32 THE SOILS AND CEOPS OF THE FARM. 



expands as it freezes aad exerts a great force in so 

 doing. When water which has collected in cracks or 

 low places on the surface of rocks freezes it may split 

 off a great mass from the mountain side or a tiny frag- 

 ment from a grain of sand. As freezing and thaw- 

 ing go on year after year the effect in breaking 

 up rocks and in pulverizing the soil is very great. 

 Water and ice in motion have a great influence in 

 the formation and distribution of soils. A little 

 stream as it flows down a hill-side carries with its 

 water, especially when swollen by rains, little stones, 

 sand and earth. As these move they loosen other 

 stones. A large stream or a great river does the same 

 work on a vastly larger scale. Streams may change 

 their course, washing out new channels and carrying 

 the rooks and soils to be distributed over lower lying 

 land. The great deltas at the mouths of the Missis- 

 sippi, the Nile and the Amazon illustrate on a large 

 scale the kind of work which running water is doing 

 in moving soil wherever there is a stream. The 

 word and rounded pebbles at the brook side, contrast- 

 ed with sharp-edged, freshly broken rook, show the 

 power of moving water to wear and pulverize as 

 well as to carry solid substances. 



Glaciers or streams of ice slowly moving down 

 mountain sides, carry with them masses of stone and 

 earth, and crush and grind the rocks over which or 

 past which they move. There is good evidence that 

 in past ages glaciers have done much work in many 

 parts of the earth. Icebergs carry masses of rock and 

 much earth, and distribute these as they melt. 



With some or all these forces acting continuously 



