The Great Barrier Reef comprises many different types' of reef formations. These can be up to 30 kilometres long and are divided by narrow passages. There are also a variety of other formations including lagoonal, crescentic and planar reefs. It has little to do with the types of corals that were important in their formation. The combined action of waves and water currents causes the deposition of finely eroded coral material which over many years may lead to the development of these islands.
Most reefs have a solid front which receives much of the force of the ocean swell.
This is called the windward side of the reef. The top of this structure, termed the reef flat, may become exposed at low tide. It is normally dotted by small, squat corals which are able to withstand the force of the sea. The substrate between the corals is covered by a hard crust of coralline algae. This is the lagoon.