B. lagoons Coastal lagoons are created when the sea level is rising to the coast, however they do not form on steep slopes or rocky terrain. Coastal lagoons are also where barrier reefs are formed, sectioning it off from the sea. They're sensitive to the tides coming in, if the tides recede too much then the lagoon is left dry, however if the tide comes in too much then the water can break the reef and engulf the lagoon.