278 NATURE STUDY AND AGRICULTURE 



heard about boring wells knows that often the well digger 

 strikes solid rock and must abandon the excavation at 

 that point. Some of the bed rock juts out of hillsides only 

 a few feet below the surface, and in some places it is found 

 on the surface. The kinds of bedrock differ in different 

 regions. 



If we look back far enough into the history of the earth, 

 we find evidences of a time when there was no soil as we 

 know it to-day. The surface was covered with solid rock, 

 wrinkled and ridged into mountains and valleys. All soil 

 has been formed from the breaking up of this solid rock. 



How many agents can you think of that have helped to 

 break up this solid rock and make it into soil ? Change of 

 temperature from hot to cold has done much to break up 

 rock and make finer the broken rock. How does heat 

 affect most substances ? How does cold affect them ? If 

 these rocks expand when heated and contract when cooled, 

 what is likely to happen if they are heated and then cooled 

 rapidly? Did you ever spatter a drop of cold water on a 

 hot lamp chimney ? Why did it break ? 



Freezing not only helps to break up solid rock, but helps 

 to pulverize soil. In your study of the effect of freezing on 

 water, what did you find out ? If water settles in the cracks 

 and crevices of rocks and then freezes, what will happen ? 



Experiment. — Make a ball of garden soil by moistening 

 it and working it like dough. Now put it in the oven or on 

 top of the stove and let it get dry and hard. Pour some 

 water over it and set it out of doors where it will freeze. 

 What is the effect? 



Rain, wind, and snow all help to disintegrate the rock. 

 Running water plays an important part, not only in helping 



