Empirical data fail to substantiate classical theories of barrier island formation from offshore bars. Specifically, the absence of open ocean beach and neritic sediments landward of barrier islands suggests that barriers have not developed from offshore bars. Formation of barrier islands from emergent bars is also rejected, because evidence from many areas of the world does not support a sea level higher than present during the Holocene. Also unacceptable is the hypothesis of continuous barrier development throughout the Holocene submergence because it does not explain the original formation. Barrier islands which form from barrier spits or, in some instances, from bars are accepted, but these methods are not regarded as the general mechanism of barrier island formation.
The hypothesis proposed here maintains that a barrier island is initiated by the building of a ridge immediately landward of the shoreline from wind-or water-deposited sediments. Slow submergence, as during the late Holocene, floods the area landward of the ridge forming a barrier and a lagoon. Once formed, the island may migrate parallel, or normal, to the coast or may remain stationary depending on sediment supply, local hydrodynamic conditions, and land-sea stability. The width of the lagoon depends on the slope of the mainland surface, amount of submergence, sediment infilling, and erosion. Slow submergence or negligible sedimentation is necessary to maintain the lagoon. Emergence in excess of lagoonal depth terminates the barrier system.