1 7 2 ERA NK LE VERE TT 



the southwestern border which claims our attention at this time, 

 since the Illinois lobe there overrode to some extent the sheet 

 of Kansan drift, formed by the western lobe which covered much 

 of Iowa and portions of the neighboring states. 



The southwestern limits of the Illinoian drift is usually marked 

 by a definite marginal ridge or by chains of knolly and slightly 

 ridged drift. Beginning at the south, in Jersey county, 111., a 

 few miles north of St. Louis, and tracing northward, the margin 

 is found to follow the east side of the Illinois River in Jersey 

 and Greene counties, and to carry only occasional knolls and low 

 ridges. It crosses the Illinois in southeastern Pike county and 

 takes a northwest course, coming to the Mississippi bluff near 

 the line of Pike and Adams counties. It there enters a district 

 which had been covered by the western lobe at the Kansan inva- 

 sion. The Illinoian border takes a northward course along or 

 near the east bluff of the Mississippi through Adams and Han- 

 cock counties. A definite ridge twenty to forty feet high is devel- 

 oped along much of the Illinoian margin in Pike and Adams 

 counties, and as far north in Hancock county as a point opposite 

 Keokuk, Iowa. For a few miles above Keokuk the Mississippi 

 River apparently follows nearly the border of the Illinoian till 

 sheet, and no definite ridges are found. At the bend of the 

 Mississippi below Ft. Madison the Illinoian border crosses into 

 Iowa. Its marginal ridge can be traced without difficulty from 

 the vicinity of the Mississippi River bluff south of West Point, 

 Iowa, northward through Lee, southeastern Henry, northwestern 

 Des Moines and western Louisa counties to the Iowa River at 

 Columbus Junction. Its course there changes to the northeast, 

 and it can be traced diagonally across Muscatine county from its 

 southwest to its northeast corner. It has been traced no farther 

 to the northeast because of concealment by a heavy sheet of 

 loess which borders the Iowan till in Scott county, Iowa. It is 

 known to extend as far north as Scott county, for the Illinois 

 till sheet has been observed in southern Scott county as far east 

 as Davenport. The concealment by the Iowan loess is very 

 great, not only in Scott county, Iowa, but also in Rock Island, 



