A beach is a shoreline of sand and other loose sediments, shaped by waves and wind. Beach materials are deposited by wave action and may include sand, shells, gravel, marine organisms such as crabs, and sometimes the cemented remains of dead animals or plants. Beach sands vary in composition depending on the geology of the surrounding area. Some are white, others black, tan, yellow or red.
A typical beach is a strip of sediment bordering a rocky or cliffy coast. Its shape and character depend on the kinds of rocks in the immediate area and the rate of delivery of sand particles from offshore. Beaches also vary in width, slope and depth.
Most beaches are built of sand, although some are composed of rock or coral rubble. Some are tidal and change size regularly with the rise and fall of the tide.
Over time, repeated actions of waves and winds wear down and smooth the sand, making it finer and creating a coastline known as a beach. The sand is then carried away by the current and may be deposited as far inland as the river mouth or even further downriver. The sand is also removed by erosion from offshore headlands or slumped from a cliff face. Beaches are also nourished by sand-producing organisms, such as mollusks, crustaceans and fish that nibble at the algae attached to rock or coral surfaces.
As the beach sand is carried downstream by wave and current action, it tends to compact or clump together. This makes it more resistant to further movement by turbulent water from succeeding wave crests. This process is especially effective when there are long periods between the breaking of wave crests.
If the sand is not redeposited, gaps develop between ridges. These gaps may fill with sand from the sea floor or with debris, such as driftwood or human-made objects. Beach sands are also cemented together by calcium carbonate precipitated from the seawater. This type of beach, called a calcareous beach, is common in the tropics and also occurs on some continental beaches.
The sand composition and shape of beaches are affected by local factors such as the geology of the region, the types of organisms that live in the area, and the amount of rain that falls nearby. The shape of a beach is also affected by the presence of vegetation that traps sand particles and slows down inland movement of dunes by winds and waves.
The shape of a beach changes seasonally as a result of differences in the energy of waves during summer and winter. The sand of beaches is usually wider and has a gentler slope in the summer, when there are calmer seas with longer periods between the breaking of wave crests. The sand of beaches is narrower and steeper in the winter, when the waves are more energetic. These seasonal changes are reflected in the different colors of beaches.