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.
【六级冲刺练习:阅读(137)】相关文章:
★ 六级深度讲义的阅读理解Passage Ten 20101004
★ 六级深度讲义—阅读理解Passage six 20100930
★ 六级经典的阅读6
★ 2013年6月英语六级考试快速阅读练习题(24)0222
最新
2016-10-18
2016-10-11
2016-10-11
2016-10-08
2016-09-30
2016-09-30