AQUEOUS AGENCIES. 61 



all these respects the effect of their action is very charac- 

 teristic, and can not be mistaken for that of water. 



Erosion. When we remember the thickness and 

 weight of glaciers, we at once see that they must rub 

 with great force on their beds. But, like water, glaciers 

 will do but little work without graving-tools. Rivers 

 erode mainly by means of sand, gravel, and pebbles, car- 

 ried along the bottom ; glaciers, by means of rock- frag- 

 ments of all sizes carried between the ice and the beds, 

 and often fixed by freezing in the ice. These graving- 

 tools are partly fragments broken off from the bed, and 

 partly fragments fallen on the surface, which become 

 ingulfed in crevasses or jammed between the sides of the 

 glacier and the bounding cliffs. In either case, by virtue 

 of the viscosity of the glacier-ice, they find their way 

 finally to the bottom. By virtue of the hardness and 

 stiffness of the ice, it tends to plane to one level ; by its 

 smoothness, it polishes ; by means of its graving-tools, it 

 scores in straight, parallel lines ; by virtue of its viscosity, 

 it conforms to the large and gentle inequalities, giving 

 rise to smooth, billowy surfaces, called roches moutonnees. 

 Thus smooth, billowy surfaces, scored with straight, par- 

 allel marks, are very characteristic of glacial action. We 

 shall call such surfaces glaciated (Fig. 30). 



Transportation. The transporting power of running 

 water increases as the sixth-power of the velocity. Even 

 at this enormous rate of increase, blocks of stone of many 

 hundreds of tons weight, such as are often found in the 

 paths of glaciers, would require, if carried by water, an 

 almost incredible velocity. But glaciers carry materials 

 resting on their surfaces, and therefore of all sizes, with 

 equal ease. Rock-fragments of thousands of tons weight 

 are carried by them and left in their path by retreat. 



Again, fragments carried by water are always more or 

 less bruised, worn, and rounded, while fragments carried 

 on the surface of glaciers are angular. Again, water- 



