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

　　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.