The Soil and Soil Water 25 



Water, too, freezing in the pores and cracks of rocks, Ice 

 breaks them as it would break a pitcher. This is be- 

 cause water expands about one eleventh of its volume 

 when it freezes. 



The roots of trees and of smaller vegetation as well Root 

 help to split rocks that are already decomposing. The gro 

 acids produced by growing roots help to dissolve rock 

 particles, and thus aid in soil formation. 



In the bed of almost any stream are to be seen smooth, The 

 rounded pebbles and boulders. These have come 

 many miles from among the hills where the stream has streams 

 its source. They were originally rough, angular frag- 

 ments of rock, but they lost their corners by being 

 jostled about and rolled over in the sandy bed of the 

 stream. Figure 17 shows fragments of brick, glass, stone, 

 and wood, that were worn smooth by this process. 

 Not only were tiny fragments broken from the pebbles 

 in the making, but the pebbles in turn wore the stream 

 bed, gouging it out deeper and deeper. Thus the bits 

 of stone acted as tools, grinding the rocky crust of the 

 earth into soil. 



Distribution of soil : the action of water. Running 

 water has helped in the distribution of soil, as well as 

 in its making. Steep mountains and hillsides often Why soil 

 are almost barren of soil, not because there never was Alley's l 

 any there, but because it has continually been carried 

 into the valleys below, by the water from rains and by 

 streams. This accounts for the great depth of valley 

 soil. 



In some parts of our country, long ago, great glaciers 

 hundreds of feet thick slowly slid over the ground. 



