As the Earth plunged into the Ice Age, a pre-Illinoian (or possibly the Illinoian) glaciation diverted the Missouri River southeastward toward its present confluence with the Mississippi and caused it to integrate into a single river system that cuts across the regional slope.
Dado que a terra afundiuse na idade do xeo, unha glaciación pre-illinoiense (ou posiblemente a glaciación illinoiense) desviou o río Missouri cara ao sueste ata a súa actual confluencia co Mississippi e causou que se integrase nun único sistema fluvial que corta a pendente rexional.