IOWA ACADEMY OF SCIENCES. 89 



FILLING AT THE WISCONSIN STAGE OF GLACIATION. 



At the Wisconsin stage of glaciation the Mississippi and 

 several of its tributaries, which flowed away from the ice 

 sheet, became so burdened by glacial detritus that they were 

 unable to completely transport their load, much less to 

 continue the erosion of their valleys. The Mississippi headed 

 in the ice sheet near St. Paul, Minn., while the Chippewa and 

 Wisconsin rivers brought material from the Chippewa and 

 Green bay lobes of Wisconsin. Rock river, also, brought 

 material from the Green bay lobe and through its tributaries, 

 Kishwaukee and Green rivers, from the Lake Michigan lobe. 

 Just above St. Louis the Illinois river contributed a large 

 amount of material, derived from the Lake Michigan lobe. 

 These streams discharged such large quantities of sand into 

 the Mississippi that its 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 vallej'^ 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 erosion in 

 the preceding stage of deglaciation. 



The filling probably began during the early part of the 

 Wisconsin stage of glaciation, but the great bulk of it appears 

 to have been contributed during the part of the Wisconsin stage 

 of glaciation represented 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 dow^n the valley as the vicinity of 

 Quincy, 111. They are a conspicuous feature above Rock 

 Island, 111. Upon following up the tributaries of the Missis- 

 sippi 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. 



