A beach is a strip of sand, gravel, clay, or other sediment that borders a rocky or cliffy coastline. It is a dynamic natural phenomenon that constantly changes, often within hours, as wave action and currents rework the beach material. It can consist of a single grain of sand, or of a much more complex mixture that includes shells, seaweed, and even marine animals such as crabs. Some beaches, called barrier reefs, are composed of sedimentary rock and may extend for dozens or hundreds of miles.
Beaches are a part of the shoreline, and their location on the coast depends on a variety of factors including the stability of the soil or rock, the presence of vegetation, and the size and power of waves. A well-stabilized beach will accrete (build up) and remain stable over time, while unstabilized beaches tend to erode, often catastrophically, causing gulfs or lagoons to form in the space where the beach once stood.
Most beaches are located on oceans, but they can also be found by lakes and alongside large rivers. Some, called barrier reefs, are composed of coral and other sedimentary rock and may stretch for dozens or hundreds of miles. Others, called beach ridges, are smaller segments of the coastline that have been formed by a combination of erosion and deposition of sediment, typically on a relatively shallower bottom.
The most common beach materials are sand and other finely-grained sediment, but they can include bits of lava or other rock, shells, seaweed, and marine organisms. Some beaches have a distinct color, which can be due to the type of sand or the presence of minerals such as silica.
In some cases, a beach is a fossil beach, where the fossils have been preserved because of a change in sea level. Fossils of ancient sea creatures, plants, and algae can be excavated from the sand at such sites, sometimes as long as a hundred meters inland.
Sand is continuously transported to and from a beach by waves, currents, and wind. It is deposited on the beach where the turbulence of breaking waves creates a trough in the sandy bottom. It is carried back into the water, however, by the rip currents that break off from the beach; the sand is then deposited again on the beach. This process of sand transport is responsible for the formation of sandbars, which are separate strips of sand that connect to the main beach or are separated from it by the water.
A beach profile is the shape and location of a beach when viewed from above, usually from a helicopter. The portion of the beach closest to the water is called the beach face, while the area inland from the beach is the backshore. The foreshore is the zone of unconsolidated sediments between the beach face and the berm, which is higher during stormy conditions. The beach profile is also affected by the weather, as wind and wave activity will affect the speed at which sediments move, how rapidly they erode, and whether they are blown inland by onshore winds to form dunes.