432 WALTER H. SCHOEWE 



The fact that the Mississippi bluffs are a few feet lower than 

 the bed of the abandoned channel does not preclude the drainage 

 of the lake by way of the abandoned channel south of Columbus 

 Junction. The lUinoian-Mississippian sag, if it really existed, was 

 at least 20 miles distant from Lake Calvin by way of the present 

 Iowa-Cedar River Valley. What the elevation of the intervening 

 topography may have been is not known. The lUinoian upland 

 at Columbus Junction is about 730 feet above sea level; at Morning 

 Sun, only 2f miles from the south valley wall of Iowa-Cedar River, 

 it is 752 feet; and at Newport, only 2 miles south of the river bluffs, 

 it is at least 720 feet. The Mississippi bluffs north of WapeUo 

 are at many places over 700 feet above sea level. The lUinoian 

 drift plain forms the upland surface on either side of the present 

 course of the Mississippi, and Illinoian drift and gumbotil are 

 exposed within several feet of the upper surface, high above the 

 present level of the river. This clearly shows that the present 

 valley of Mississippi River is incised below the general Illinoian 

 upland plain and thus is post-Illinoian gumbotil in age. The same 

 is true of the Iowa-Cedar River Valley. 



On the basis of erosion alone it is not apparent why the Missis- 

 sippi should have shifted its course to its present valley between 

 Clinton and Fort Madison "with the recession of the Illinoian ice 

 sheet. "^ All pre-IUinoian drainage lines must have been filled with 

 drift, if not completely, at least partially, by the depositing Illinoian 

 ice sheet. This is indicated by the presence of the gumbotil along 

 the vaUey walls of the streams. According to Leverett, the altitude 

 along the Mississippi bluffs is but a few feet lower than the bed 

 of the abandoned channel which at Columbus Junction is 120 feet 

 above the level of Iowa River. Hence an enormous amount of 

 material must have been removed before the displaced Mississippi 

 River could have returned to its former course. The process of re- 

 moving this material must have involved a long time. 



' F. Leverett, "Outline of Pleistocene History of Mississippi Valley," Jour. ofGeol. 

 Vol. XXIX (1921), p. 622. 



