1 8 FRA NK LE VERB TT 



These streams discharged such large quantities of sand infta the 

 Mississippi that the valley was greatly filled as far down as the 

 head of the broad valley of the lower Mississippi at Cairo. 

 Throughout much of the interval between St. Paul and Cairo the 

 valley was filled to a height of fifty to seventy-five feet above 

 the present stream. In the vicinity of the rapids it reached 

 nearly fifty feet above the level of the erosion in the preceding 

 stage of deglaciation. 



The filling probably began during the early part of the Wis- 

 consin stage of glaciation, but the great bulk of it appears to 

 have been contributed during the part of the Wisconsin repre- 

 sented by the Kettle morainic system. The transportation of 

 sand down the valley no doubt continued for a long time after 

 the ice-sheet had ceased to contribute material to the headwaters 

 of the present Mississippi. The filling may, therefore, have 

 occupied a longer time than that involved in the formation of 

 all the moraines which cross the headwaters of the Mississippi. 



The greater part of this filling consists of sand of medium 

 coarseness. This, however, is interbedded with thin deposits 

 of very fine gravel, and pebbles are also scattered through the 

 sand. The pebbles seldom exceed one half inch in diameter 

 and are usually one fourth inch or less. They have been noted 

 by the writer as far down the valley as the vicinity of Ouincy, 

 Illinois. They are a conspicuous feature above Rock Island, 

 Illinois. Upon following up the tributaries of the Mississippi 

 toward the head of these valley trains, the material becomes 

 markedly coarser, as is to be expected on the theory of their 

 derivation from the ice-sheet. 



It scarcely needs to be stated that so great a filling has greatly 

 interrupted the removal of the rock barriers of the Mississippi 

 at each of the rapids. A stream with the present volume of the 

 Mississippi and its comparatively low gradient of about six inches 

 per mile, can scarcely do more than remove the material brought 

 in by its tributaries, to say nothing of removing the great amount 

 of material deposited at the Wisconsin stage of glaciation. 

 There appears, however, to have been a long period succeeding 



