474 THE ILLINOIS GLACIAL LOBE. 



DEFLECTIONS SOUTH OF GLACIAL BOUNDARY. 



It seems necessary to refer briefly to two slight deflections of the Mis- 

 sissippi, one of which is below the limits of glaciation and the other near the 

 point where the glacial boundary bears eastward away from the Mississippi 

 Valley. The former is certainly independent of obstruction by the ice, and 

 the 'latter may be also. The deflection near the glacial boundary is at 

 Fountain Bluff, Jackson County, Illinois, where a rocky point similar to 

 that at Fulton has been cut off from the west bluff. The broad preglacial 

 valley, as at Fulton, is still occupied by the stream at flood stages, but is 

 much less direct than the course across the rocky point. In explanation of 

 this peculiar feature it is suggested that the encroachments of the Mississippi 

 upon the rocky point had so broken down the crest that a moderate silting 

 up of the valley, which probably occurred during the loess deposition, 

 enabled the stream to cross it. The persistence of the stream in this course 

 is probably attributable to the more direct line of discharge. Possibly the 

 presence of the ice in the valley immediately north and east of this rocky 

 point was influential in diverting the stream across it. The deflection farther 

 down the river is at Thebes, Illinois, where for a distance of about 6 miles 

 the river is excavating a new course across a rock point which projects 

 westward into Missouri. An inspection of this district shows that the alti- 

 tude of the rock in the rocky point crossed by the stream is so low that it 

 is necessary to assume but little silting up of the preglacial channel to have 

 made it possible for the stream to take the present direct course, probably 

 not more than 50 feet above the present level of the river. A silting of this 

 amount may be supposed to have occurred during the loess deposition. 



ROCK FLOOR AND PRESENT STREAM COMPARED. 



In this discussion each of the preglacial valleys utilized by the present 

 Mississippi are included. The data concerning the rock floor are obtained 

 from well borings or from tests for bridges, but are not sufficient^ full to 

 show whether the lowest part of the valley floor has at any point been 

 reached, much less do they indicate the lowest part at all points. It seems 

 probable, however, that the majority are in the deep part, since they are 

 located near the middle of the preglacial channel. They at least indicate 

 approximately the depth of preglacial excavation, and it is thought that they 

 throw some light upon the slope of the rock bottom. 



