Barrier islands are separated from the mainland by a lagoon . They are commonly associated with tidal flat deposits. Lagoons are bodies of water on the landward side of barrier islands. Lagoons are also present behind reefs, or in the center of atolls. Tidal flats border lagoons.
Tidal flats are areas of low relief, cut by meandering tidal channels.
Intense burrowing is common. Stromatolites may be present if conditions are appropriate. Most modern reefs are in warm, clear, shallow, tropical seas, between the latitudes of 30 o N and 30 o S of the equator. Sunlight is required for reef growth because of the presence of symbiotic algae called zooxanthellae which live in the tissues of corals. The continental shelf is the flooded edge of the continent.