Rocks are eroded by mechanical and running water weakens the rock. Some types of rocks, such as limestone dissolve slowly in water. Other rocks may be eaten away by chemicals in the water.
Running water in the form of waves, rain, or streams and rivers, carries off loose particles of rock and deposits them in the beds of rivers and oceans. Loose soil is also carried off and deposited in this way.
Such deposits of clay, sand and gravel are called alluvium. Erosion of soil cause dangerous mud slides or makes vast areas of land unsuitable for growing anything.
Trees and plants reduce erosion by holding the soil in place and by absorbing much of the excess water. If this protection is removed by the careless cutting down of forests, large amounts of soil can be washed away.
In recent years, scientists have developed ways to prevent soil erosion. They include planting of young trees to replace those cut down. Farming methods that promote soil conservation include crop rotation, contour planting, and soil-drainage systems. The building of dams helps to collect excess water and prevents it from flooding the land.
Other kinds of erosion include the wearing away of sea coasts by the action of waves, the removal of rocks by large glaciers, and the shifting of soil and sand by wind. Valleys, ravines and gullies are often formed by streams and rivers, which cut through rocks and soil and carry them away. The course of the river can be changed by the power of running water.
