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Erosion in nature is a beneficent process without which the world would have died long ago. The same process, accelerated by human mismanagement, has become one of the most vicious and destructive forces that have ever been released by man. 'Geological erosion' or 'denudation' is an early and important process in soil formation, whereby the original rock material is continuously broken down and sorted by wind and water until it becomes suitable for colonization by plants. Plants, by t h e binding effects of their roots, by the protection they afford against rain and wind and by the fertility they impart to the soil, bring denudation almost to a standstill. Nevertheless, some slight denudationis always occurring. As each superficial film of plant-covered soil becomes exhausted it is removed by rain or wind, to be deposited mainly in the rivers and sea, and a corresponding thin layer of soil forms by slow weathering of the underlying rock. The depth is sometimes only a few inches. occasionally several feet deep, but within it lies the whole capacity of the earth to produce life. Below that thin layer comprising the delicate organism known soil is a planet as lifeless as the moon.