14-1 THE ILLINOIS GLACIAL LOBE. 



PROBABLE EXTENT OF THE IOWA PORTION OF IOWAN DRIFT. 



[nasmuch as there is some evidence suggesting- a slight extension of 

 ice from Iowa into northwestern Illinois at the Iowan stage, the question 

 of the extent of that ice sheet is here considered. 



The "upper till" of northeastern Iowa now classed largely as Iowan 

 drift is represented by McGee to have its eastern border near the western 

 edge of the Driftless Area from the Minnesota-Iowa State line southeastward 

 to the southern point of the Driftless Area near Sabula, Iowa, 1 Generally it 

 tails short a few miles of reaching the Driftless Area, but in southern Jackson 

 and in Clinton County, Iowa, it is represented to extend beyond the earlier 

 shirt and to constitute the border of the Driftless Area. It is represented to 

 extend to the Mississippi Valley from northern Clinton County southward 

 to Scott County, and to fall short but a few miles of reaching that valley 

 in Scott and Muscatine counties. A tectonic map 2 represents the ice sheet 

 to have extended across the Mississippi and rested on the east bluff for a 

 few miles below Clinton. From Muscatine Count}' westward the limits are 

 not definitely given. The upper till, however, is represented to extend to 

 the limits of the district reported upon. 



The investigations carried on by the Iowa survey have supported the 

 mapping and results of McGee in a general way, but not in all details. 

 Very little disagreement as to the boundary is found from the Minnesota- 

 low a State line southward to Delaware County, Iowa. But from that 

 county southward to eastern Jones County Professor Calvin, of the Iowa 

 survey, places the limit of Iowan drift a few miles inside McGee's limit. 

 There are certain extramarginal phenomena recognized in the disputed ter- 

 ritory which he considers closely related to the Iowan invasion, but not 

 requiring the presence of Iowandce, chief among which are heavy accumu- 

 lations of loess and a tendency to ridging of the loess in lines trending from 

 YVXW. to FSF.. as in the undoubted Iowan area. 



From northern Jones County eastward to the Mississippi the border 

 has not vet been investigated by the Iowa survey, but the writer made 

 some examinations in this district in 1894 while engaged in tracing the west 

 border of the Illinoian drift. The examinations were begun in Clinton 

 County and carried westward. On the uplands northwest of Clinton a belt 



Eleventh Ann. Kept,. U. S. Geol. Survey, PI. XLIV. - Op. cit., PI. LVI. 



