430 WALTER H. SCHOEWE 



by erosion. From this one is forced to the conclusion that the gum- 

 botil formerly extended across the valleys. It is conceivable, 

 although there is no evidence for it, that Lake Calvin might have 

 had an outlet across the Illinoian upland plain immediately after 

 the ice withdrew without having involved much erosion and inter- 

 fered with the formation of the gumbotil. That such could have 

 been the case for any considerable length of time, however, is not 

 probable. The level of the lake during its maximum height stood 

 1 20 feet above the present surface of the Iowa-Cedar River at 

 Columbus Junction. The surface of the lake could not have been 

 lowered appreciably without having resulted in pronounced erosion, 

 which would have been fatal to the formation of the gumbotil. 

 The presence of the lake, on the other hand, is fully in accord with 

 the theory of the formation of the gumbotil, for the levels of the lake 

 and of the gumbotil as they are shown at Columbus Junction are 

 separated by not more than 10 feet, a difference in height which 

 would hardly permit pronounced erosion. 



The Illinoian gumbotil underlies the flat tabular stream divides 

 and outcrops near the valley walls of Iowa-Cedar River and along 

 both sides of the Mississippi, high above the present level of the 

 rivers. This together with the fact that the gumbotil lies horizon- 

 tally clearly shows that the present vaUeys of Iowa-Cedar and 

 Mississippi rivers are incised below the general Illinoian upland 

 plain and thus are post-IUinoian gumbotil in age. Therefore 

 these valleys through which the lake was eventually drained could 

 not have served as the outlet of Lake Calvin very soon after the 

 ice had retreated. Thus Lake Calvin existed for a long time, and 

 the displaced Mississippi must have followed the course south of 

 Columbus Junction until the lake was finally drained by way of the 

 Iowa-Cedar River Valley. 



It appears that the earlier investigators based their conclusion 

 of an early abandonment of the temporary course of Mississippi 

 River south of Columbus Junction on three facts: (i) that there are 

 no unusual deposits of sand and gravel in the now abandoned channel 

 south of Columbus Junction; (2) that the elevation of the Illinoian 

 upland bordering the present course of the Mississippi is only a 

 few feet lower than the bed of the abandoned channel; and (3) that 



I 



