94 THE ILLINOIS GLACIAL LOBE. 



the frozen condition of the ground may have prevented the stream from 

 eroding more material than it could readily transport. In that case the 

 material should be accumulated in portions of the Mississippi Valley to the 

 south, where the gradient became too low to admit of its being swept along. 

 In apparent support of this view there is found, immediately below the Des 

 Moines or lower rapids of the Mississippi, a marked filling of the valley 

 with deposits of sand and fine gravel. This filling may be seen to good 

 advantage at and below the village of Warsaw, which stands on a terrace 

 of aggradation antedating the loess in its formation and apparently sepa- 

 rated from the Kansan glacial stage by an erosion interval of considerable 

 length. 



The Warsaw exposures were examined by the writer in 1894 and 

 reexamined by Professor Chamberlin, Dr. H. F. Bain, and the writer in 

 the summer of 1896, when the relationships given above were worked out. 

 The Kansan till has suffered erosion to a level but little above the present 

 stream and a bowlder bed marks the junction of this till with the overlying 

 sand and gravel. This bowlder bed is continued on ,the north side of the 

 river in Keokuk, as pointed out by Prof. C. H. Gordon. 1 The sand and 

 gravel deposits are typical fluvial material and are built up to a height of 

 about 80 feet above the river. At their top is an ashy silt resembling a soil 

 but perhaps redeposited as flood-plain material, and above this a deposit of 

 sand grading upward into loess, the sand and loess together being 20 to 25 

 feet in thickness. Opposite Warsaw near the mouth of the Des Moines 

 River there is a somewhat different exposure of fluvial filling known as the 

 "Yellow banks." This has been examined, both by the writer and by Pro- 

 fessor Gordon, and the following section was published in the Geology of 

 Iowa in 1895 : 2 



" Yellow-banks" section, near Keokuk, Iowa. 



Ft. in. 



Clay, yellow, pebbleless 5 



Silt, drab, pebbleless . . . '. ' 1 3 



Earth, black, with a few small pebbles; apparently an old flood-plain deposit 12 



Clay, yellowish (local) 6 



Sand, with a few small pebbles ; layers of bowlders 1 foot thick at base 20-25 



Earth, black, with yellow streaks; apparently au old flood-plain deposit 3- 6 



Gravel, with some sand beds; pebbles 2 inches or less in diameter 20 



Blue clay, till, exposed 15 



Total - 85 



' Iowa Geol. Survey, Vol. Ill, 1895, pp. 252-254. - Op. cit.. p. 243. 



