894 EMILIANI AND FLINT [CHAP. 34 



Much of the evidence on which glacial units are based consists of outwash 

 rather tlian of sediments deposited by glaciers directly, and most of it has been 

 drawn from a few valleys on the northern flank of the mountains. Data from 

 other parts of the mountains have been gradually embodied in the scheme, but, 

 as the sediments are discontinuous, correlation from one valley to another is 

 based in some instances only on similarity of sequence. 



The interglacial or "warm" units are based on both zones of weathering and 

 soils, and, more helpfully, on fossil-bearing sediments in which pollen is out- 

 standingly conspicuous. Some of the fossil-bearing strata occur in such a fashion 

 that although they are unquestionably "warm", their position in the sequence 

 as a whole is in doubt. A possible exception is the fine succession of lacustrine 

 deposits at LeflFe, northeast of Milan. The pollen content of this long succession 

 (Lona, 1950; Lona and Follieri, 1957) reveals an alternation of "warm" and 

 "cold" climates, reaching from near the base of the Pleistocene into a "cold" 

 unit considered as possibly equivalent to the Mindel (Table II). The sequence 

 at Lefife is tentatively fitted into the general Alpine succession through local 

 stratigraphic relations (Venzo, 1955). 



The units commonly recognized in central North America, the region where 

 the peripheral drift of the great ice sheet is best developed, are fewer than in the 

 regions of Europe discussed above. The column (Table III) includes only four 



Table III 

 Stratigraphic Column for Central North America 



STAGES 



Wisconsin Glacial 



Sangamon Interglac ial 

 Illinoian Glacial 



Yarmouth Interglacial 

 Kansan Glacial 



Afton ian Interglacial 

 Nebraskan Glacial 



glacial and three interglacial stages. Correlation of American with European 

 units is insecure except for upper parts of the Wisconsin stage, which are 

 equivalent, through i^Q dating, to upper parts of the Weichsel. Stratigraphic 

 units in other parts of North America have been commonly correlated with this 

 sequence. In the Cordilleran region, however, the use of local names has been 

 the general practice, 



C. The Last Major Glaciation 



The last major glaciation should be the most useful for study because its 

 record has survived in greatest detail. Actually, however, its stratigraphy has 

 proved difficult to establish, and much of the record is still unclear. The earliest 



