372 GRASSES OF IOWA. 



ice has left the state. During these intervals the state, and at least much 

 of the country to the north, was wholly free from ice and the climate 

 probably did not differ much from that now prevailing. 



Pre-Kansan. The earliest ice invasion was the Pre-Kansan. But 

 little is known of it since only scattered patches of the Pre-Kansan drift 

 are found. So far as is known it nowhere outcrops except where the later 

 drift has been eroded. At many points it is absent, presumably having 

 been destroyed or so worked over as to be unrecognizable in the later 

 glacial periods. Between it and the Kansan is the Af tonian interval rep- 

 resented by but few deposits which can be certainly recognized. The 

 length of this interval and the climate prevailing are almost wholly un- 

 known. 



Kansan. It is the Kansan drift which covers the greater portion of 

 southern and western Iowa. It consists largely of bowlder clay, usually 

 blue at the base but alternating above first to yellow and then to a deep 

 reddish brown. This change of color and the accompanying phenomena 

 of oxidation, ferrugination, decalcification, etc., are believed to indicate 

 that the drift was long exposed to weathering before the beds resting on 

 it were deposited. 



Illinoian. In the extreme southeastern portion of the state there is 

 an area covered by a later drift which in general constitution and char- 

 acter is similar to the Kansan. It is, however, quite distinctly younger 

 and is separated from the Kansan by a well developed soil horizon repre- 

 senting the Yarmouth interval. The drift itself is called the Illinoian 

 and the interval which followed it is known as the Sangamon. 



Ioican. Succeeding the Sangamon interval was the Iowan invasion 

 which covered the most of the northeastern portion of our own state and 

 certain parts of Illinois. The Iowan ice behaved in a very peculiar fashion. 

 It seems to have been very thin, particularly along its edge, and stretched 

 out in a series of long tongues, giving a very crenulate margin. The 

 drift itself is very thin but the area covered by the Iowan is characterized 

 by the immense size and number of the surface bowlders. It was appar- 

 ently during the Iowan period that much of the southern portion of the 

 state became covered by the loess. The latter is a fine silt or clay, usu- 

 ally buff in color rarely showing any signs of stratification, and carrying 

 no pebbles other than lime balls. It spreads out in an irregular sheet 

 forming the surface material throughout the entire state except in the 

 portion covered by the Iowan or Wisconsin ice sheets. 



Wisconsin. Later than the Iowan, and separated from it by an in- 

 terval presumed to be represented by certain fossiliferous beds near To- 

 ronto, is the Wisconsin drift. In Iowa we have no representatives of the 



