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The Recent deltaic mass of the Mississippi River is a huge pile of seaward-thickening deposits which underlies the deltaic plain and continental shelf. It rests upon and buries a rugged surface sculptured during the last ice age when sea-level was 450 feet lower than at present. The sediments were deposited as the last Pleistocene ice sheets melted and sea-level rose, and their main lithologic characteristics reflect the gradual lowering of stream gradients. The mass can be divided into a substream of permeable, gravel-bearing sands and a finer-grained topstratum of relatively impermeable, more heterogeneous sands, silts, and clays. Lithologic, textural, and faunal characteristics of depositional units within the topstratum are similar to those of sediments now accumulati g within depositional environments of the region.
These environments are mainly marshland and pro-delta marine, but they also include more restricted ones such as the fluviatile and brackish-water channels, bay and lake bottoms, and local beaches and spits. Interfingering and overlapping relationships of the various facies in the top-stratum show that the Mississippi River changed its position many times while sea-level was rising. That active subsidence accompanies the deltaic accumulation is shown by the seaward tilt of mappable marine beds within the mass and by the slope of the late Quaternary surface underlying the Recent deposits.
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