Definition: An oceanic island, often having a characteristic ring-like shape surrounding a lagoon. Atolls are formed when coral reef grows around a volcanic island that later subsides into the ocean.
Definition: Gravel is an environmental material which is composed of pieces of rock that are at least two millimeters (2mm) in its largest dimension and no more than 75 millimeters.
Definition: A sheet of saline water separated from the open sea by sand or shingle banks. The sheet of water between an offshore reef, especially of coral and mainland. The sheet of water within a ring or horseshoe shaped atoll.
