202 



ROCKS AND SOILS 



In this way you may be able to determine some of 

 the factors that help prevent erosion. 



4. Read the available reference materials and make 

 a list of the advantages of weathering and erosion. 

 Make another list of their disadvantages. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS' 



Experiment 126. Why are streams yellow after heavy 

 rains ? 



After a heavy rain, fill a two-quart jar with the yellow 

 water from a stream. Let it stand undisturbed for several 

 days. Record your findings in your notebook. 



Where did the sediment that was in the water come from ? 

 Running water is an agent of 



Experiment 127. How do changes in temperature help 

 to break up rock? 



Heat a piece of glass. Plunge it into cold water. What 

 happened? Might rocks be crumbled in this way? Explain. 



Fill a bottle with water. Insert the stopper and tie it 

 securely with a string. Pack the bottle in a mixture of ice 

 and salt. Examine later. What happened? 



Does the freezing of water break rock? 



Activity 128. What evidences of erosion can you find 

 after a heavy rain? 



After a heavy rain take a trip to a country district and 

 observe the many evidences of the work of running water. 

 Keep a record of your observations and report them to class. 



Try to find evidences of the work of plants and animals 

 in breaking up rocks and soil. If glaciers have crossed the 

 region in which you are living, try to find some of the 

 effects. Perhaps you can find scratches on exposed rock 

 beds or transported boulders. Record your observations. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



The surface of the earth is ever changing. One of 

 the great truths of science is that the earth's surface 



is constantly undergoing change. As we travel across 

 the earth's surface we are impressed by the variety of 

 conditions that we find. Great oceans, high mountains,, 

 hills, valleys, rivers, plains, plateaus, deserts, and lakes 

 attract our attention. Have you ever wondered how 

 the earth came to be as it is today? Have you ever 

 wondered whether it was the same in past ages or 

 what changes we may expect in the future? Scientists 

 are at work trying to find answers to these very ques- 

 tions, and they have much knowledge to impart to 

 you if you will but read, study, and think. 



Great contending forces are at work constantly 

 changing the surface of the earth. Some of these agen- 

 cies like rivers, wind, freezing water, and changes of 

 temperature wear down the high places and tend to 

 fill the low places. The processes by which rocks are 

 broken down are called -weathering and erosion. In ad- 

 dition to weathering and erosion, great forces are at 

 work within the earth that elevate parts of the earth's 

 crust. The present surface features of the earth are the 

 result of all these forces. You will learn more about 

 these things as you study this unit of work. 



What are the agents of mechanical weathering? 

 Weathering is produced by changes of temperature, 

 by ice, by plants and animals, and by the force of 

 gravity. 



Temperature changes affect rocks. Have you per- 

 formed the experiment of heating a piece of glass and 

 plunging it into cold water? Did the glass crack? Tem- 

 perature may vary more than fifty degrees Fahrenheit 

 between the heat of day and the cold of night. During 

 the day the rocks become strongly heated by the rays 

 of the sun and expand. At night they cool rapidly and 



Courtesy American Museum of Natural History 

 FIG. 325. PIECES OF ROCK BEING PUSHED APART BY GROWING TREE 



1 See workbook, p. 74. 



Keystone View Co. 



FIG. 326. A STUDY OF EROSION 



The effects of running water and of wind as agents of erosion 

 are apparent ; the work of vegetation to prevent erosion may be 

 seen in the right foreground. 



