ABSTRACT

Comparative field and aerial photographic studies of the Mississippi-Alabama Gulf Coast barrier islands reveal that Cat Island, the westernmost barrier island in the series, is unique in having a large number of prominent, forested beach ridges. The Cat Island ridge complex is composed of three distinct sets of sub-parallel east-west trending ridges. The ridge set on the south side of the island is younger and less well developed than the older two sets.

There is considerable evidence that the barrier island system predates the eastward progradation of the St. Bernard lobe of the Mississippi delta complex, which began its eastward progradation about 3,000 years before present, and continued until its abandonment approximately 1,500 years ago. The low, poorly developed ridges along the southern shore of Cat Island and the low ridges and broad swales of Middle Spit mark the transition to a more protected, energy-reduced environment that accompanied the eastward progradation of the St. Bernard Delta into the area to the immediate south and southeast of Cat Island.

The development of the St. Bernard Delta complex not only modified the previously existing longshore current and littoral sediment transport; it also protected Cat Island from the erosional effects of waves and swell of the open Gulf of Mexico, with the exception of a narrow "window" to the southeast. The "window" lies between Ship Island, the barrier island east of Cat Island, and the Chandeleur Islands, which are reworked distributary sands of the St. Bernard Delta. This restriction in the direction of wave approach has resulted in the partial reworking of the eastern end of the progradational ridge complex into a large northeast-southwest trending spit, thus giving Cat Island its unique "T" shape.

It is likely that during the pre-St. Bernard Delta period of robust sedimentation, the other islands in the barrier chain also exhibited progradational ridges similar to those now found only on Cat Island. Unlike Cat Island, which has been protected and preserved by the St. Bernard Delta, the other barrier islands have subsequently suffered a general reduction of sediment supply due to the weakening and eastward displacement of the littoral currents, and have been modified and reworked during the past 1,500 years by processes of island and inlet migration and storm wave washover.