much 

 smaller readvances. 



Taking their history together, they seem to have been 

 productive partly of corrasion and partly of aggradation. 

 Along the main wide valleys which possess marked declivi- 

 ties they appear to have formed small glacial canons by 

 recession. The latter terminate in cirques enveloped by 

 the older forms. Frequently two glacial canons may now 

 be observed trenching an old or single glacial floor. (The 

 Upper Evolution Valley for example). 



Along the spurless and evenly sloping sides of the valleys 

 formed by the previous ice flood, the reduced glaciers 

 formed a long line of erosion, the line so made correspond- 

 ing with the surface limits of the glacier. Side valleys 

 having steep declivities along "narrows" also show the 

 erosion lines well. These are the " schrund lines " men- 

 tioned by Gilbert (Systematic Asymmetry* p. 582). 



A similar end result may be seen along the margins of 

 fairly rapid water streams, when confined between soft 

 banks. The strong central current causes an undulatory 

 motion to be set up towards the sides, where a wave of 

 translation is set up, resulting in the formation of a bench 

 and a line of erosion. To Professor R. A. Daly the writer 

 is indebted for an explanation of the moraine-like material 

 which covers these benches, and the still later talus cones 

 over them in turn. When the Glacial Period was waning 

 snow banks still clung to the benches and became laden 

 with debris, with the final disappearance of these snow 

 banks the debris was left, and talus formed thereon in 

 turn. 



1 Many of the present " steps" and " treads " are the result of the 

 later ice visitations. 



