The great Barrier Reef began to form where the shallow area of the sea floor had favorable conditions to encourage and support colonization of benthic plants and animals.  These conditions are the presence of a hard bottom free from excess mud and sand.  The area must be clear, warm sea water with enough movement to circulate oxygen and plankton to the colonial organisms.  The skeletal limestone remains of corals and other are then cemented together by calcareous algae.  The process continues as the reef "grows" towards the surface.  Particular to the Great Barrier that encouraged the continued growth of the reef is the large continental shelf off the coast of Queensland which provided the base for the reef system.  A second factor was the fluctuating sea levels that occurred in the Pleistocene era.  A time when the continental shelf was exposed to the air.  At this time the colonial organism were able to keep pace with the raising sea level until the present reefs were formed.  Warm waters and favorable oceanic and tidally induced currents maintain flourishing coral growths throughout the Great Barrier Reef at the present time.  The Great barrier system is a cluster of shelf reefs which are found in shallow waters as opposed to oceanic reefs which are found in deeper waters.