230 PHYSICAL GEOLOGY 



SUMMARY 



A chief function of glaciers is to return to lower and warmer 

 levels moisture which otherwise would be imprisoned in- 

 definitely as snow and ice. Geologically, glaciers, like rivers, 

 have as their principal mission the wearing of the land and 

 the moving of the waste toward the sea. In the aggregate, 

 however, they are much less important agents of change 

 than rivers. Streams are, and since the very early history 

 of the earth have always been, at work nearly everywhere 

 upon the land. Even in deserts there are very few large 

 areas without valleys, although such valleys may be occu- 

 pied only by temporary streams. At present, glaciers affect 

 but a small fraction of the land surface, and while, as we 

 have seen, their extent has been much greater than now at 

 various times in the past, this was true, so far as known, 

 for only comparatively short periods. Glaciers are at a 

 disadvantage, too, from the fact that their work is entirely 

 mechanical. On the other hand, their activities are not so 

 conditioned by the hardness and structure of the surfaces 

 upon which they work as are those of streams. 



Although not so important geologically as they, ice takes 

 its place with air and water as one of the three great grada- 

 tional agents which modify land surfaces. 



QUESTIONS 



1. Why is the snow line much lower on the southern (sunny) 

 side of the Himalaya Mountains than on the northern (shady and 

 cooler) side ? 



2. Why have the Sierra Nevada and Cascade Mountains more 

 glaciers than the Rocky Mountains? Why are there more in the 

 northern than in the southern Rockies? 



3. What are the factors upon which the size of a given valley 

 glacier will depend ? 



4. What things limit the height which rock-capped ice pillars 

 such as those shown in Figure 209 may attain ? 



5. Do all parts of the medial surface line of a valley glacier 

 move at the same rate ? Why ? 



