208 PHYSICAL GEOLOGY 



European areas referred to, and since no other agent is 

 known capable of doing so, it has been concluded confidently 

 that these regions were formerly covered by glacier ice. 

 These glaciers were as extensive as the till is widespread, 

 and therefore are known to have covered at their maximum 

 development the areas shown in Figures 217 and 218. In a 

 similar way other great areas in various parts of the world 

 are known to have been glaciated at still earlier, but widely 

 separated times (pp. 337, 394). Some of these areas are within 

 the tropics, and now enjoy very warm climates. Glaciers are, 

 then, one of the great geological agents that have modified 

 numerous ancient as well as present land surfaces. Various 

 phases of their work may be studied in most parts of northern 

 United States. 



THE GEOLOGICAL WORK OF GLACIERS 



Like winds and rivers, glaciers transport rock waste, 

 wear the surfaces over which they move, and deposit their 

 loads to form characteristic features. 



TRANSPORTATION AND DEPOSITION 



As snow gathers to form a snow field it surrounds and 

 covers loose pieces of rock on the surface, and incloses pro- 

 jecting ledges of firm rock. When the snow field becomes 

 an ice field, and begins to move, it carries much of the loose 

 material in its bottom with it, and may also break off and 

 remove pieces of the bedrock, so that the glacier has a load 

 from the beginning. Wherever the water in the soil upon 

 which the glacier advances is frozen, it cements the soil 

 particles into a firm mass. Wherever this ice-cemented soil 

 is frozen to the glacier ice above, it becomes, in effect, a part 

 of the glacier, and is likely to be carried on by its further 

 movement. Most of the material carried by ice sheets, and 

 possibly also much of that transported by many valley gla- 

 ciers, is gathered in these and other ways by the under sur- 



