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 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 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 down 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. 
