HOW IS SOIL MADE? 



Amer. Mus. of Nat. Hist. 



This tree started in a crack in a granite rock and has 

 exerted force enough to split it. 



in rock freezes, the ice formed presses with a force of 

 2000 pounds per square inch on the rock surrounding it. 

 No wonder the 

 rock chips off ! 

 When we add the 

 heat of the sun to 

 the action of frost, 

 and remember 

 that on the desert 

 there may be dif- 

 ferences of 100 

 between the day 

 and night tem- 

 perature, we can 

 see why the out- 

 side of the rock becomes larger than the inside and strain 

 results which causes it to crack and break. 



The tree in the picture has split the rock just as a wedge 

 will split a block of wood. As trees grow, their roots 

 press the rock apart more and more and thus allow the 

 other agents of weathering to act upon it. Burrowing 

 animals, earthworms, ground squirrels, gophers, and 

 woodchucks break up the soil into finer particles. Wind 

 and rain also help to break down rocky soil and distribute 

 it so that other agencies will also act on it. 



Chemical Weathering. We do not realize that rocks 

 decay, but such is the case. Pure water will not have 

 much effect upon rock, but add to it a little acid and it 

 will soon eat away limestone rock. Plants give off acid 

 through their roots, thus causing certain kinds of rock 

 to break down. Then oxygen and carbon dioxide in rain 

 water cause rocks to oxidize and decay, or the rock may 

 dissolve. The beautiful red and brown coloring of rocks, 

 such as at Bryce, Zion, or in the Grand Canyon of the Colo- 

 rado River, is caused by the action of oxygen and water. 



