ABSTRACT

Wave power gradient studies along the mainland in the St. Joseph Bay area (panhandle coast of Florida) indicate that the dominant wave approach direction, responsible for littoral drift towards S-SE, is from the west. With this direction, attempts have been made to arrive at a relative depositional sequence in this region consisting of: bay, spit with beach ridges and mainland beach ridges.

On a smoothed coast (without the beach ridges which now give it a curvature), but with the present day bathymetric conditions, the wave power gradient was determined. There was no noticeable drift component indicating deposition. Then, the bathymetry was made smooth by the removal of the offshore shoal, the effective littoral power was computed and plotted to get an idea of the littoral drift. As the third stage, successive sets of beach ridges were removed from the spit one after the other and the littoral power was determined in each case. At a certain stage, after the removal of a specific number of ridges in the spit, making the bathymetry smooth, the effective littoral drift showed a strong drift toward S-SE, accounting for deposition, in turn leading to the growth of the mainland beach ridges (which originally had been smoothed out).

This produced one possible depositional history of this region (within a short period in the late Holocene): initially, the simultaneous growth of the mainland beach ridges (sometime after the closing of the channel east of the Cape San Blas) and the northern part of the spit extending northward from the cape, with the associated beach ridges; then the growth of the remaining part of the spit and the formation of the shoal attached to it; and finally the termination of growth of the mainland beach ridges. In the absence of any datable material, the age of the beach ridges on the mainland is thought to be between 400 and 600 years (deduced from the average growth of the beach ridges along the northern tip of the spit).