A beach is land along a body of water, made up of any number of materials, including sand, pebbles, cobbles, shells and rock. Beach material can be deposited during periods of accretion, or removed during erosion. Beaches are constantly changing in size and composition, sometimes on short timescales of weeks, days or even hours during major storms.
The formation of a beach involves weathering, the process by which rocks are broken down into smaller and smaller pieces, erosion by waves, and transport by currents. Beaches also act as a buffer, dissipating the energy of waves that would otherwise impact coastal terrains.
Most beaches, especially in mid-latitudes, are composed of terrigenous (mineral) siliceous sand. In the tropics, however, many beaches consist of biogenic (organic) sand, which is mostly comprised of skeletal fragments and frameworks built by marine organisms.
Beaches are generally classified according to their size and shape, the type of sediment they contain, and their morphological features. Flat beaches are generally composed of fine to medium sand, while steeper beaches are typically composed of coarser particles such as pebbles or cobbles. Beaches that contain large-sized grains often show a characteristic feature known as particle imbrication, where the lower surface of one grain is partially resting on the upper surface of another.
Many of these characteristics can be determined by the amount and speed of sediment supply, the turbidity and flow speed of the water, wind speeds and direction, and the presence or absence of vegetation that slows the movement of sediment. In sand bars that are the result of longshore drift, the slope of a beach can be affected by the amount and type of sediment that is carried across it during high-tide intervals.
Other morphological beach features include the spit and tombolo. A spit is an island that forms where a bar extends into a bay, or at points of reentrance along a coastline, such as coves and rias. In these types of locations, sediments that are carried by a longshore drift tend to fall out of the water in front of the bar and form a spit. Eventually, the spit may be eroded by the sea, forming a lagoon. Lagoons can then silt up and become salt marshes.