210 E. C. ANDREWS. 



matched in miniature by the channel profiles formed by 

 flood waters. 1 Furthermore it was well known that: — 



1. Many glacierets and glaciers to day exhibit such 

 feeble flow phenomena that they have preserved their 

 original banded structure from head to foot. Along the 

 same valley for many miles below the present glacial foot 

 abundant signs of violent glacial action in recent times are 

 shown by the deep tortuous glacial grooves which wind 

 round and over the rock masses. (Such strong glacial 

 markings characterise basal constrictions in canons such as 

 those of the Oalifornian Sierras). 



2. Glaciers, in recent times, were of much greater volume 

 than they are at present. 2 This is implied by the state- 

 ments in the preceding paragraph. 



3. Present day glaciers, wherever they are amenable to 

 such examination, appear to be the least competent as 

 corraders at those places where, on the assumption of the 

 ability of glaciers to corrade channel structures, they 

 should be excavating most rapidly. (In this connection see 

 Culver and Fairchild quoted under Literature). 



These facts suggested for ice streams that increased 

 volume tended to produce increased mobility or plasticity, 

 and that increase of velocity was productive of corrasive 

 strength rising in a high geometrical ratio. 3 



But for the same channel this would imply a marked 

 decrease in plasticity or mobility upon the decrease of stream 

 volume. This would imply also a still greater reduction in 

 corrasive power. In other words, upon reduction of volume, 

 the glacier would now commence to aggrade the channel 



1 Andrews, (b). 



a See however, Gregory, p. 6, lines 19 - 21. Nevertheless in every 

 glacial region examined up to the present, the evidence of the pronounced 

 deglaciation of valleys is everywhere present. 



3 Gilbert has shown this for ordinary streams (a) pp. 89-91. 



