THE lOWAN MARGIN 117 



Wisconsin lobe. More recent studies in this region have cast some doubt 

 on the lowan age of the till in question. The erosion, oxidation, and 

 leaching of the surface is very much greater than in the typical lowan 

 of northeastern Iowa, and furthermore a mantle of loess is moulded over 

 the irregularities of the surface. The lowan drift in the typical area is 

 all but universally free from loess. 



The Iowan Margin 



its sinuosities and digitations 



There are reasons for believing that the lowan ice was very thin at the 

 margin, and the same attenuation must have characterized it for a num- 

 ber of miles back from the margin. This thin condition of the marginal 

 portions of the ice-field is inferred from the fact that the ice seems to have 

 been incapable of overcoming hills or prominences of an}^ considerable 

 altitude. Furthermore, the ice for some reason was unusually motile 

 and seems to have been capable of flowing completely around obstacles 

 that it did not surmount, even when the obstacle was a highland area 

 embracing a score or more of square miles. The same motility is indi- 

 cated by the fact that, along its free margins, it had the habit of flowing 

 in numerous long, digit-like lobes wherever the ground was low and favor- 

 able, the marginal lobes being usually separated from each other by re- 

 entrant angles occupied by slightly higher ground that the ice did not 

 overflow. Accordingly the margin of the lowan area is very sinuous, 

 and many unexpected eccentricities of distribution are encountered in 

 tracing it in the field. 



LOESS RIDGES ALONG THE MARGIN 



The North Liberty and Solon lobes of Johnson county, separated by 

 the loess- covered Kansan highland traversed by the Iowa river, afl'ord a 

 good illustration of this peculiar characteristic. The Cedar river, from 

 a point above Cedar Rapids, has carved its valley in another highland 

 of the same kind, and it is this loess-covered Kansan ridge that separates 

 the Solon lobe from a narrow and very typical tongue of lowan that runs 

 down to the little town of Buchanan, in Cedar county. Tipton is sit- 

 uated near the western edge of a low, broad, interlobular, loess-covered 

 region of Kansan drift. The Wapsipinicon valley was followed in its 

 lower course by a long, narrow lobe of lowan ice which pushed down to 

 Clinton and across the Mississippi river. On the east side of the river 

 this ice-lobe encountered an insurmountable barrier in the low bluffs 

 which mark the boundary of the floodplain. 



The most erratic lobes observed occur in Dubuque county. One hav- 



