A beach is a landform that lies mostly above water and is more or less actively influenced by waves at some point during the tide cycle. The composition of a beach depends upon the nature and quantity of sediments washed up from the nearby ocean, the speed of water and wind flow and turbidity. Sediment that is fine, has a low viscosity, and is compacted will be more resistant to erosion. Established vegetation such as sea grasses and dunes also resist erosion by slowing the fluid flow at the surface layer.
As waves crash into beaches they carry sand south, a process known as longshore drift. This is especially true in the surf zone where waves break near the shore. However, sand may be carried farther offshore to sandbanks where it is deposited by submarine canyons. The net movement of sand along coasts is from north to south, but this motion depends on many factors such as sea swell direction and the size of the resulting wave.
In the temperate parts of the world, most beach sand is primarily quartz with some feldspar and a few heavier minerals such as calcite. In the tropics, tropical beach sands are composed of calcium carbonate that has been deposited by the skeletal remains of marine organisms. Beach sands that are composed of calcareous materials tend to have a darker color than other types of beach sand.
The beach is a habitat for a variety of plants and animals, as well as a place for recreational activities such as swimming, sunbathing, and walking. Beach flora such as sea grasses and dunes provide food for crabs, insects, birds, and other small animals. Birds use beaches to nest and sea turtles lay their eggs. Beaches serve as a filter for runoff from the coastal plain and may become enriched with water borne silt and organic material.
Some minor relief forms are visible on sand beaches, such as oscillation ripples (ripples on the surface of still water), swash or rill furrows, and the famous beach cusps that form when wave crests parallel the beach. In high-energy wave conditions, beach cusps may have a crescentic shape, while they can lose their shape in lower energy conditions.
Underneath most beaches are layers of coarse sand and rock, sometimes cemented together with calcium carbonate, called beach rock. If the rocks and sand are exposed, they often have a yellowish color. Beach rock is common on sandbars that form when two pushing currents meet. It is also found in bays and other enclosed bodies of water that ocean water flows into.