Erosion

Transport of sediment occurs by one or more of four agents: gravity, wind, flowing water, or flowing ice. A simple principle controls transport; movement of sediment occurs only as long as the force exerted on the sediment grain by the agent exceeds the force that holds the grain in place (friction due to gravity). For example, the wind can only move a grain of sand if the force generated by the wind exceeds the frictional force on the bottom of the grain. If the wind's force is only slightly greater than the frictional force, the grain will scoot along on the ground. If the wind's force is much greater than the frictional force, the grain will roll or perhaps bounce along on the ground. The force produced by flowing air (wind), water, or ice is a product of its velocity.

When gravity alone moves rocks or sediment, this is a special type of transport known as mass wasting (or mass movement). This name refers to the fact that most mass wasting involves a large amount of sediment moving all at once rather than as individual grains. Along a highway, if a large mass of soil and rock from a hillside suddenly gives way and rapidly moves downhill, this would be a type of mass wasting known as a landslide. Mudflows and rockfalls are two other common types of mass wasting.