IOWA ACADEMY OF SCIENCES. 79 



of the Mississippi above the rapids. It did not reach the 

 section of the preglacial valley west of the rapids. The inflec- 

 tion from the preglacial channel must, therefore, be due to the 

 Kewatin ice field. 



But, since the Kewatin ice field may have twice invaded this 

 region, it is necessary to inquire into the probable effect of 

 each of its two invasions. If it be found that the earlier 

 invasion extended beyond the line of the preglacial valley, 

 and deposited sufficient material to prevent the re-establish - 

 ment of the river along the preglacial line, a deflection at this 

 early date must have occurred. The deflection, however, need 

 not necessarily have thrown the stream into its present course 

 across the rapids. That course may have been taken as a 

 result of the later invasion of the Kewatin ice field, if not as a 

 result of the still later encroachment of the Labrador ice field. 

 It is reasonable to suppose that the deflection caused by the 

 Kewatin ice field might give the stream a course farther to 

 the east than the lower rapids, since the region across which 

 the rapids have been opened appears to have been entirely 

 covered by the Kewatin ice field at each of its invasions. It 

 will be necessary, therefore, to determine whether the 

 Kewatin field did not establish the Mississippi in a course east 

 of the rapids, and whether that course was not held by the 

 Mississippi until the Labrador ice field forced it westward into 

 its present course across the lower rapids. 



Turning now to the question of the influence of the sup- 

 posed earlier invasion of the Kewatin ice field, a few remarks 

 seem necessary concerning the deposits made by that ice field. 

 The lowest conspicuous member of the drift series in eastern 

 Iowa is a sheet of dark blue till, often nearly black, which is 

 thickly set with fragments of wood and coal. This is overlain 

 by a sheet of blue-gray till which difl'ers from the blue black 

 till in texture and rock constituents as well as in color. It 

 shows a decided tendency to break into rectangular blocks, 

 and often presents vertical fissures extending to a depth of 

 many feet, which are filled with sand or deeply oxidized clay. 

 The blue-black till is very friable, and seldom shows a 

 tendency to break into rectangular blocks, while the few 

 fissures which it contains traverse it in oblique, raHier than 

 vertical, lines. The blue-gray till carries much less vegetal 

 material and coal fragments than the blue black till. It dift'ers 

 also from the blue-black till in containing a larger percentage 



