470 THE ILLINOIS GLACIAL LOBE. 



while the new channel has an estimated rock excavation ranging from 60 to 

 135 feet in depth and scarcely 1^ miles in width. Probably the average 

 depth of rock excavation in the new channel is less than 100 feet. 



The lower rapids have a length of 11.1 miles, the head being at Mon- 

 trose Island and the foot a short distance above the river bridge at Keokuk. 

 From the foot of the rapids there is a narrow valley leading westward about 

 4 miles to the broad preglacial valley. The total descent in the rapids is 

 22.17 feet, or very nearly 2 feet to the mile. The rate of descent, as in the 

 upper rapids, is greatest in the lower part, there being- a fall of about A\ 

 feet in the lower mile and nearly 8 feet in the lower 2 miles. 1 Above this 

 part the fall, though not uniform, is less definitely broken into rapids and 

 pools than at the upper rapids. Indeed, there appears to be a rock floor 

 formiug the river bed throughout the entire length of the lower rapids. 



A question of much importance is found in the determination of the 

 date at which the Mississippi entered upon the work of excavating its new 

 channel past the lower rapids. The difficulties attending the solution of 

 this question are great, and have been discussed in a special paper by the 

 writer. 2 The main points set forth in that paper are outlined below. The 

 deflection being toward the east, the drift filling which caused the displace- 

 ment in all probability took place as a result of the invasion of the Kewatin 

 ice field at the Kansan stage of glaciation. But it is not definitely settled 

 that the present course across the rapids was adopted upon the retirement of 

 the Kewatin ice, since there is a possibility that the displacement was farther 

 to the east. The coincidence of the present course with the western border 

 of the Illinoian drift suggests the question whether the river may not have 

 adopted this course as late as the time of the Illinoian invasion. 



Attention has already been called to the great erosion of Kansan drift 

 along the Mississippi below the lower rapids which occurred prior to the 

 valley filling that seems to have accompanied the Illinoian stage of glacia- 

 tion. The pre-Illinoian excavation appears to have been nearly as great as 



'TUe following data are obtained from Greenleaf'a report on Water Power of the Mississippi 

 and Tributaries : Tenth Census of United States, 1880, Vol. XVII, p. 60. "In the first 4,800 feet from 

 the lower lock there is a rise of 4.21 feet, then 2.22 feet in the nest 3,600 feet, and 1.67 feet in the suc- 

 ceeding 3,600 feet to the middle lock, making the fall in ordinary low water from a point opposite the 

 middle lock to the foot of the rapids 8.1 feet." 



: The lower rapids of the Mississippi River, by Frank Leverett: Jour. Geol., Vol. VII. 1899, 

 pp. 1-22. 



