THE GLACIAL PERIOD 



57 



atmosphere which would shut out much of the sun's heat -all 

 the*se have been suggested as possible causes, and each has been 

 judged more or less incompetent to produce the results. Possi- 

 bly several causes co-operated to produce the glacial age. It is 

 estimated by Penck that a reduction in the average annual 

 temperature of less than 15 

 F. would bring on a return 

 of the glacial period in North 

 America, a slight change 

 apparently to produce such 

 far-reaching consequences. 



If the exact causes still 

 are matters of debate, the 

 fact is certain that the Chi- 

 cago area was covered with 

 a deep glacier and that not 

 once but several times, for 

 there is good evidence that 

 a succession of glacial 

 periods have followed each 

 other in North America. 

 These have been designated FlG " 38.-Ma P of Morth America at 



. the time of the glacial period. Note that 



theAftomanorJerseyan,the the southern limit of the glacier was a 



Kansan, the Illinoian, the little short of the southern boundary of 



lowan, and the Wisconsin. what is now Illinois and Indi ana and that 



, f ,. , T7 . there was an unglaciated area in Wiscon- 



The oncoming of the Wis- sin and northwestern Illinois . ( SeeFig. 5 i.) 

 consin ice sheet occurred 



possibly twice as long ago as has elapsed since the last ice dis- 

 appeared completely here, a matter of some ten thousand years. 

 The lowan began, roughly, four times as long ago; and the 

 Illinoian, eight times or more. The time in which we live may 

 be one of the interglacial periods, and it may be that many of 

 the mighty works of our much vaunted civilization will, in the 

 distant future, be blotted out by the renewed ruthless advance 

 of the great ice cap. 



