252 GEOLOGY 



Conditions influencing rate of erosion. (1) A somewhat obstruc- 

 tive configuration of the surface over which the ice irfoves is neces- 

 sary for effective erosion. Ice wears a flat surface relatively little 

 since there is little for it to get hold of. Ice sometimes overrides 

 such a surface, burying its soil and even more or less of its herbaceous 

 vegetation. Erosion is probably at its maximum, so far as in- 

 fluenced by topography, when the surface is rough enough to offer 



Fig. 203. Striae on bed-rock. Kingston, Des Moines County, la. 



notable catchment for the base of the ice, but not so rough as to 

 impede its motion seriously. The amount of roughness favorable 

 for the greatest erosion increases with increasing thickness of 

 the ice. 



Other conditions which influence erosion by ice are (2) the 

 amount of loose or slightly attached debris on the surface; (:>) the 

 resistance of the rock; (4) the slope of the surface; (5) the thickness 

 of the ice; (6) the rate of its movement; and (7) the abundance and 

 character of the debris which it has to work with. The effect of 

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

 less simple. Clean ice passing over a smooth surface of solid 

 rock would have little effect upon it; but a rock-shod 



