Soil erosion is the wearing away of soil, a primary cause of concern in agriculture. There are two types: Geological erosion denotes those naturally occurring erosion processes that constantly affect the earth's surface features; it is usually a fairly slow process and naturally compensated for. Accelerated erosion describes erosion hastened by the intervention of man. Sheet erosion occurs usually on plowed fields. A fine sheet of rich topsoil is removed by the action of rain water. Repetition over the years may render the soil unfit for cultivation. In rill erosion, heavy rain may run off the land in streamlets: sufficient water moving swiftly enough cuts shallow trenches that may be plowed over and forgotten until, after years, the soil is found poor. In gully erosion, deep trenches are cut by repeated or heavy flow of water. Wind erosion is of importance in exposed, arid areas.