SOIL FORMATION AND TRANSPORTATION 13 



It is as a transporting agent that water is most active. 

 From the time when raindrops beat down on the surface 

 of the soil, while they are gathering into rivulets and the 

 rivulets are becoming rivers that discharge into the ocean, 

 they are engaged in moving particles of rock debris and 

 soil. It is estimated that the United States is being planed 

 down at the rate of one inch in seven hundred and sixty 

 years. This is rapid enough if it were applied at one point 

 to dig the Panama Canal in seventy-three days. 



The carrying power of water has resulted in the formation 

 of the rich river valley soils that have been deposited by 

 the streams flowing through them. The coastal soils and 

 lake soils have also been transported by water. 



13. Action of ice. — In former times a considerable part of 

 the northern United States was covered by huge masses of ice, 

 known as glaciers. These ice masses were of enormous vol- 

 ume and moved slowly in a southerly direction. The great 

 thickness of the ice mantles, amounting to several thousand 

 feet at some places, caused them to cover hills, valleys and 

 mountains, and their enormous weight ground rock surfaces, 

 pushed forward heaps of soil and transported huge boulders. 

 The southern limit of the glaciers corresponded roughly to 

 the lines now marked by the Ohio and Missouri rivers, and 

 again extended farther southward along the Pacific coast. 

 It met the Atlantic coast at about the present location of 

 New York. Changes of climate caused an alternate reces- 

 sion and extension of the ice sheets several times, and during 

 all this period soil was being formed and worked over by 

 the ice and the water that melted from it. When the glacier 

 melted, stranded ice masses remained .behind. These formed 

 lakes in which soil was reworked and shifted, and as the 

 lakes finally drained off, the reworked soil was left behind. 

 These glacial soils are, as a rule, productive, because of the 

 thorough pulverization and mixing they have received. 



