THE WORK OF SNOW AND ICE 119 



Glaciers make scratches, or stria (Fig. 2, PL XXXIII, p. 112), 

 on the rocks beneath, and under favorable conditions great grooves 

 are formed. The striae are made by the stones carried in the bottom 

 of the ice. The grooves are sometimes made where the bed-rock is 



Fig. 124. A hill near the last not overridden and smoothed by ice. (From 

 photo, by Chamberlin.) 



softer, or where great bowlders are held firmly in the bottom of the 

 ice, and urged along under great pressure. Fine, clayey material 

 in the bottom of the ice polishes the rock below. The polish, the 

 striae, and the grooves left on the surface of the rock after the ice 

 has melted are among the clearest marks of the former existence of 

 glaciers. In any limited area, the striae are generally parallel to one 

 another, and show the direction in which the ice moved. 



The stones in the bottom of the ice are rubbed against one an- 

 other, as well as against the bed of the glacier, and are scratched 

 much as the bed-rock is (Fig. 1, PL XXXIV, p. 113). Since the 

 stones in the ice shift their positions from time to time as the ice goes 

 forward, they are frequently striated on two or more sides. 



As the materials carried by the ice rub against one another and 

 against the bed over which they pass, they are worn smaller and 

 smaller. The finest products of the grinding have been called 

 rock flour. The materials carried by the ice are therefore of all 



