THE MISSISSIPPI VALLEY. 



475 



In the table which follows it will be observed that the rock bottom has 

 a lower altitude at St. Paul, Minnesota, than at several points farther down 

 the present stream, a feature which is thought to sustain the interpretation 

 made by Hershey, that a divide has been crossed between St. Paul and 

 Dubuque. In a paper prepared by the writer a few years ago 1 this low 

 altitude at St. Paul and vicinity was suggested to be due either to north- 

 ward differential depression or to a local deepening, such as might have 

 resulted from a waterfall or from subglacial erosion by water or ice. How- 

 ever, no evidence of the operation of these agencies has been discovered, 

 and they could not have been operative within the Driftless Area. These 

 suggestions do not, therefore, seem so pertinent as the interpretation made 

 by Hershey. 



Altitudes of rock bottom and present Mississippi. 



St. Paul, Minn 



Late City, Minn 



Winona, Minn 



Lacrosse, Wis 



Prairie du Chien, Wis 



Dubuque, Iowa 



Sabula, Iowa 



Fulton. Ill 



Leclaire, Iowa (new channel) 



Rock Island, 111. (new channel) 



Muscatine, Iowa (new channel) 



Near Wilton, Iowa (old channel) 



Near Muscatine, Iowa (old channel) 



Mouth of Iowa River 



Burlington, Iowa 



Fort Madison, Iowa 



Feet. 

 683 

 658 

 639 

 628 

 604 

 585 

 572 

 566 

 562 

 542 

 531 



Feet. 

 702 

 (?) 

 656 

 643 

 623 

 607 

 592 

 587 

 576 

 56 D 

 547 



530 547 



523 

 511 

 502 



539 

 527 

 518 



Feet. 

 a 483 

 6495 

 c503 

 <J 504 

 (J 492 

 rf453 

 e429 



400± 



550 



530 



506 

 /400- 

 g 388 



445— 



430— 

 7i365 



aN. H. Winchell, Am. Geologist, Aug., 1892. 



&Geol. of Minnesota, Vol. II, p. 17. 



cData furnished by Dr. U. S. Grant in letter. 



d Chainberlin and Salisbury, Sixth Ann. Rept. U. S. Geol. Survey, p. 223 



e Data given by W E. Oake, ex-mayor of Sabula. 



/A well at this point, reported by Udden, failed to reach rock at elevation 4U0 feet above tide. 



g Two wells in the valley below Muscatine, reported by Udden, reached rock at a level about 142 feet below the 

 low water level of the Mississippi. 



AC. H.Gordon: Geol. of Iowa, Vol. Ill, 1893, p. 246. A well one-half mile north of Fort Madison reached a level 

 about 365 feet above tide without entering rock. The channel may, therefore, be deeper than that shown by the Fort 

 Madison wells. 



1 Jour. Geol., Vol. Ill, 1895, pp. 740-763. 



