Erosion is the wearing away of material on the surface of the land by wind, water, or gravity. In nature, erosion occurs very slowly, as natural weathering and geologic processes remove rock, parent material, or soil from the land surface. Human activity, on the other hand, greatly increases the rate of soil erosion.

Water plays an important role in soil erosion. When an area receives more water (in the form of rain, melting snow, or ice) than the ground can absorb, the excess water flows to the lowest level, carrying loose material with it. Gentle slopes are subject to sheet and rill erosion, in which the runoff removes a thin layer of top soil.

Glaciers are important agents of erosion. Although a

Gully formation, a severe form of soil erosion, is a natural geologic process that can be greatly accelerated by human activities such as urbanization, deforestation, overgrazing of cattle, and poor agricultural practices. Erosion attacks the moisture-bearing ability of soils and adds deposits to waterways. These destructive processes continue at an increased rate on every continent, as overpopulation and industrialization tax the remaining soil.

Without human activities, losses of soil through erosion would in most areas probably be balanced by the formation of new soil. On virgin land a mantle of vegetation and trees protect the soil. When rain falls on a surface of grass or on the leaves of trees, some of the moisture evaporates before it can reach the ground. Trees and grass serve as windbreaks, and a network of roots helps to hold the soil in place against the action of both rain and wind. Agriculture and lumbering, as well as housing, industrial development, and highway construction, however, partially or wholly destroy the protective canopy of vegetation and greatly speed up erosion of soils.

Wind is another active agent of erosion, especially in arid climates with

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