A sand bar is a long, narrow ridge of sand found in shallow water. It is typically formed by the action of waves and currents. A SHOAL is a similar formation, but it is usually composed of gravel or stones instead of sand. A REEF is an underwater formation of coral or rock that is often home to a variety of marine life. An OASIS is an area of lush vegetation surrounded by desert, often with a source of fresh water. All of these terms are related to sand bars in that they are all formations of sediment in water.