HOW IS SOIL MADE? 235 



blowing sand. We might even have a sandstorm and have 

 the windshield of our car etched and pitted by the wind- 

 driven grains. And as we got near the eastern coast, we 

 would find mountains again wooded to their summits, 

 mountains with smooth rounded outlines which the 

 geologists tell us show that they are very, very old and 

 have had time to lose their angular steep sides so char- 

 acteristic of the younger bare mountains of the far West. 

 In some places we would find soil just where the forces 

 of weathering had produced it. This is called residual 

 soil. In other places the soil has been brought by moving 

 ice, water, or wind and is called transported soil. 



SELF-TESTING EXERCISE 



Select from the following list the words which best fill the spaces in the 

 sentences below and arrange the words in proper numerical order. A 

 word may be used more than once. 



springs more ice colder transported 



wells less steam outside geysers 



rivers greatest water inside weathered 



expand mechanical cooling soil residual 



contract chemical heating rocks erosion 



move air hotter lava weathering 



iron oxygen separation dissolved dioxide 



Ice occupies (1) , space than the water from which it is 



formed. Freezing (2) is an important (3) agent in break- 

 ing down rocks in the process of (4) formation. When the 



sun's rays beat down upon a rock, the (5) layers become 



(6) than those below and as they (7) they tend to loosen. 



Alternate heating and (8) finally results in the (9) of 



fragments. Limestone rock is (10) by water containing 



carbon (11) Rocks that contain only a minute quantity of 



iron remain unchanged deep in the earth, but when exposed to the 



air, turn brown and rusty because the (12) of the (13) has 



combined with the (14) of the rock. Such (15) rock 



crumbles easily, producing (16) The wearing down of rocks 



and transportation of the material is called (17) Water is 



a powerful agent of (18) Underground waters in some parts 



