THE GLACIAL OR PLEISTOCENE PERIOD 219 



and Columbia rivers; (3), the whole of northern and western Alaska, embrac- 

 ing the Yukon River Valley, besides other smaller streams; and (4), the drift- 

 less area in southern Wisconsin bordering the Mississippi River. A fifth region 

 of survival was provided in New Jersey and Pennsylvania east of the 

 Appalachian chain and south of New York. 3 



3. THE ICE INVASIONS 



Five distinct ice invasions, separated by four well-marked interglacial 

 intervals are now known, with a possible sixth invasion and a fifth interval. 

 For many years the Glacial Period was thot to be a unit, the complexity of 

 the margin being attributed to minor fluctuations in the ice sheet. Accumu- 

 lated evidence first noticed in the west by Chamberlin, McGee, Salisbury and 

 other geologists, and more recently recognized in the east by later investigators, 

 have conclusively demonstrated the complexity of the Glacial Period and have 

 established beyond question the reality and significance of the interglacial 

 intervals, which are now believed to have been, in part at least, as extensive 

 and far reaching as is the period in which we live. 



The six ice invasions and the five interglacial intervals, as they affected 

 America, may be tabulated as follows: 4 



1 Nebraskan and Jerseyan, first recognized invasion. 



a. Aftonian, first interglacial interval 



2. Kansan, second glacial invasion. 



b. Yarmouth or Buchanan, second interglacial interval. 



3. Dlinoian, third glacial invasion. 



c. Sangamon or Toronto, third interglacial interval. 



4. Iowan, fourth glacial invasion. 



d. Peorian, fourth interglacial interval. 



5. Early Wisconsin, fifth glacial invasion. 



e. Fifth interval of deglaciation (unnamed). 



6. Later Wisconsin, sixth glacial invasion. 



f. Glacio-fluviatile sub-stage. 



g. Champlain sub-stage (marine). 



The extent of the interval between the earlier and later Wisconsin invasions 

 is not definitely known. No soil horizons referable to this interval are at 

 present known. 



a. Interglacial Life and Conditions 



The data upon which to reconstruct the fauna and flora of the interglacial 

 intervals are necessarily meagre, consisting of scattered records of imperfectly 



3 There is reason to believe that favorable conditions for the survival of many boreal 

 mollusks existed in Greenland, Newfoundland, Anticosti and other places along the Atlantic 

 coast and also along the Pacific coast (vide Scharff, 1907, and Adams, 1905). 



' Chamberlin and Salisbury, Geology, III, p. 383. The table has been reversed. The 

 name Nebraskan of Shimek replaces the old Sub-Aftonian. Whether this is to be correlated 

 with the Jerseyan of the east is not at present decided. 



