72 IOWA ACADEMY OF SCIENCES. 



the glacial boundary from St. Louis eastward as indicated 

 above. It is the southwestern border which claims our atten- 

 tion 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 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, Illinois, a few miles north of St. Louis, and tracing 

 northward, the drift 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 invasion. The 

 Illinoian border takes a northward course along or near the 

 east bluff of the Mississippi, through Adams and Hancock 

 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 oppo- 

 site Keokuk, Iowa. 



For a few miles above Keokuk the Mississippi river appar- 

 ently follows nearly the border of the Illinoisan 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, north- 

 ward through Lee, southeastern Henry, northwestern Des 

 Moines, and western Louisa counties, to the Iowa river at 

 Columbus JunctioQ. Its course there changes to the northeast 

 and it can be traced diagonally across Muscatine county from 

 its southwest to its northeast corner. It is traced with difficulty 

 farther to the northeast because of concealment by a heavy 

 sheet of loess which borders the lowan till in Scott county, 

 Iowa. It is known to extend as far north as Scott county, fer 

 the Illinoian till sheet has been observed in southern Scott 

 county as far east as Davenport. The concealment by the lowan 

 loess is very great, not only in northern Scott county, Iowa, but 

 also in Rock Island, Whiteside and Carroll counties, 111. It 

 becomes a dilficult matter, therefore, to decide upon the posi- 



