IOWA ACADEMY OF SCIENCES. 87 



whereas it is now confined to an inner valley scarcely one-hall' 

 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 lOWAN STAGE OF 

 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 Du'ing the lowan stage of glaciation, 

 as long since pointed out by McGee* and elaborated by Calvin 

 and others, f 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 asolian. 



In the region under discussion the valleys, as previously 

 indicated, were opened only to shallow depths htnce 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 plaias were covered to a depth of 

 six to ten feet by this silt. 



* The Draluage 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. Rep't U. S. Geol. Survey, 1890, pp. 435-471. 



tGeolog-y 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. Geolog-y of Linn County, by W. H. Norton, Iowa Geol. Survey, Vol. IV. 1894. pp. 

 168-181. Geolog-y of Marshall County, by S. W. Beyer, Iowa Geol. Survey Vol. VII, 

 1896, pp. 234-238. Geology of Plymouth County, by H. F. Bain, Iowa Geol. Survey, Vol. 

 VIII, 1837. pp. 335-351. 



