The barriers which skirt the greater part of the northern Gulf Coast constitute sand bodies with widths up to several miles and thicknesses of 20 to 60 feet. In most places the sand bodies are bordered on both sides by muddy sediments. The larger barriers have at least four fades— beaches, dune belts, barrier flats or marshes, and inlets. Each of these has sediment characteristics which are usually distinctive, one from the other. The beach sands can generally be separated from the dune sands by lower grain roundness, lower silt content, and more even stratification. The barrier fiats and marshes have higher silt and clay content than the other environments and commonly contain calcareous aggregates. The inlets are intermediate in sand content and can be recognized by their mixture of bay and open-gulf organic remains. The barrier islands have formed either during or since the rise in sea level at the end of the last glacial epoch. Contrary to a long-held opinion, they are not related to coast lines of emergence but either to slow submergence or to a steady state. Accordingly, it is reasonable that the ancient barriers should have been preserved in the stratigraphic column of a subsiding area like the Gulf Coast.
The source of sand necessary to maintain the barriers is to a great extent the sand deposits of the continental shelf, supplemented by the sands of the few rivers which enter directly into the gulf rather than into bays. In places where the shelf sand has been covered by mud, the barriers have been known to be completely eroded. Some of the barriers are growing seaward and others have retreated, while still others have been quite stable in position during historical times.
Barriers are common along other coastal lowlands around the various continents, although they may develop to a moderate extent along mountainous coasts. They are particularly common along the flanks of the large deltas of the world, occurring especially at points where the delta deposition has temporarily ceased and where the area is subsiding and being reworked by the waves.