IOWA ACADEMY OF SCIENCES. 85 



to a level scarcely fifty feet above the present stream, was 

 built up to eighty or ninety feet above the river at that point. 

 The depth of filling is found to increase upon passing 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 one-fourth 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 fifteen 

 or twenty feet in depth. Sand deposits are also found at a 

 corresponding level in Hamilton, 111., 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 eighty feet above the 

 river, or to within twenty-five 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 ten 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 

 Labrador ice field at the Illinoian stage of glaciation, there 

 appears to be no such sand filling as is found below the rapids, 

 although it has nearly as low a gradient. This feature con- 

 firms the above interpretation, that the sand filling occurred 

 during this stage of glaciation. 



In explanation of the small amount of material deposited in 

 the bed of the temporary Mississippi, Professor Chamberlin 

 has suggested to me that the ground in which this channel 

 was excavated may have been frozen at the time of excavation, 

 its situation being on the immediate borders of the ice sheet, 

 and that this frozen condition of the ground may have pre- 

 vented the stream from eroding more material than it could 

 readily transport 



The time involved in the valley filling is a question of much 

 interest, but one on which an estimate is very difficult to 

 make. The filling of any given section is not a measure of 

 the full work of the stream, but simply an index to the excess 

 of material above the limits of transportation by the stream. 



