i 5 8 



PHYSICAL GEOLOGY 



and that a very thin glacier or one with clear ice at its bottom may, 



indeed, not only be unable to wear down its rock bed, but may even 



override loose sand or glacial drift. 



In moving over a projection in its valley a glacier smooths off the 



side upon which it impinges (the stoss side) and plucks angular frag- 



^ ments from the lee side, often 



leaving it rough and jagged. 

 It also smooths rough sur- 

 faces into forms (Figs. 142, 

 143) which, because of their 

 rounded shapes, have been 

 given the name roches mouton- 



Fig. 143. — Diagram showing a projecting 

 rock smoothed and rounded on the side upon 

 which the glacier impinged (stoss), and rough- 

 ened on the lee side by plucking — a roche n ' ees (French for rock sheep), 

 moutonnee. Tr 117 i • 1 



It one looks down a glaciated 



valley, the smooth stoss slopes of the roches moutonnees are very 

 striking. If, however, one glances up the valley, the rough lee sides 

 of the roches moutonnees are often so conspicuous as to seem to con- 

 tradict the statement that glaciers deepen their valleys by abrasion. 

 Effect on the Material Carried. — Not only are the rock beds of 

 glaciers grooved by bowlders, scratched by pebbles, and polished by 



Fig. 144. — Glaciated pebbles. (After Blackwelder and Barrows.) 



clay, but these tools are, in turn, scratched and polished (Fig. 144). 

 If one axis of a glaciated pebble is much longer than the other, it is usu- 

 ally found that the striations are parallel to the longer axis. This is 

 due to the fact that the pebble was held in this position in the ice, as it 

 offered less resistance in this way than in any other. If the axes of a 

 pebble are approximately equal, it may have scratches running in 

 many directions, as this shape would enable it to turn more easily 

 in the ice and therefore to be carried onward in various positions. 



