Elongate accumulations of sediment formed in the shallow coastal zone and separated from the mainland by some combination of coastal bays and their associated marshes and tidal flats. Barrier islands are typically several times longer than their width and are interrupted by tidal inlets. There are at least three possible explanations for their origin: (1) longshore spit development and subsequent cutting of inlets; (2) upward shoaling by wave action of subtidal longshore sand bars; and (3) drowning of coastal ridges. All three modes of origin may have occurred. The first two have been observed in recent times, the third is yet to be conclusively demonstrated.