大学英语四级阅读的200篇: Unit 35 passage 1_大学英语阅读

大学英语四级阅读的200篇: Unit 35 passage 1

　　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 rivers 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 rivers 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.