The coast, i.e., the zone where land, sea, and atmosphere meet, is one of earth's most varied and variable environments. Its tectonic and climatic history have produced a configuration which is being altered by physical, chemical, and biological processes. Because these processes operate at different intensities in various parts of the world, the degree of alteration varies greatly.Coastal processes cause both erosion and deposition but the overall effect is a tendency toward smoothing out the coastline. Some of the most common forms along coasts of today are cliffs, beaches, lagoons, estuaries, deltas, dunes, and reefs. Many of these are ephemeral and are present today mainly because coastal processes are operating at a level only recently reached by a rising sea. Even though stillstand has prevailed only 3000–5000 years, there already has been a major reduction in the size and variety of many of these forms and also a major reduction in the quantity of sand available for the nourishment of the world's beaches and coastal dunes. As long as stillstand continues, variety in coastal forms is destined to be reduced even more.