Soil erosion is the process weathering of soil caused by the water, tillage or the wind. Pesticides and other chemicals can get trapped in the soil, polluting streams and rivers as the soil breaks apart. Mass movement is a major part of the erosional process and is usually the first stage in the breakdown and transportation of weathered elements in mountainous areas moving the material from the higher elevations to lower elevations where other disintegrating agents such as rivers and glaciers can then pick up the rock and move it to even lower elevations.