Deltaic sedimentation by the Mississippi and its ancestral streams has been an essentially continuous process since at least the beginning of the Tertiary, so that it is difficult, if not impossible, to assign a definite date to the beginning of Mississippi River delta history. Tertiary deltas, located in Texas and Louisiana, are indicated by thick lenses of interfingering marine and continental sediments. Pleistocene deltas probably roughly coincided in position with the present shore line; their distal portions are buried, but their landward parts (delta-floodplains) are now elevated as terraces. The Recent delta extends from an apex at the head of Atchafalaya River to a base of 150 miles along the Louisiana coast, with the now active portion lying in the eastern part o the delta proper.

The sedimentary load of the Mississippi River (probably more than two million tons per day) is concentrated by deposition in the vicinity of the active passes. This concentration of load results in rapid subsidence, eight feet or more per century, as shown by subsiding bench marks and other lines of evidence. Though land is added in the vicinity of active passes, it is lost to adjacent areas dominated by subsidence. Delta-flank and subdelta-flank depressions thus become sites of marine deposition well inland from positions contemporaneously receiving continental deposits.

Delta sedimentation results in a characteristic pattern consisting of: (1) a framework of natural levees, with associated channel and crevasse deposits; (2) within this framework, deposits of the marshes, lakes, and bays; (3) an outer margin of beach and bar deposits; and (4) deposits of the open Gulf beyond. Natural levees, the dominant land form of the delta, start forming below water beyond the ends of distributaries, and gradually increase in height upstream. Owing to subsidence during growth, more natural levee material is present below than above the surface. Natural levees grow higher during general overtopping by flood waters. Breaks in the levees result in crevasse deposits. Most crevasses are not permanent; a crevasse is maintained and becomes a significant distributary only when its angle of bifurcation with the main channel is relatively small.

The dominance of natural levees in the delta pattern is vigorously contested by shore-line processes. Wave erosion results in a concentration of sand in the form of a fringing arc of bar and beach deposits around the whole outer margin of the subaerial delta. In the vicinity of active distributaries, this concentration occurs as a submerged bar. In recently abandoned subdeltas it forms a barrier beach, whereas in older subdeltas wave erosion has resulted in retreat of the barrier beach to the main coast line, forming an ordinary shore beach which is still rapidly advancing landward.

Sediments of the Lower Delta, though exceedingly variable in character, show a definite relationship between sedimentary type and environment of deposition. The coarser elements tend to be concentrated in active channels, at the crests of natural levees, and where winnowing has been effective in removing the finer constituents, as on beaches, bars, and in open bays. Finer elements are concentrated in quiet water farther from active distributaries, such as marshes and stagnant bays and channels.

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Sorting varies from very poor in the case of many of the fine sediments and flood-water deposits in channels and on bars, to extremely good in beach, dune, and open water sands subjected to a long period of winnowing.

The usual picture of delta structure as a series of top-set, fore-set, and bottom-set beds is not supported by evidence in the Mississippi delta. A more correct picture is furnished by a pile of roughly superimposed leaves, in which the veins of the leaves represent natural levees and associated channel and crevasse deposits. Intervein areas represent deposits of the marshes, lakes, and bays, whereas the outer margins of the leaves indicate beaches and bars,--the buried ones probably being similar to the "shoe-string" sands of Kansas and Oklahoma. Beyond the fringing bars and beaches occur deposits of the open Gulf. Where advance of the sea over old abandoned subdelta surfaces has partially destroyed the delta pattern, marine beds, easily recognized by their faunal content, interfinge with the delta complex. In view of this intimate association between marine and non-marine sediments, some of the deposits in the open Gulf probably should also be classed as deltaic.

The most characteristic feature of large deltas appears to be the interfingering of marine and continental sediments plus a greatly over-thickened section in the zone of interfingering, the latter resulting from combined subsidence and deposition in the distal portion of the subaerial delta. Thus, in a broad sense, the delta is a thick lens, with a relatively thin non-marine inner portion, a correspondingly thin outer marine portion, and a thick central zone composed of alternating marine and continental sediments.