THE WORK OF SNOW AND ICE 



117 



over or against which it passes. Clean ice, moving over smooth, 

 solid rock, would erode little, but ice carrying pieces of rock in its 

 bottom wears the surface, even when it is smooth and of solid rock. 



Valleys through which valley glaciers pass are widened and 

 deepened, and their walls made smoother (compare Fig. 118 with 

 Fig. 119). 



A mountain valley glacier erodes effectively at the head of its 

 valley in many cases, and this makes the valley deeper and wider 

 at its head (Fig. 1, PI. XXXIII, p. 112, and PL XXVI, p. 91). The 

 big, blunt, steep-sided heads of valleys developed by the erosion of 

 valley glaciers are cirques. Cirques are numerous in the Uinta, 

 the Bighorn, and many other mountains of the West in which 

 there were formerly large glaciers. In the bottoms of the cirques 



Fig. 120. Island Lake, near Telluride, Colo. The lake is about 12,500 

 feet above sea-level. (Hole.) 



there are often basins in the solid rock. Not a few of the beautiful 

 little lakes (PL XXXIII, Fig. 1, and Fig. 120) which add so much 

 to mountain scenery are in such basins. 



