468 



THE ILLINOIS GLACIAL LOBE. 



the mouth of the Iowa River. But in the section below the mouth of the 

 Iowa, as far as the mouth of Skunk River, it coincides in width with the old 

 valley. For a few miles in Lee County, Iowa, in the vicinity of the city 

 of Fort Madison, only a part of the width of the preglacial valley has been 

 opened (see fig. 4), but just before entering the rapids the present valley 

 expands to about the full width of the old valley. 



Fig. 4.— Sketch map of the ] 



l bordering the lower rapids of the Mississippi Ki 



The present bed of the Mississippi is 100 feet or more above the rock 

 floor of the preglacial valley throughout the entire distance between Musca- 

 tine and the lower rapids. A rough estimate of the relative sizes of the 

 present valley and the preglacial valley in this section shows the preglacial 

 valley to be fully twice as large as the present valley. The present valley 

 has an average depth of about 150 feet and an average breadth of about 6 



