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of the deposits which now lie between the Illinois valley bluffs 

 over the ancient rock bed are due to the Illinoian glaciation. 



The next ice invasion, probably the Iowan, came from the 

 northwest, and to this stage have been assigned deposits in 

 Illinois in the vicinity of the lower Rock River watershed. It 

 seems to have had no marked effect in our state. 



Long ages after the close of the Illinoian period, came an 

 event which profoundly affected the future of Illinois, and 

 caused, among other things, the sand deposits studied by us. 

 The Wisconsin glacial invasion was not so extensive as the 

 Illinoian, but it is characterized by voluminous deposits, deeply 

 covering and almost entirely obliterating the previous surface 

 contours; by large and continuous ridges thrown up along the 

 terminal lines and during the successive stages of its retreat; 

 and by abundant and powerful water action. Upon or near 

 the terminal ridges in Illinois are the cities of Princeton, 

 Peoria, Pekin, Atlanta, Clinton, Shelbyville, Mattoon, and 

 Charleston. 



A later substage of less extent but even greater intensity 

 was that called the Bloomington, whose terminal ridges branch 

 off eastward near Peoria, and pass under Bloomington and Gib- 

 son City, and to the north of Danville, 111., and Covington, Ind. 

 The enormous outwash of sand and gravel has left more or less 

 of its deposits in all the valleys leading away from these ridges. 

 The west side of the rock valley above Peoria was covered by 

 the terminal ridge to a height, near Peoria, of about 350 feet 

 above the present river, making a total depth of about 450 feet 

 of glacial deposits upon the bottom of the ancient rock valley. 



The Illinois, clogged with more than it could carry, filled 

 its bed with sediment to a depth of about 170 feet above pres- 

 ent levels at Peoria within the gap in the terminal ridges, and 

 spread out in a vast detrital fan over the great expansion of 

 the valley below, with a comparatively rapid gradient of de- 

 scent. At the mouth of the Sangamon the filling reached 

 about 75 feet above present levels; and at the river's mouth, 

 about 50 feet. As the ice retreated and the water cleared, 

 nearly all of this vast deposit was swept out of the valley, leav- 



