-!92 W. UPHAM STAGES OF THE ICE AGE 



drift sheet and melting later from the Mississippi, was the third, or Illi- 

 noian, icefield, belonging to the far extended Labradorian area of glacial 

 outflow. After the Kansan and Illinoian stages, which appear to have 

 been mainly contemporaneous, another retreat of the ice-sheet took place 

 in the Sangamon interglacial stage. Both the Yarmouth and Sangamon 

 areas of wavering glaciation, and the duration of their stages, were slight 

 in comparison with the Aftonian stage, which was the most prolonged 

 interruption of the Ice Age on this continent. 



Next were the Iowan and Wisconsin stages of glaciation, the fourth 

 and fifth in the list of wide climatic fluctuations that successively amassed 

 and melted back the North American icefields. The Iowan drift was 

 followed by the chief deposit of the loess, a peculiar silt, which was weath- 

 ered and eroded in the vicinity of Peoria, in Illinois, previous to the re- 

 advancing Wisconsin glaciation. Hence the name Peorian is applied to 

 the interglacial time of recession of the ice-border between these stages of 

 colder climate and ice-advance. Depression of the land during the Iowan 

 glacial stage had given decreased slopes of the Missouri, Mississippi, and 

 Ohio valleys, permitting them to receive the thick formation of the loess. 

 The land subsidence appears to have remained, or even to have progressed 

 for the northeastern part of our continent, during the Wisconsin stage, 

 while the most prominent marginal moraines were amassed at times of 

 pause or often some readvance that accompanied the general melting and 

 final departure of the ice-sheet. 



After the ice had melted far back, the sea covered the depressed coast 

 and reached inland along the Saint Lawrence, Ottawa, and Lake Cham- 

 plain valleys, forming fossiliferous marine beds, from which this latest 

 and relatively short part of the Glacial period is named the Champlain 

 stage. It is thus seen that the alternating Iowan, Peorian, Wisconsin, 

 and Champlain stages, comprising the time of greatest subsidence of the 

 continental glaciated area, were attended by the growth, fluctuations, and 

 disappearance of the Iowan and Wisconsin icefields. These four late 

 stages seem to have been somewhat closely continuous with the preceding 

 Kansan and Illinoian maximum glaciation, which was widely separated 

 from the Nebraskan stage by the Aftonian retreat of the ice boundaries. 



A similar sequence of glacial and interglacial stages has been traced in 

 the British Isles, northern Europe, and the Alps, which indicates that the 

 climatic conditions producing the Ice Age on the continental areas at 

 opposite sides of the Atlantic, were approximately contemporaneous, and 

 that their fluctuations of glaciation were caused by like secular changes 

 of climate. 



