48 



SOIL PHYSICS AND MANAGEMENT 



the Sangamon stage (Fig. 44). Peat deposits have been found as 

 much as 22 feet in thickness that were formed during this period, 

 (d) lowan Glaciation, Loess Deposits and Peorian Inter- 

 glacial Stage. The Sangamon interglacial stage was followed 

 by the lowan advance from the Keewatin center and covered a con- 

 siderahle part of Minnesota, Wisconsin, northeastern Towa and the 

 northern part of Illinois. The conditions at the time of the melting 

 of this glacier gave rise to extensive loess deposits. 



During sum- 



EUROPE 



'"lnc distribution of Ice'durlnK Epoch o'f M 

 [lactation, and chief areaa occupied bj.snow. 

 local ice-sheets, 'and glacleri during 

 Fourth Glacial Epoch. / 



I""""] Eporh of Maiitnum Olaclatloo: 

 ' (So,,J UUciil Epoch) 



I I Epoch of flreat Baltic'Giacler: 



1 ' "" nK ou. i.i - 



Fio. 43. Map showing extent of ice-sheet, Europe. (Reproduced from Dana's Manual of 

 Geology, by special arrangement with American Book Company.) 



mer the melting was very rapid so that the flood plains of streams 

 draining from the glacier received deposits of rock flour during 

 these periods of overflow. During times of little melting the 

 streams contracted to their ordinary channels, leaving the material 

 exposed on their flood plains. This was picked up hy the wind and 

 distributed over the upland where it occurs as a deposit from 3 to 

 150 feet in thickness over part of the states bordering the Mississippi 

 and Missouri Rivers. The loess buried the Sangamon soil. 



The Peorian interglacial stage followed the lowan glaciation, 



