The late Quaternary river-mouth deposits of the Mississippi were laid down during the cycle of sea-level change that has occurred since the beginning of the Late Wisconsin glacial epoch. Well data from the coastal Louisiana marshlands and the adjacent continental shelf, together with cores and samples from the Gulf floor, permit generalizations as to the nature, distribution, depositional history, and volume of these deltaic deposits. Carbon-14 analyses of wood and shells provide dates for younger deposits of the cycle.

More than 8000 cubic miles of sediments have been carried to the Gulf by the Mississippi during the late Quaternary cycle. This huge mass has been deposited within a 44,000-square-mile area, comprised of the deltaic plain and adjacent portions of the continental shelf and slope. The continental margin has subsided during deposition, forming a trough-like depression which is localized to the depositional area. This trough has been downwarped more than 350 feet near the present shoreline and more than 500 feet offshore on the continental shelf.