Erosion goes on continuously and it should be the concern of all of us. There is evidence of this process of erosion all about us. Both the small gullies(溪谷) in the side of a nearly hill and the immense Grand Canyon of the Colorado River and other gorges represent results of erosion. A muddied river after a heavy rain and a dust storm in a parched area of farmland both are carrying particles of soils from one place to another. Likewise, a creeping glacier in a mountain valley, the pounding waves of the ocean, and an underground river in a limestone cavern are all agents of erosion.
Erosion levels the earth's surface. Essentially, erosion is the natural process that ceaselessly tries to level the earth's 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 earth's surface are being raised by volcanic action and by balancing movements of the earth's crust. For example, great pressures exerted by the water and sediments (沉积物) 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 earth's 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 earth's 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.
Running water causes erosion on hillsides. The way in which erosion takes place by running water depends on many factors The steeper the slope down which the water flows, the faster it flows and generally the more rapidly the soil is washed away. If the topsoil is loose, it will carried away faster than if it is packed down or held together by vegetation. Erosion takes place more rapidly as the volume of water running over the surface increases. As in the formation of a river valley, the gullies in a hillside become both deeper and wider as the eroding action continues. A gully also grown longer with each rainfall as the water runs into the head and washes in some of the surrounding soil.
Weathering and erosion change valleys. Weathering and erosion in valleys cause large-scale changes in the formation of big river valleys. A river valley tends to deepen as the bed of the river is eroded away. This deepening continues until the river reaches its base level, which is the level of the body of water into which the river flows. Weathering and erosion of the river's banks are going on at the same time, however, and the river generally widens as it deepens to form a V-shaped valley. In areas where weathering takes place rapidly, the shape of the V changes as the widening process "catches up" with the deepening process. Generally, a "young" river is one flowing swiftly through a narrow valley with steep sides, while an "old" river is one flowing slowly through a wide valley with gently sloping sides. In dry climates, however, weathering may take place quite slowly and then the deepening process results in deep canyons such as that of the Colorado River. Rapids and waterfalls are also characteristic of young rivers. These eventually disappear as the river's bed is eroded. A river such as the Mississippi shows not only the characteristics of a young river in its headwaters but also other stages of maturity and old age as it flows from its narrower northern valley through the broad flood plains of the southern portion. The Colorado River is considered a young river, even though it has probably been eroding its way through the hard rocks to form its canyon for million of years.
Some erosion, such as that described in connection with river valleys, takes place slowly. The destructive soil erosion discussed in the largest part of this selection, however, goes on rapidly. One of the most important problems facing our country is the conservation of the soil lost by rapid erosion. Scientists have directed their attention to this problem and have found several effective solutions to it.
1. The passage mainly deals with causes of erosion.
2. Erosion is caused by natural forces such as a creeping glacier in mountain valley, the pounding ocean waves.
3. Wind is the most effective agent of erosion.
4. Vegetation increases the speed of running water on hillsides.
5. Young rivers are characterized by rapids and waterfalls.
6. Erosion can change rocks into fertile farm lands.
7. The movements of the earth's crust is more destructive than erosion.
8. Erosion is a natural phenomenon to______the earth's surface.
9. The cooler, denser*air moves downward toward the earth because of______.
10. Even a short rain may result in ______ if the soil is loose and not protected by
答案：1. Y 2. Y 3. N 4. N 5. Y 6. N 7. NG 8. level 9. the force of gravity 10. small gullies