The outer seaward edge of the reef slopes rapidly to a great depth. The outer edge of the reef has a massive expanse of coral growing just beneath the surface containing huge boulders and pinnacles which extend seaward under the breakers to form a steep rough cliff. This cliff merges beneath the active zone of reef formation into a slope of dead coral, which is known as the talus slope. Behind the main platform of the reef there are crevices and gullies filled with fragments of rock, coral and sediment which forms a crude cement and gives a base for the coral to grow upon. Atolls The third type of reef is known as an atoll.
Atolls form far from land typically.
An atoll is similar in construction to a barrier reef. The difference is that an atoll is roughly circular in shape with steep outer sides that slope down into very deep water. Within the walls of the reef exists a shallow lagoon, which rarely reaches 100 feet deep. Also there exists a type of reef known as a bank reef. This occurs when a coral reef grows in a marginal area during an inter-glacial period.