The drift material carried into Minnesota and left brhind I)y the 

 melting: ice was picked up between the center from which the glacier 

 radiated and the northern border as well as along- its course across 

 the state. In the case of the later ice sheets glacial debris left by the 

 earlier invasions was to a greater or less extent incorporated in the 

 mass of fresh material l)rought by the advancing ice. a factor which 

 complicates such a comparison as that mentioned above. 



From eleven to thirteen successive stages of the glacial period are 

 generally recognized as applving to the United States (13, p. 383). 

 These are as follows, the most recent (XIII) being given first and the 

 oldest last (I). 



XIII — The Champlain sub-stage (marine). 

 XII — The glacio-lacustrine sub-stage. 

 XI — The later Wisconsin, the sixth advance. 

 X — The fifth interval of deglaciation, as yet unnamed. 

 IX — The Earlier Wisconsin, the fifth invasion. 

 VIII — The Peorian, the fourth interglacial interval. 

 VII — The lowan, the fourth invasion. 

 VI — The Sangamon, the third interglacial interval. 



V — The Illinoian, the third invasion 

 IV — The Yarmouth, or Buchanan, the second interglacial inter- 

 val. 

 Ill — The Kansan, or second invasion now recognized. 

 II — The Aftonian, the first known interglacial interval. 

 I — The Sub-Aftonian, or Jerseyan, the earliest known inva- 

 sion. 



There is some doubt as to the existence of the lowan. The late 

 FVofessor Calvin and members of the Iowa Geological Survey have 

 regarded this as a distinct stage of glaciation but little older than the 

 Wisconsin. Leverett (22, p. 282) was at first inclined to regard this 

 as possibly of Illinoian age but has later come to consider it as part of 

 the Kansan. In this case the Sangamon and the Peorian interglacial 

 intervals would be identical. In the present thesis the views of Lev- 

 erett are accepted. 



• 



Of the six glacial invasions five crossed what are now the borders 

 of this state (22, p. 248 ff ; 13, p. 382 flf). The first of these was the 

 Pre-Kansan, or Nebraskan, which corresponds to the Sub-Aftonian. 

 This radiated from the Keewatin field and covered the greater part of 

 the state but none of the drift is now naturally exposed in Minnesota, 

 it having been entirely buried by the following invasions and being 

 now encountered only in the deeper wells. 



The second ice sheet to reach Minnesota, the Kansan (Fig. 1), also 

 radiated from the Keewatin field, covering every part of the state 

 either previously or subsequently glaciated. The mantle left behind is 

 designated as the Old Gray Drift and is still exposed in parts of Dakota, 



