THE MISSISSIPPI VALLEY. 



469 



miles, while the preglacial valley has a breadth 

 ranging from 6 to 15 miles and a depth of about 

 250 feet below the level of the bordering' rock 

 bluffs. 



DEFLECTION AT THE LOWER RAPIDS. 



The preglacial channel, as long since pointed 

 out by Gen. G. K.Warren, 1 and subsequently elab- 

 orated by Prof. C. H. Gordon, 2 passed southwest- 

 ward across southern Lee County, Iowa, entering 

 the broad valley of the Mississippi just above the 

 mouth of the Des Moines River. (See sketch 

 map, fig. 4.) Its east bluff, as determined by Gor- 

 don, is only 3 or 4 miles west of the present stream. 

 The deflection, therefore, is much smaller than in 

 the case of the upper rapids. In its new course 

 across the rapids and thence west into the old val- 

 ley the channel has a length of about 15 miles. 

 The filled portion of the preglacial channel west of 

 it has a length of only 8 miles, being more direct 

 than that of the new course. 



The preglacial channel has, in its abandoned 

 portion, about the same width as in the reestab- 

 lished portions above and below, and is shown by 

 an artesian well at Montclare to reach a level about 

 125 feet below that of the present river bed at the 

 head of the rapids. The accompanying diagram- 

 matic section (fig. 5), prepared by Gordon, sets 

 forth the relative size of the preglacial and present 

 valleys and the position of this boring. The depth 

 of rock excavation in the old channel is estimated 

 by Gordon to be 245 feet, and the width 6 miles, 



1 Bridging of the Mississippi : Ann. Rept. Chief of Engineers, 

 U. S. Army, 1878, Appendix X, pp. 916-917, Diagram E ; also Dia- 

 gram 1, sheet i. 



-Buried river channels in southeastern Iowa: IowaGeol. Sur- 

 vey, Vol. Ill, for 1893, pp. 239-255, figs. 5 and 6, PI. XXVI. Published 

 in 1895. 



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