Natural forces are constantly changing our world. Some rocks are dissolved by rainwater and others are split by frost or rapid temperature changes. Worn rock is then carried away by running water, glaciers and, in dry regions, by the wind. Weathering, running water, glaciers and winds are all forces of erosion.
Natural erosion is slow. For example, it takes 30,000 years for 1 m (39 in) of ground to be worn away from the eastern USA. However, these natural processes can be speeded up. When people cut down forests or plough up and overgraze grasslands, they expose the soil. The forces or erosion then operate much faster, stripping away the soil and making once-fertile land useless.