GLACIAL OR ICE-LAID DEPOSITS 



47 



and organic matter accumulated as peat in some swampy areas. 

 This stage is known as the Yarmouth. 



(c) The Illinoisan Glaciation and Sangamon Interglacial 

 Stage. The third glacial advance was from the Labradoreau 



Fio. 42. Map .showing the three renters of ire accumulation in North America. (Cham- 

 berlain ami Salisbury. Courtesy Henry Holt & Co.) 



center and was Hie most extensive in the middle west during the 

 entire period of glaciation. The greatest area of surface deposits is 

 in Illinois, hence the name. It is exposed in Ohio, and Indiana, also. 

 This glaciation was followed by a long interglacial stage during 

 which weathering and soil formation occurred. It is known as 



