Wave motion of the water extends to about one-half of the wavelength. When a wave enters water shallower than one-half the wavelength, its forward motion is slowed by drag on the bottom. This causes refraction of the wave so that it approaches the shore line in a nearly perpendicular direction. It may also cause the wave crest to overtake the trough to form a breaker . Breakers are collectively called surf .
Wind waves are very efficient agents of size-sorting .
Wave motion can keep silt and clay sized particles in suspension preventing their deposition in the near-shore environment. In temperate regions, the principal mineral in beach sand is quartz , whereas in tropical regions with reefs the principal mineral is calcite . Active volcanic islands may have sands composed of basalt and glass shards. When waves arrive at an angle ot the beach they may cause a long-shore current. Sand particles moved by this current is the long-shore drift .