UNIT IX. ROCKS AND SOILS OF THE EARTH 



As we travel over the earth's surface we notice a 

 variety of conditions. Large continents with great 

 oceans surrounding them, high mountains along the 

 borders of the continents, hills, valleys, plains, pla- 

 teaus, deserts, rivers, and lakes attract our attention. 

 How has our earth come to be as it is? Scientists are 

 at work trying to answer this very question. 



The earth's surface has been, for a long time, and 

 is now constantly changing. Agents of erosion such 

 as running water, wind, and moving ice wear away 

 the surface of the earth at higher elevations and 

 transport the material to regions of lower elevation. 

 In addition to these forces that are wearing down the 

 surface of the earth, certain internal strains and forces 

 are at work elevating parts of the land. The irregular 

 surface of the earth which we see today is the result 

 of these two opposing forces. The struggle between 

 these contending forces is still going on, and what the 

 final result will be no one knows. 



While erosive agents sometimes prove detrimental 

 to man's welfare, they have, at the same time, been 

 of inestimable benefit to him. They wear away the 

 rocky hillsides and transport the material to the val- 

 leys and help to produce our fertile fields. Nature 

 has been working for millions of years producing the 

 soils upon which we raise our crops. Practically all 

 of the earliest civilizations were established in fertile 

 river valleys, and even today most of the people of 

 the earth are congregated in or near crop-producing 

 regions. 



There are other interesting things such as volcanoes 

 and earthquakes which we shall study in this unit. 

 Early people had fanciful and superstitious notions 

 which they invented to explain these phenomena. 

 Earthquakes, for example, were thought to be the 

 result of struggles between gods or giants beneath 

 the earth's surface. Early peoples failed to make care- 

 ful observations. Today our ideas about earthquakes 

 and volcanoes are based upon carefully collected facts 

 about them. And while we cannot control earthquakes 

 or volcanoes, we can understand what causes them 

 and attempt to make an intelligent adjustment to 

 them. 



Living things are dependent upon soil for their ex- 

 istence. The food materials of plants and animals, for 

 the most part, come directly or indirectly from the 

 soil. For several thousands of years agriculture has 

 been one of our basic industries. Man is constantly 

 occupied with the problem of maintaining soil fertil- 

 ity and finding new lands for farming. In this unit 

 you will have the opportunity to study the factors 

 which must be regulated in order to raise crops suc- 

 cessfully. 



In this unit we wish you to learn more about the 

 points so briefly sketched in this introduction. The 

 storyof the earth is an interesting one, and the topic 

 of soils is as important and closely related to human 

 welfare and existence as any in the entire course. 

 What do you already know about rocks and soils? 



1. What different kinds of rocks do you know? 



2. Have you ever collected rocks and minerals? 

 What did you learn from the experience? 



3. Did you ever see a rock with shells or other 

 animal remains in it? How did they get there? 



4. Did you ever see rock that could be separated 

 into layers? How was this formed? 



5. How was coal formed? 



6. What are some of the agents at work wearing 

 away the earth's surface? 



7. Large round boulders are often found on the tops 

 of mountains. Can you explain how they may have 

 got there? 



8. Can you account for the formation of soil if the 

 earth was once a barren, rocky surface? 



9. How was the earth formed? 



10. Why are rivers and streams always muddy at 

 flood stage and after rains? 



11. What is a nebula? 



12. How old is the earth? 



13. How are forces at work building up the earth's 

 surface? 



14. Do volcanoes open to the center of the earth? 

 Give reasons for your answer. 



15. Are the Rocky Mountains older than the Ap- 

 palachian Mountains? Give reasons for your answer. 



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