GEOMORPHIC IMPLICATIONS OF INCREASING MULTIPLICITY OF RECOGNIZED PLEISTOCENE GLACIAL ADVANCES

The classical fourfold subdivision of the Pleistocene Epoch has generally been abandoned. Wisconsinan and Illinoian Glaciations remain acceptable, but beyond that everything is now lumped into the Pre-Illinoian. However, new units are appearing as field studies find new evidence, and the "one-million-year Pleistocene" now encompasses more than twice that span. Evidence of 8 Pre-Illinoian glaciations has been recognized in Nebraska and Iowa (Boellstorf, Easterbrook), 4 in central Missouri (Guccione, Rovey, Balco),and 3 (perhaps 5) in Kansas (Bayne, Dort). Oldest dates are in the range of 2.2 to 2.4 million years. Verification of this increased number of glaciations forces consideration of several direct implications regarding geomorphic history. Purely for convenience suppose that there have been 10 separate Pleistocene glaciations (8 Pre-Illinoian plus Illinoian and Wisconsinan). That would require that 10 times the Upper Mississippi drainage developed, only to be obliterated 9 times. Nine out of 10 generations of all other northern continental drainages had the same fate. Drainageways in unglaciated areas maintained courses, but they had to adjust downstream multiple times to remain accordant with master-stream redevelopment. Ice-dammed lakes formed and drained 10 times; were there 9 preceding generations of Lake Agassiz? Ten times the land surface was subjected to isostatic depression, then rebound, contributing to ubiquitous development of jointing in surface rocks and promoting stream diversions and piracies. Ground water was affected by augmentation, then depletion everywhere because the glacial/interglacial cycles reflect basic changes in terrestrial climate. And, of course, sea level fell and rose again, shorelines migrated, and coastal streams were rejuvenated, then blocked in the same recurrent cycles. Because glaciation is basically an erosive event, evidence from the oldest cycles is most sparse, being preserved in scattered parts of the deepest cuts in the bedrock surface, unseen, often unsuspected. Cycles in alpine areas were equally numerous, but evidence will have been largely destroyed.