SOIL-FORMING PROCESSES 19 



riding Greenland to-day. Of great thickness and weight and 

 impelled southward by tremendous pressure, these ice sheets 

 swept away the old soil mantle and ground the underlying 

 rocks with irresistible energy. The heterogeneous debris, im- 

 bedded in the ice, only served to enhance the cutting power 

 of the slowly moving mass. Hundreds of square miles were 

 covered and as the ice was often several thousand feet thick, 

 mountains as well as hills were over-ridden. (See Fig. 3.) 



In the melting back of these tremendous ice sheets, the 

 accumulated debris was of necessity left behind. When the 

 ice retreat was rapid, the deposit was comparatively thin and 

 uniform. When a halt occurred, the material was left in 

 irregular hummocks. It is hardly necessary to state that the 

 soil developed from the former deposit is the more important 

 agriculturally, due to its level topography and wide extent. 

 The area of the latter is fortunately small. The streams 

 flowing from the ice fronts were no insignificant feature of 

 the glacial phenomena. Such streams were heavily laden with 

 sediment, which was distributed far and wide in regions miles 

 beyond the ice front. 



In whatever manner the glacial debris was laid down it is 

 necessary to note that such deposits were soil material, not 

 soil. Chemical action in all its complexity and the interven- 

 tion of plants and animals, especially the former, were neces- 

 sary before a true soil could be born, a soil still in its youth 

 and covering in the United States alone over 500,000 square 

 miles. (See Fig. 3, page 20.) 



13. The influence of wind. — Wind, like water and ice, 

 has both cutting and carrying power. The fluting of rocks, 

 the polishing of stones, and the undermining of cliffs are of 

 such frequent note as to require but brief mention. There 

 seems no escape from the conclusion that wind is engaged in 

 rock disintegration. Its geological function in arid regions 

 seems similar to that of running water in humid lands. 



It is, however, as a transporting agency of fine materials 



