IOWA ACADEMY OF SCIENCES. 
87 
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 sluggish 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 suffered 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 OP 
GLACIATION. 
The period of low gradient and slack drainage, just dis- 
cussed, was followed by even less favorable c )nditions for the 
opening of a channel During the Iowan stage of glaciation, 
as long since pointed out by McGee‘S and elaborated by Calvin 
and others,! the deposition of a sheet of silt occurred, not only 
along the main valleys, but over much of the low country in 
the interior of the Mississippi basin. This silt is the problem- 
atical loess. Its mode of deposition is still a matter of dispute, 
the deposit being thought by some glacialists to be largely 
aqueous, while by others it is thought to be chiefly seolian. 
In the region under discussion the valleys, as previously 
indicated, were opened only to shallow depths, hence but 
slight accumulation of the silt was necessary to fill them or to 
cause the streams to spread over the bordering plains. The 
depth of the silt in the vicinity of the lower rapids seldom 
reaches thirty feet and probably averages not more than 
fifteen feet. Its bulk, therefore, does not, so far as the 
valleys are concerned, greatly exceed that of the filling which 
occurred below the rapids during the Illinoian stage of glacia- 
tion. If, however, the deposits on the bordering plains are 
taken into consideration, the amount of material deposited is 
very much greater, for the plains were covered to a depth of 
six to ten feet by this silt. 
* The Drainage System and Distribution of the Loess of Eastern Iowa, by W. J. 
McGee, Bull. Wash. Phil. Soc’y. Vol. VI, 1883, pp. 93-97. Also see discussion in Eleventh 
Ann. Eep’t U. S. Geol. Survey, 1890, pp. 435-471. 
t Geology of Jones County, by S. Calvin, Iowa Geol. Survey, Vol. V, 1895, pp. 63-69. 
Geology of Johnson County, by S. Calvin, Iowa Geol. Survey, Vol. VII, 1896, pp. 39-45, 
86-89. Geology of Linn County, by W. H. Norton, Iowa Geol. Survey, Vol. IV, 1894. pp. 
168-184. Geology of Marshall County, by S. W. Beyer, Iowa Geol. Survey Vol. VII, 
1896, pp. 234-238. Geology of Plymouth County, by fl. F. Bain, Iowa Geol. Survey, Vol. 
VIII, 1897, pp. 335-351. 
