Ground-breaking work on the Quaternary of the Lower Mississippi Valley was published in 1944, in which Fisk identified four terrace formations above the contemporary floodplain. He assumed that base level changes which resulted from glacioeustatic sea-level changes were the principal controls on aggradation and degradation in the Lower Mississippi Valley.
Based upon this assumption, he assigned each terrace formation to a period of glacial minima (high sea level), and presumed that each terrace rested upon an erosional surface which was carved during the preceeding glacial maxima (low sea level). As such, the Williana Terrace was post-Nebraskan, the Bentley was post-Kansan, and so on. The lowest terrace was assigned to a brief mid-Wisconsin glacial interlude, with the modern floodplain being post-Wisconsin.
Fisk's foundation has required extensive revision. Aggradation is now associated with early stages of deglaciation, and glacioeustatic base level changes are regarded as unimportant above the delta. Contemporary researchers report that a very sparse and uncertain record exists for early and mid-Pleistocene geology of the Lower Mississippi Valley, and thick, extensive valley train deposits were laid down by a braided river system during Wisconsin glaciation.
Adapted from pp. 547-561 of Autin, W. J., et. al. 1991. "Quaternary Geology of the Lower Mississippi River Valley." Chpt. 18 in Quaternary Non-glacial Geology: Conterminous United States. Volume K-2 in The Geology of North America series. Boulder, CO: Geological Society of America.