622 FRANK LEVERETT 
It seems probable that the Illinoian glaciation was the first to 
throw the upper Mississippi out of its course through the Illinois 
Valley. At the culmination of the Illinoian stage of glaciation 
the Mississippi opened a temporary course across eastern Iowa, 
just outside the limits of the ice sheet.t This connected with the 
present Mississippi at the head of the gorge across the Des Moines 
rapids below Fort Madison at a level about 120 feet above the 
present stream; so nearly all the cutting in rock at these rapids 
has been done since that time. The Illinoian drift extends a few 
miles beyond the present course of the Mississippi from Clinton 
to Fort Madison, but farther north and south its border lies east 
of the river. With the recession of the Illinoian ice sheet the 
Mississippi shifted to its present course between Clinton and Fort 
Madison. 
It seems to have been at this time that the course across the 
rapids above Rock Island was initiated, the ice being still present 
on the lower land to the east. ‘The channel across these rapids, 
like that across the Des Moines rapids, has the appearance of being 
much younger than that between Muscatine and Rock Island. 
Goose Lake channel, which branches off from the present Mis- 
sissippi at the mouth of Maquoketa River and passes southward 
across Clinton County, Iowa, to the Wapsipinicon Valley, was 
probably opened in Illinoian time, and may have continued down 
to Wisconsin time. It seems, however, to have been abandoned 
before the waters of the glacial lakes. began their work of excavation 
discussed below. 
The glacial drainage outside the Illinoian ice sheet in south- 
eastern Iowa carried enough material into the Mississippi Valley 
below the Des Moines rapids to fill it to a level about too feet 
above the present low-water level of the river. But within a few 
miles it dropped to a level so nearly the same as that of the filling 
at the succeeding, Wisconsin, stage of glaciation that it cannot 
well be differentiated. In each the level of filling is about 60 feet 
above present low water, and 30 to 4o feet above flood stages. 
Frank Leverett, ‘“The Illinois Glacial Lobe,” U.S. Geol. Surv., Monograph 
XXXVIII (1899), pp. 89-97, Pl. VI. 
