476 THE ILLINOIS GLACIAL LOBE. 



Altitudes of rod; bottom and present Mississippi — Continued. 



Montclare, Iowa (old channel) 



Montrose, Iowa (new channel) 



Keokuk, Iowa (new channel) 



Quincy, 111. (rock shelf) 



Hannibal, Mo 



Louisiana, Mo 



Mouth of Illinois River 



Bellefontaine, Mo. (on Missouri River) 



East St. Louis, 111 



East Carondelet, 111. (on rock shelf ?) . . 



Fountain Bluff, 111. (new channel) 



Near Wolf Lake, 111 



Thebes, 111. (new channel) 



Cairo, 111 



9 



12 

 38 

 17 

 25 

 68 

 17 

 24 

 6 

 100 

 15 



Feet. 



a 374 

 490 

 475 



6413 



c362 



d380- 

 (?) 



e295 



/284 

 330 

 300 



</255- 

 280 

 (?) 



a Beck's artesian well, at Montclare ; see Geol. of Iowa, Vol. Ill, p. 247 ; also this report, fig. 5. 

 b Bridge piers rest on a rock shelf 35 to 40 feet below low water: Kept. 0". S. Army Engineers, 1878. 

 cData concerning channel piers furnished by W. S. Lincoln, chief engineer of Wabash Railroad, St. Louis, 

 Missouri. 



d Bed of present stream is 380 feet above tide : Kept. IT. S. Army Engineers, 1878. 

 e Missouri Kiver Commission, Kept, for 1890. The low-water altitude here given is on the Missouri. 

 /Data concerning depth to rock at bridge piers furnished by Robert Moore, C. E., St. Louis, Missouri. 

 <7 A well made by Boliu Sublette failed to reach rock at 50 feet below river level. 



The fact that the rock bottom in this and other valleys of the Upper 

 Mississippi region lies considerably below the present streams has often 

 been cited in evidence of a great preglacial altitude of the region. This 

 interpretation seems questionable, inasmuch as there appears to' be an 

 adequate fall to the seaboard from the rock floors of these valleys, even 

 though the altitude were no greater than at present. In the valley under 

 consideration the rock floor in the 210 miles between Fort Madison, Iowa, 

 and St. Louis, Missouri, makes a descent of about 80 feet, or 4i inches to 

 the mile, and stands sufficiently high at St. Louis to maintain a similar fall 

 to the Gulf were a direct channel to be opened. A somewhat similar 

 gradient appears also to be maintained in the portion above Fort Madison. 

 Although the gradient is somewhat lower along the rock floor than that of 

 the present Mississippi, it is about as great as that of the present Ohio, which 

 has a fall of but little more than 5 inches to the mile in the 967 miles from 

 Pittsburg to Cairo. 



