THE PLEISTOCENE OR GLACIAL PERIOD. 



387 



the Lafayette formation, upon which it rests unconformably (Fig. 513); but 

 it also contains crystalline pebbles and calcareous clays assignable to wash 

 from the glacial regions, all other assignments seeming to be excluded by a 

 special investigation. A marked interval between its deposition and that of 

 the overlying loess is indicated. As the sub-Aftonian and Aftonian deposits 

 are the only older ones with which great gravel deposits are known to be asso- 

 ciated, and as the Natchez deposit must be referred to an early Pleistocene stage 

 because the great Mississippi trench, 60 miles more or less in breadth, has been 



Fig. 512.— Section just east of Oelwein, la. 1, Sub-Aftonian (Jerseyan); 2, Aftonian 



3, Kansan; and 4, Iowan. 



excavated since it was formed, reference to one of these two stages is more plausi 

 ble than to any later one. This reference is strengthened by the fact that almost 

 the whole formation— which was clearly a valley train leading back to the drift 

 area — has been removed. 



Assuming the correctness of this reference and combining it with other data, 

 the following tentative conception of the sub-Aftonian and Aftonian stages is 

 reached. The ice-sheet spread from the Keewatin and Labradorean centers to 

 the approximate limit of the known drift in the Mississippi valley, and deposited 

 a typical sheet of bowlder clay (sub-Aftonian) and also gave rise to great valley 

 trains of glacio -fluvial material that stretched from the drift border to the Gulf, 

 filling the low-gradient valleys of the time to depths of 30 to 50 feet near the 

 drift border, and of 200 feet near the Gulf (Natchez formation). The invasion 



