2011年英语四级阅读备考材料（21）_第2页

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.考试网

　　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 vegetation.

　　I. Y 2. Y 3. N 4. N 5. Y 6. N 7. NG 8. level 9. the force of gravity 10. small gullies