462 THE ILLINOIS GLACIAL LOBE. 



The precise position of the supposed divide is not given, but is sug- 

 gested by Hershey to be south of the mouth of the Wisconsin River, where 

 the "Military Ridge" is crossed by the present river. Northward from this 

 supposed divide the valley now occupied by the Mississippi is thought by 

 him to have been occupied by a stream "flowing toward central Minnesota 

 instead of away from it." 



The geological elate of the change is regarded as somewhat uncertain, 

 though Hershey thinks it probable that it occurred during the advancing 

 stage of the earliest glaciation in the region to the north. The advance of 

 the ice sheet into the lower course of a northward-flowing stream would 

 naturally cause the turning of the drainage southward over a low divide. 

 This may have occurred long before the culmination of the Kewatin ice 

 sheet An alternative view, also suggested by Hershey, refers the reversal 

 of drainage to an uplift of the northern portion of the Mississippi Basin. 

 By either view the date of reversal is placed by him near the close of the 

 Ozarkian (or post-Lafayette) epoch of erosion, rather than at an earlier 

 time. 



MINOR DEFLECTION AT PULTON, ILLINOIS. 



The preglacial valley, thus enlarged by accession of waters from the 

 north, has been but partially filled with drift as far south as Clinton, and is 

 occupied by the present stream except for a space of about a mile, just 

 above Clinton, where a rocky point on which the village of Fulton, Illinois, 

 stands, has been cut off from the Iowa side. The old valley still furnishes 

 escape for flood waters around the eastern end of this rocky point. These 

 features are well displayed on the Clinton topographic sheet of this Survey, 

 which is here reproduced (PI. XVIII). 1 The cause for this course across 

 a projecting point of the west bluff is not eas)^ to determine, and seems 

 especially remarkable because of the unobstructed condition of the old 

 valley. Before speaking further concerning the cause, a similar feature in 

 a neighboring township is considered. 



An equally singular deflection of a stream, shown also on PL XVIII, 

 is found in the lower course of Elk River, which enters the Mississippi 

 from the Iowa side about 7 miles above Fulton. The stream leaves a 



1 In the inspection of this sheet it is necessary to allow for 20 to 30 feet of loess on the higher 

 part of the rocky point, thus reducing the rock to about 70 feet above river. 



