The predominant landform consists of flat, weakly to moderately dissected alluvial plains. The plains were formed by deposition of continental sediments into a submerged, synclinal trough, which was later exposed by sea level subsidence. Lithology and Stratigraphy. Rock units formed during the Cenozoic Era. The remainder consists of about equal amounts of Quaternary marine deposits and glacial outwash.
Recent alluvium consists of silt deposited during floods.
Soils are Aquepts, Aqualfs, Aquents, Udolls, and Udalfs. Haplaquepts, Ochraqualfs, Fluvaquents, Natraqualfs, Ochraqualfs, and Hapludalfs are found in back-swamp areas and older natural levees. Dystrochrepts, Udifluvents, and Fluvaquents occur in smaller areas. Hydraquents and Medisaprists are in southern Louisiana. These soils have a udic or aquic moisture regime, a thermic temperature regime, and montmorillonitic or mixed mineralogy.