LOWER RAPIDS OF THE MISSISSIPPI I 3 



leaves the Iowa valley, 75 miles to the north. The portion 

 above the point where the Iowa is crossed has been so modified 

 since the Illinoian stage of glaciation, that very little is known 

 concerning its condition at the close of that glacial stage, but 

 the portion south from the Iowa valley has been only slightly 

 modified. 



Very little material was deposited on the bed of the tem- 

 porary channel of the Mississippi in the 75 miles from the Iowa 

 valley to the head of the rapids, but a great filling occurred in 

 the broad valley below, and some filling along the rapids, espe- 

 ally at their lower end. The valley which, at the foot of the 

 rapids, had been cut down to a level scarcely 50 feet above the 

 present stream, was built up to 80 or 90 feet above the river at 

 that point. The depth of filling is found to decrease upon pass- 

 ing down the valley and becomes scarcely noticeable at Hannibal. 

 It is, therefore, much like a delta, formed where a rapid stream 

 emerges into a sluggish lake-like body of water. It consists 

 mainly of fine material, sand or silt, with few pebbles greater 

 than % inch in diameter. A fine gravel, however, appears at an 

 exposure called "Yellow Banks " near the mouth of the Des 

 Moines River. The bowlder bed in Keokuk, described above, 

 received at this time a capping of sand 15 or 20 feet in depth. 

 Sand deposits are also found at a corresponding level in Hamil- 

 ton, Illinois, near the foot of the rapids, capping a low part of 

 the rock bluff. Another possible remnant of the sand filling is 

 found at Sandusky, Iowa, six miles above Keokuk, immediately 

 back of the bowlder-strewn slope, noted above. It there rises 

 about 80 feet above the river or to within 25 feet of the level of 

 the bottom of the channel of the temporary Mississippi, ten miles 

 to the north. No remnants of the filling have been noted in this 

 interval of 10 miles, and it is thought probable that the rate of 

 fall was so great above Sandusky that but little lodgment of 

 material occurred. 



In the portion of the Mississippi valley covered by the Lab- 

 rador ice-field, at the Illinoian stage of glaciation, there appears 

 to be no such sand filling as is found below the rapids, thus con- 



