C being much lower than A — the descent of the profile B O 

 is attended by rapid increase of stream strength, and thus 

 sapping becomes a most important factor in the production 

 of BO. 



Again the lee seite of a roohe moutonnee must suffer less 

 from abrasion in proportion as its slope is increased, until 

 iu the extreme case when the profile B O is vertical, the 

 abrading action of tht; glacier vanishes, and corrasion is at 

 a maximum by sapping processes. This is so because of 

 the tendency of a stream to parabolic motion. A homely 

 illustration may be borrowed from the ordinary methods 

 of the petrologist. in the accompanying iigure (Fig. 11) a 



block, say of granite i< represented, from which it is 

 desired to secure specimens. Will the collector steadily 

 hammer at a point such as A so as to secure the best results, 

 or will he strike in directions such as those indicated bv 

 the arrows BCD? By hammering at A he will simply 

 compress the granite structures, unless indeed he possess 

 enough force to smash the block clean through from A to 

 I) or 13. if he strikes long and hard enough at the end A 

 he will produce an abraded surface in the direction A B. 

 On the other hand by equally heavy blows delivered at B C 

 and D, he will cause a relatively rapid recession of the 

 face, because instead of compressing the structures at 



