146 S. CALVIN PRESENT PHASE OF PLEISTOCENE IN IOWA 



by any trustworthy criterion, the Sangamon interval, though shorter than 

 the Yarmouth, was long. 



UNDULATING SURFACE OF THE IOWAN 



The surface of the Iowan is more undulating than that of the Illinoian. 

 The amount of material carried by the Iowan ice within the territory 

 covered by the Iowan lobe was very small : the deposit is thin and meager 

 as compared with the great thickness of the Kansan till ; it is very much 

 thinner than even the Illinoian. There was not enough of the Iowan 

 completely to disguise the topography of the eroded surface upon which 

 it was laid down. The thickness varies from zero on the summits of the 

 old Kansan ridges to a probable maximum of 20 feet over the Kansan 

 valleys. There are large spaces, scores of them, within the Iowan area 

 where there is not now a trace of Iowan drift. The Iowan ice passed 

 over them, but left none of its load. Such a space occupies a few square 

 miles east and north of Independence, in Buchanan county. Another, a 

 larger one, lies south of Lime creek, in Cerro Gordo county. There are 

 many similar cases, especially near the marginal limits of the Iowan 

 lobe, and all demonstrate the fact that Iowan drift may be absent from 

 a fairly large area and present in all the area surrounding it. For ex- 

 ample, the Iowan drift is absent for a mile or more east of Independence, 

 but at the old Illinois Central gravel pit, 4 miles east of the city, the 

 Iowan is about as well developed as it is anywhere in the state. Here the 

 drift in question overlies weathered Buchanan gravels, post-Kansan in 

 age, which have been worked for railway ballast; it varies from 1 foot 

 to 8 feet in thickness (plate 4, figure 1). The meagerness of the Iowan 

 till, even when normally present, and its entire absence from large areas 

 which must have been covered with Iowan ice have made it very difficult 

 in some localities to determine the exact position of the border of the 

 Iowan lobe. The Iowan border, as drawn on the drift maps published in 

 the reports of the Iowa Geological Survey, in the light of clearer knowl- 

 edge of the peculiarities and behavior of the Iowan ice, will require a 

 considerable amount of rectification. 



INDIVIDUAL CHARACTERISTICS 



Notwithstanding its meagerness and occasional absence, the Iowan 

 drift exists, and it has its distinctive individual characteristics. It is 

 light yellow in color, lighter than the Illinoian. It is less calcareous than 

 any of the other drifts. Its characteristic boulders are large, light- 

 colored, coarse-grained granites, rich in feldspar, and ranging from 10 or 

 12 to 40 or 50 feet in diameter. These great rough boulders, numerous 



