152 THE ILLINOIS GLACIAL LOBE. 



to Rock River. 1 He has represented the Illinois lobe to have subsequently 

 extended to the border of the Mississippi Valley in Whiteside and Rock 

 Island counties, and to have led to the flooding of the Driftless Area, 

 forming what is termed Lake Hennepin. 2 It would appear from McGee's 

 discussion that the dates of culmination of the two lobes were separated by 

 only a brief interval, an interval which in no wise compares in length with 

 that which has been found to have occurred between the Kansan invasion 

 of the Iowa lobe and the Illinoian invasion of the Illinois lobe, and that one 

 lobe held nearly its maximum extent until the other culminated. The rela- 

 tions of the two lobes of the Iowan invasion appears to have been largely a 

 matter of conjecture, for it is now found that the Illmois lobe fell far short 

 of reaching the limits assigned to it by McGee. It is doubtful if evidence 

 has yet been collected by which it will be possible to demonstrate clearly 

 either the space or the time relations of the two ice lobes. 



The space relations as well as time relations of the two lobes being still 

 unsettled, the influence upon drainage is at best a matter of conjecture. 

 The Iowa ice lobe appears to have reached as far east as the Mississippi 

 River in southern Clinton and northern Scott counties. Possibly it reached 

 a few miles beyond the river in southwestern Carroll, western Whiteside, 

 and northern Rock Island counties, Illinois, though such an extension is, 

 as already noted, but weakly supported. The Illinois ice lobe certainly 

 extended a few miles beyond Sterling on the north border of Rock River 

 Valley and probably extended about to Geneseo, on the south border of 

 that valley. The debatable tract east and south of Morrison, in central 

 and southern Whiteside County, was apparently covered at the Iowan stage, 

 either by the Illinois or by the Iowa ice lobe. If the latter is found not to 

 have extended beyond the Mississippi Valley, it would follow that this 

 district was occupied by the Illinois lobe. 



The question of a coalescence of the two lobes, or the overlapping of 

 their fields, depends, therefore, upon the determination of the extent of the 

 Iowa lobe. If that lobe did not extend beyond the Mississippi, there would 

 remain a strip about 8 miles in width along the east border of the Missis- 

 sippi Valley which remained uncovered by ice throughout the Iowan 

 invasion. By including the valley its width would be increased to 10 or 



1 Eleventh Ann. Rept., U. S. Geol. Survey, p. 570. 

 : 0p. cit., PI. LVIII; also pp. 570-577. 



