A beach is a coastal landform composed of a line of sand or other sediment. Beaches are continually changing and evolving, shaped by a wide range of natural processes. They can include weathering, where rocks break down into smaller pieces; erosion, where the waves wear away at the coast; and deposition, where the waves carry sediment onto the shore. These processes can happen slowly or quickly. Beaches are also constantly rearranged by the tides and currents that bring in new materials and take them away.
Beaches can be made of almost anything, from sand and shells to seaweed and even calcified algae. They can have a very steep slope or a gentle one. Some beaches have large rocks scattered across the beach, and others have no stones at all. Beaches are usually found at the ocean’s edge, but they may be on lakes, rivers, and in freshwater bays.
Most of a beach’s material comes from the erosion of rock, by waves and wind, in a process called beach drift or longshore drift. Beaches also contain a lot of debris, which can include garbage and the remains of dead organisms. These materials are moved around by tidal and ocean currents, which can travel a few meters or hundreds of kilometers.
A sandbar is an offshore ridge of sand or coarse sediment. The swirling turbulence of waves breaking off a beach excavates a trough in the sandy bottom. Some of this sand is carried forward on the beach, but much more is deposited off-shore as the flank of the trough. The sand suspended in the backwash and in rip currents as well as sand moving shoreward from deeper water add to the bar. Over time, the sandbar can grow to be very prominent.
There are many kinds of sandbars, ranging from small ones that only appear in calm weather to large berms that form on the low beach during a storm. A sandbar that is very close to the shoreline is sometimes called a groin or sand ridge. These are useful for reducing the power of ocean waves, and they often provide safe boat anchorage in dangerous areas.
Some sandbars occur at the mouth of a bay, and are called bay-mouth bars. These are especially important because they can protect the bay from incoming waves and tides.
Generally, a beach is divided into four zones. The zone closest to the water is the swash zone, which includes the runnel, berm, and beach face. The beach face contains the finer sand and shell fragments. The wrack line is the area that has been covered by the seaweed and other material washed up by the swash and waves. The berm is the higher, more solid sand and rock formation that often forms on the high-tide line. A beach can also be distinguished by the presence of a bay-mouth bar or a sand-drift bar in its nearshore region. These are the most important landmarks on a beach.