四级冲刺练习阅读137

　　Erosion levels the earths surface. Essentially, erosion is the natural process that ceaselessly tries to level the earths surface. If it were not for other forces also constantly at work, the mountains and hills would eventually disappear and all of the land would be under water. Fortunately, however, many areas of the earths surface are being raised by volcanic action and by balancing movements of the earths crust. For example, great pressures exerted by the water and sediments on the ocean floor along many shorelines result in forces that cause the land area to be uplifted. Thus erosion never completely succeeds in leveling the earths surface.

　　Gravity is influential. The force of gravity is responsible for both the falling of rain from the clouds and for the water running off the land as it always seeks a lower level. Likewise, the great moving currents of air that we call winds are the result of unequal heating of the earths surface and the convection currents that are set up in the atmosphere. Here again the force of gravity is responsible for the downward movement of the cooler, denser air toward the earth which starts the air moving.

　　As long as the winds blow and rain falls, erosion will continue to take place. How is erosion useful to man and when does it become a serious problem?

　　Erosion works with weathering. Erosion works hand in hand with the process of weathering in causing rocks to be broken up and changed into soil. It is the agents of erosion that carry away this newly formed soil and expose fresh rock surfaces to the agents of weathering. Thus new rock surfaces are exposed to the air, to rapid changes in temperature, to the pounding rain and driving wind, to dissolved chemicals in ground water, and to the other agents that help in making more soil. This transported soil often ends up in the fertile river valleys and plains where we find our most productive farmlands. In this process of transporting new soil and minerals from where they have been made to where they can be used, erosion is useful to man.

　　Winds cause dust bowls and sand dunes. Winds erosion in semi-arid regions may remove millions of tons of fine topsoil from fertile fields. In the Dust bowl area of the Great Plains such dust storms during the long fry spells known as droughts have caused tremendous losses and many farms have had to be abandoned. In other areas wind-blown sand forms dunes which may bury fertile farms, forests, and sometimes even towns. It is obvious that erosion by the action of winds is most effective where the soil is bare and unprotected by natural vegetation. Even more effective than winds as an agent of erosion is water.

　　Rain loosens soil. Rain falling on level land loosens the soil and carries it short distances by the spattering. On the side of a slope, however, such splashing slowly moves the soil downhill. This process of raindrop erosion proceeds quite slowly in comparison with the erosion resulting from streams of water that flow down the hillside as the water runs off. If the soil is loose and not held together by the roots of trees or other plants, small gullies will be visible after even a short rain. We see these gullies along the side of a road where a new cut has been made through a hill. We often see them in a recently planted, sloping lawn. They are also visible in many cultivated hillside fields. If left unchecked, gullies grow larger after each rain or when the winter snows melt and the water runs off the land. Each gully represents valuable topsoil that has been carried away and deposited elsewhere.