A beach is an area of sand, rock and gravel that is deposited along a body of water by the action of waves and tides. It is often lined by shrubs, grasses or other plant life that has adapted to the salty environment.
Beach formation is a complicated process that involves many different factors over a long period of time. Weathering, or the breaking down of rocks into smaller pieces, can happen naturally, but it is also accelerated by the constant action of waves on the coastline. These waves wear away the surface of the rocks and, if they are particularly large, may even carry them off into the ocean. As these rocks travel, they can be worn down again, but they will eventually be carried back to the beach, where they are once again deposited in a new location.
As sand is moved along beaches by wind and waves, it is usually deposited on the areas nearest the water. The parts of a beach that are completely covered by water at high tide are known as the intertidal zone. The sand that is dry at low tide may be private property and people are generally prohibited from entering it. However, some states have legislation that allows the public to cross private land to access a section of beach deemed by the government to be public.
In places with very powerful waves, the sand is deposited into an upper terrace of ridges and depressions, called a dune. In places where waves are less energetic, the sand is deposited into lower terraces of sandbars and troughs. These troughs are sometimes called embayments. The shape of a beach can be influenced by how much sediment is being produced upstream, the speed and turbidity of the moving fluids, the size and state of the particles involved, the presence of vegetation, and how stable the foreshore is. In well-stabilized areas, sand tends to accrete (accumulate). In unstabilized areas, it is more likely to erode.
Cusps, or hummocks, are ephemeral features that form in the spaces between the sandbars and sandbanks when conditions are right. They are a result of waves passing through a space between the sandbanks and the shore, and they change in shape and height with changing wave conditions.
Occasionally, beaches are replenished by importing and depositing sand from riverbeds or from sand quarries. This practice is referred to as beach nourishment. It is controversial because it can significantly alter the natural characteristics of a beach and its environment, but it is common in some locations. The sand being used for beach nourishment must be substantially different in composition from the naturally occurring beach sand. The resulting beach may not be as beautiful or as ecologically diverse as a natural one.