530 THE PLEISTOCENE EPOCH 



ther east the till-sheet passes into the Saskatchewan gravels laid 

 down by the waters derived from the ice -front. The materials of 

 this drift are either local or came from the Rocky Mountains. 

 The lowest drift deposits of the Mississippi valley are provisionally 

 regarded as equivalents of the Albertan drift ; they are best dis- 

 played in southern Iowa, but their extent has not yet been deter- 

 mined. According to some authorities the Lafayette formation 

 should be correlated with the earliest glacial stage, rather than 

 with the Pliocene. 



A great retreat of the ice, if not its entire disappearance, brought 

 about interglacial conditions at least in the Mississippi valley 

 (Aftonian stage). The surface exposed by the retiring ice was 

 rapidly occupied by vegetation, which in many places in Iowa, 

 Minnesota, etc., formed accumulations of peat, sometimes to the 

 depth of 25 feet. The Kansan stage represents the greatest 

 extension southwestward of the ice-sheet, when the glacier de- 

 scended from the north (perhaps the Keewatin glacier) nearly to 

 the mouth of the Ohio River, and spread across Iowa and Mis- 

 souri far into Kansas. Eastward, the sheet extended across the 

 Mississippi River into Illinois. Again came a time of retreat, when 

 the Kansan till was eroded, soil was formed, and peat deposited 

 upon it (Buchanan stage). A renewed extension of the ice laid 

 down the Illinois till-sheet, which is found not only in that state, 

 but crossed into Iowa also. A fourth recrudescence of the glacier 

 (lowan stage) occasioned the deposit of another till-sheet, of an 

 extent not yet determined, which is best displayed in northeastern 

 Iowa, where it is intimately associated with the largest accumula- 

 tions of loess in the Mississippi valley. The lowan till-sheet is 

 followed by interglacial deposits which are perhaps contempora- 

 neous with those which are so well shown near Toronto on Lake 

 Ontario. The latter beds form a succession of fine shales and 

 sandstones that lie between two sheets of glacial drift, and con- 

 tain fossil plants which indicate a milder climate than that of 

 the present time at Toronto, near which these beds are found. 

 Such a fact is difficult to explain, except as the result of truly 

 interglacial conditions. The Wisconsin stage is much the most 



