The Mississippi submarine trench differs from other submarine valleys in being "trench-like" with a broad, flat floor rather than "V-shaped." Domes and ridges stand out in topographic relief along the southwest margin of the trench; others, less prominent, lie along the northeast margin. These domes and ridges may be related to underlying salt structures.

The Iberian trough is flanked by the "Five Islands" along its southwest margin and by another series of domes along the northeast flank. The "Five Islands" are unique among onshore South Louisiana salt domes in having pronounced topographic expression. They are described briefly to illustrate the similarity of the Five Islands and the Iberian trough to the Mississippi submarine trench and the salt domes along its margin. The two major structural troughs have the same trend and are in alignment. This similarity and their probable common or related origin are discussed.

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