Date of this Version
The first of four North American Ice Ages commenced about 2.8 m.y. ago, reached a maximum between 2.4 and 2.6 m.y. ago during which continental glaciers invaded southwestern Iowa and southeastern Nebraska, and ended about 2.1 m.y. ago. This first Ice Age may span the Pliocene-Pleistocene boundary herein placed at 2.5 ± 0.1 m.y. The second Ice Age occurred about 1.9 to 1.5 m.y. ago. However, apparently it was not severe enough for glaciers to reach into Nebraska or central Iowa. The third Ice Age occurred between about 1.0 and 0.4 m.y. ago. Glaciers invaded Nebraska and Iowa at least four times during this Ice Age, and periods between these advances were of sufficient duration for the formation of paleosols and gleys. The classic Nebraskan-Aftonian-Kansan sequence was deposited during this Ice Age. The fourth Ice Age commenced about 100,000 years ago. Deposits representing this Ice Age reached central Iowa but apparently did not enter Nebraska.
Conceptual application of the classic North American Pleistocene stage terms has resulted in the stages having overlapping time spans. In view of this and because ≈ 60 percent of the Pleistocene is pre-Nebraskan, the classic stage terms need to be revised or abandoned.
Interpretations of events leading up to the Ice Ages in Nebraska have previously been based on faunas and sediments from what was considered a relatively complete sedimentary and faunal sequence spanning late Pliocene and early Pleistocene time (the "Kimballian fauna" from the uppermost Ogallala Group and the Broadwater Formation and fauna, respectively). Data presented here indicate these sediments and faunas are separated in time by about 4 m.y., and the significance of the marked differences between "Kimbal1ian" and Broadwater faunas needs re-evaluation.