A beach is a zone of unconsolidated material along the shore of an ocean, lake or river. It can consist of sand, gravel, cobbles, shells, marine algae or even a mixture of these materials. Beach sediment may be deposited or eroded by waves and wind, and it can move long distances up and down the coast. Some beaches are characterized by their fossils, while others are inhabited by modern animals and plants.
Sand, or sediment, is the most common material found on a beach. It can come from a variety of sources, including erosion by waves and currents or from rivers. Typically, the sand on a beach is fine, with particles that range in size from a few inches to a few feet. Shells, seaweed and even calcite from the sea floor are also common. On Sanibel Island in Florida, for example, beaches are largely composed of intact or broken fragments of shells. Beaches are also home to ooids, light-colored ovals of calcium carbonate that look like mint candy (Tic Tacs).
Beach sediment is deposited by ocean waves and currents. As waves reach shallow water, their height decreases and the space between individual waves increases. This process is known as shoaling, and it can lead to the formation of sand bars and lagoons. A sand bar is a ridge of sand located offshore from a beach. It may be the result of a buildup of sediment in an embayment or bay, a delta formed by a river flowing into a body of salt water, or a deposit of sand relocated by waves and currents from a nearby beach.
Some sand bars are quite large, covering an entire beach or even parts of several beaches. In these cases, they are often described as a barrier spit or baymouth bar. The area of water left behind such a bar is often silted up and becomes salt marshes.
In other places, such as the entrance to a bay or inlet, an ocean bank may form. These are typically smaller, but they can be as wide as a beach or more. They are usually made of sand, but they can be any type of granular material that wave and current action can affect, such as a mix of soil, silt and gravel.
During the low tide, many animals live buried in the sand on a beach. Clams, crabs and other crustaceans, shellfish and marine worms all live in the sandy intertidal zone on beaches. The sand also provides habitat for birds and marine mammals. Beachgoers can help to protect the environment of a beach by reducing pollution from litter and garbage. Floating plastic may choke or entangle seabirds and other wildlife, while debris can block the growth of grasses and marine algae. Beachgoers should also leave animals and vegetation alone, and avoid taking shells from the beach. This helps the habitat of these organisms and prevents the spread of invasive species.