One of three main geomorphological types of coral reefs, the other two being barrier reefs and atolls. These are formed close to shore on rocky coastlines by the growth of corals and associated hydrozoans, alcyonarians and calcareous algae. Fragments of limestone derived from such bioherms are welded together by the encrusting calcareous algae as well as by the deposition of interstitial calcium carbonate cement, the latter brought about by geochemical reactions and possibly bacterial action. The zone of living corals is separated from the shore by a shallow reef flat where reduced circulation, periods of tidal emersion, and the accumulation of sediments inhibit coral growth. See Barnes and Hughes (1988).