<P> Soil erosion is the displacement of the upper layer of soil, one form of soil degradation . This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans . In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolean) erosion, zoogenic erosion, and anthropogenic erosion . Soil erosion may be a slow process that continues relatively unnoticed, or it may occur at an alarming rate causing a serious loss of topsoil . The loss of soil from farmland may be reflected in reduced crop production potential, lower surface water quality and damaged drainage networks . </P> <P> Human activities have increased by 10--40 times the rate at which erosion is occurring globally . Excessive (or accelerated) erosion causes both "on - site" and "off - site" problems . On - site impacts include decreases in agricultural productivity and (on natural landscapes) ecological collapse, both because of loss of the nutrient - rich upper soil layers . In some cases, the eventual end result is desertification . Off - site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment - related damage to roads and houses . Water and wind erosion are the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems worldwide . </P> <P> Intensive agriculture, deforestation, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion . However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils . </P> <P> Rainfall, and the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion . Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four). </P>

What are the two main forces that cause soil erosion
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