Mississippi valley. Successive terraces that formed in lower Mississippi Valley during Pleistocene glaciations are shown in this highly schematic cross section. The terraces, with the oldest at the top, were flood plains laid down between glaciations. During each glacial period the river rejuvenated by the consequent fall in sea level and the consequent drop in its mouth, cut deeply into the preceding floodplain. The Prairie terrace represents between the early Wisconsin and late Wisconsin glaciation. Haag, 1962.