A beach is a landform that stretches along the shore of an ocean, sea or lake and is formed from the accumulation of sediments. The sediments that make up a beach may consist of sand, gravel or crushed seashells and organic materials that have been carried by waves and deposited by the tides. Beaches are constantly changing in size, shape and composition. They are also subject to significant erosion and deposition by waves and currents.
The sediments that form a beach vary, depending on the type and location of the source area from which they are derived. In some cases, beaches are made of terrigenous (rock-derived) materials, whereas in others, the sediments may be biogenic (formed by the shells and skeletons of marine organisms). The origin of a beach is also important in understanding its dynamics. Beaches that are subject to intense erosion and transport by the ocean, longshore drift, rip currents and tsunamis will tend to be coarser and less well-stabilized than those that are relatively protected from these forces.
Beaches are usually dominated by siliceous (quartz) sediments in temperate regions, but in the tropics they can be made up of biogenic sands derived from the skeletal remains of shells and other marine organisms. These sands typically have a high cation-to-anion ratio, meaning that they are very alkaline in nature.
Some beaches are very wide and shallow, while others have a much steeper slope and can be covered in boulders. Beaches can also have a very varied color. Some are white, while others are dark gray or even black. In some regions, beaches are extremely polluted and contain toxic chemicals from industrial, agricultural and residential waste. Waves and winds wash this waste from inland areas, and it is often washed up on the beach after strong storms.
When a beach is viewed from the air, it can be divided into several segments. The portion that is closest to the water is known as the foredune or low-tide terrace and can be composed of sand, gravel or cobbles. Beyond the foredune are a series of ridges called berms, which are typically formed by the waves of a past storm. Behind the berms are submerged sandbars and troughs that can be quite deep at low tide.