144 S. CALVIX PRESENT PHASE OF PLEISTOCENE IN IOWA 



ice, so far as it affected Iowa, came from the northwest, as the Kansan 

 and the pre-Kansan had done in the earlier stages of the Pleistocene; 

 but the Iowan glaciers stopped a long way short of the limits reached by 

 their ancient predecessors from the Kewatin centers. The main body of 

 the Iowan failed to reach Iowa. A broad lobe was all that passed the 

 northern boundary of the state, and this covered an area approximately 

 100 miles in length from northwest to southeast and 80 miles in width. 

 The area thus affected lies in the northeastern quarter of Iowa, in a re- 

 gion that had not been reached by the Illinoian. It was the eroded and 

 weathered surface of the old Kansan, therefore, that this ice invaded and 

 on which the Iowan was deposited. The latest observations show that 

 within the limits of the area under discussion the Iowan nowhere touches 

 or overlaps the Illinoian. Along a line drawn from Marshall county to 

 the northwest corner of "Worth the Iowan is itself overlapped by the 

 younger Wisconsin. 



RELATIVE AGE OF THE IOWAN 



The Iowan is a very young drift when compared with the Illinoian. 

 Its surface remains as the ice left it. Not even along the larger water- 

 courses has it been trenched to any notable extent, as is so conspicuously 

 the case in the Illinoian. Effects of erosion are practically zero, or they 

 were about zero until tl\e agriculturist came to disturb the adjustments 

 of slope and run-off and vegetation, which maintained a fair degree of 

 topographic stability. The opening of artificial drainage courses, the 

 destruction of the prairie sod, and the general cultivation of the surface 

 have resulted in more erosion during the last fifty years than had taken 

 place under natural conditions during all the millenniums since this till 

 was deposited. And yet, with all the help that destructive art has fur- 

 nished, the surface as a whole remains essentially unchanged. If we 

 may judge by the comparative erosion in the Iowan and the Illinoian, 

 the Sangamon interval, though shorter than the Yarmouth, was fairly 

 long. If the interval is measured by the comparative extent of the 

 changes wrought by weathering, the conclusion is the same, but the con- 

 viction is stronger. The brown or red weathered zone of the Illinoian is 

 from 3 to 4 feet in thickness. In the Iowan there is a deep, black soil 

 developed on the surface, but changes due to weathering can not be recog- 

 nized. In particular instances where the Iowan till is thin, the whole 

 thickness of the deposits belonging to this stage is included in the humus 

 layer; but where the thickness amounts to several feet, apart from the 

 dark soil band there are no changes which can be measured or described. 

 The till presents the same color and has the same composition from its 



