1 6 FRANK LEVERETT 



the main valleys, but over much of the low country in the inte- 

 rior of the Mississippi basin. This silt is the problematical 

 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 aeolian. 



In the region under discussion the valleys, as previously indi- 

 cated, were opened only to shallow depths, hence only a 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 dur- 

 ing the Illinoian stage of glaciation. 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. 



Whether the deposition took place by water or by wind, 

 there seems to have been a suspension of erosion on the lower 

 rapids, and the length of this suspension must certainly be suffi- 

 cient to affect materially their duration. An estimate of the 

 time involved seems at present impossible, there being fewer 

 data for an estimate than in the filling which occurred at the 

 Illinoian stage. 



Erosion following the loess filling. — After the deposition of 

 the loess, the valleys throughout much of the Mississippi basin 

 experienced a marked deepening, which brought their bottoms 

 to a lower level than before the loess filling. In the portion of 

 the Mississippi valley, which lies within and near the rapids, 

 the deepening seems to have proceeded continuously to a level 

 nearly as low as the present stream, or fifty to seventy-five feet 

 below the excavation which occurred in the interval following the 



63-69. Geology of Johnson County, by S. Calvin, Iowa Geol. Surv., 1896, Vol. VII, 

 pp. 39-45, 86-89. Geology of Linn County, by W. H. Norton, Iowa Geol. Surv., 

 1894, Vol. IV, pp. 168-184. Geology of Marshall County, by S. W. Beyer, Iowa 

 Geol. Surv., 1896, Vol. VII, pp. 234-238. Geology of Plymouth County, by H. F. 

 Bain, Iowa Geol. Surv., 1897, Vol. VIII, pp. 335-351. 



