barrier reef

An elongated accumulation of coral lying at low-tide level parallel to the coast but separated from it by a wide and deep lagoon or strait. The coral is thought to have formed initially on a flat surface: then as the sea-level rose in post-glacial times, thereby submerging the irregular wave-cut platform, the coral growth kept pace with the rising ocean level, so creating the great thickness witnessed today in such places as the Great Barrier Reef off the East coast of Queensland, Australia. This stretches for more than 1900 km and varies in width from about 30 km to 150 km. (Source: WHIT)