LOWER RAPIDS OF THE MISSISSIPPI 15 



concerned. This is true, not only in the region about the lower 

 rapids, but throughout the entire exposed portion of the Illinoian 

 drift sheet. 



The erosion on the lower rapids appears to have been scarcely 

 sufficient to remove the sand filling which occurred during- the 

 Illinoian stage of glaciation. It could have amounted to scarcely 

 20 feet in depth and was mainly in loose material. The limits 

 of the erosion are determined by the level down to which the 

 loess extends. That deposit appears nowhere in situ at a lower 

 level than 65 to 70 feet above the head of the rapids. Its lower 

 limits in the portion of the valley above the rapids are also as 

 great as 70 feet above the present stream. 



A study of tributary valleys in this region has shown that 

 the streams meandered widely and performed a large amount of 

 work, notwithstanding the shallow depth of erosion. For exam- 

 ple, Skunk River, in southeastern Iowa, at that time meandered 

 over a width of about two miles (see Fig. 2), whereas it is now 

 confined to an inner valley scarcely one half mile in average 

 width. It should be noted, however, that the erosion of fifteen 

 or twenty feet, over a width of two miles, by a stream with slug- 

 gish current, may involve more time than is required for the 

 cutting of the inner valley, which has an average depth of nearly 

 100 feet and a width of about one-half mile. In this interval, 

 as in the interval of filling which preceded it, the rapids suf- 

 fered but little modification, yet the time involved was sufficiently 

 long to affect materially the estimates of the duration of the 

 stream in its present course. 



The loess filling accompanying the Iowan stage of glaciation. — 

 The period of low gradient and slack drainage, just discussed, 

 was followed by even less favorable conditions for the opening 

 of a channel. During the Iowan stage of glaciation, as long 

 since pointed out by McGee, 1 and elaborated by Calvin and oth- 

 ers, 2 the deposition of a sheet of silt occurred, not only along 



1 The Drainage System and Distribution of the Loess of Eastern Iowa, by W 

 J McGee, Bull. Washington Phil. Soc'y, 1883, Vol. VI, pp. 93-97. Also, see discus- 

 sion in Eleventh Ann. Rep. U. S. Geol. Surv., 1889-90, pp. 435-471. 



2 Geology of Jones County, by S. Calvin, Iowa Geol. Surv., 1895, Vol. V, pp. 



