THE QUATERNARY PERIOD 333 



positions of the moraines there clearly prove that the ice front 

 was more or less distinctly lobate. 



Successive Ice Invasions 



The front of the great ice sheet, like that of ordinary valley 

 glaciers, must have shown many advances and retreats. In the 

 northern Mississippi Valley, however, we have positive proof of 

 several (perhaps five of six) important advances and retreats of 

 the ice which gave rise to true interglacial stages. The strongest 

 evidence is the presence of successive layers of glacial debris, a given 



Fig. 210 

 Diagram to show how successive glacial drift sheets are 

 distinguished. N to m, younger drift; m to S, surface of 

 older drift. Surface of younger drift almost unaffected by 

 erosion and weathering, while the older drift is notably 

 dissected and its surface considerably weathered. A 

 distinct terminal moraine at m marks end of younger 

 drift sheet. Heavy black bands represent deposits of 

 organic matter. 



layer often having been oxidized, eroded, and covered with vegeta- 

 tion before the next (overlying) layer was deposited (see Fig. 210). 

 In drilling wells through the glacial deposits of Iowa, for example, 

 two distinct layers of vegetation are often encountered at depths 

 of from 100 to 200 feet. Near Toronto, Canada, plants which 

 actually belong much farther south in a warm climate have been 

 found between two layers of glacial debris. Thus we know that 

 some, at least, of the ice retreats produced interglacial stages with 

 warmer climate and were sufficient greatly to reduce the size of the 

 continental ice sheet or possibly to cause its entire disappearance. 

 By applying the principles just laid down, at least five advances 

 and retreats of the ice, with distinct interglacial intervals have 

 been recognized in North America as follows: (1) Pre-Kansan or 

 Jerseyan; (2) Kansan; (3) Illinoian; (4) Iowan; and (5) Wisconsin. 1 



1 The Wisconsin invasion is sometimes divided into two — an early 

 and a late Wisconsin. 



