148 



WORK OF SNOW AND ICE 



attached debris on the surface; (3) the slope of the surface; (4) the 

 thickness of the ice; (5) its rate of movement; (6) the resistance of 

 the rock; and (7) the amount and kind of debris the ice carries. 

 The effect of most of these conditions is evident, but the last two 

 call for a word of explanation. 



So far as concerns the resistance of the rock, it should be noted 

 that resistance is not a matter of hardness simply. Rock which is 

 affected by cleavage, whether joints or bedding planes or both, is 

 eroded readily, expecially on steep slopes, even if very hard. In 



Fig. 145. Striae on bed-rock. Kingston, Des Moines County, Iowa. 



such situations, the removal of rock in large blocks (plucking) is 

 probably more important, on the whole, than wear by the debris 

 carried. 



Clean ice passing over a smooth surface of solid rock would 

 have little effect upon it; but a rock-shod glacier abrades the same 

 surface notably. The effect of this abrasion is shown in the grooves 

 and scratches (stria) which the stones in the bottom of the ice inflict 

 on the surface of the rock over which they pass (Figs. 145 and 146). 

 At the same time, the stones in the ice are worn by abrasion both 

 with the bottom, and with one another (Fig. 147). It does not fol- 

 low, however, that erosion is greatest when there is most material 

 in the bottom of the ice; for with increase of debris there may be 



