THE MISSISSIPPI VALLEY. 473 



Kansan stage of glaciation, and has proceeded very slowly down to the 

 present stream level. Except at a few points where the river in rounding 

 a curve is encroaching on its bluff, the rise is gradual from each bank to 

 the brow of the bluff, so that a large part of the slopes may easily be cul- 

 tivated. Although the bluffs are largely a firm limestone, they recede 

 about as much as the bluffs formed in glacial deposits at the upper rapids. 

 Their age, therefore, appears to be several times as great as that of the 

 upper rapids. 



The work performed in cutting away the rock barrier is many times 

 greater than at the upper rapids. No "chains" rise above the general level, 

 as at those rapids, and the fall has been reduced to a very moderate rate. 1 

 Cutting now proceeds very slowly at these rapids, for the river carries but 

 little sediment except at flood stages, and these extend over but a few weeks 

 of the year. If all the features of the new channel be considered, together 

 with the fact of the occurrence of long stages of interruption from cutting, 

 there seems little to oppose the view that the opening or selection of this 

 course may have been at so remote a date as the Kansan stage of glaciation. 



REESTABLISHED STREAM BELOW THE LOWER RAPIDS. 



The disturbance of the Mississippi Valley drainage, below the lower 

 rapids is mainly restricted to the first 50 miles, between Keokuk, Iowa, and 

 Hannibal, Missouri, where the western or Kewatin ice field at the Kansan 

 stage of glaciation seems to have covered the valley and extended a few 

 miles beyond it. This portion of the valley was left open at the Illinoian 

 stage of glaciation. There is, however, a bare possibility, as noted above, 

 that the Illinois lobe encroached slightly upon the Mississippi Valley just 

 above St. Louis. The disturbance of drainage was but temporary at either 

 place, and the present river is fully reestablished in the preglacial course. 

 Its valley bottoms are as wide as those of the preglacial river and range 

 from 5 to nearly 10 miles in width. The rock bottom of the preglacial val- 

 ley, like that of the section above the rapids, is considerably below the level 

 of the present stream, as shown by the table presented below. 



1 It is probable that in the early part of the opening of this new channel the gradient was much 

 steeper than now, and possibly falls of some height existed. But as yet few definite criteria bearing 

 upon this early condition of the valley have been recognized. 



