PAPERS ON GEOLOGY AND GEOGRAPHY 217 



SAND AND GRAVEL FILLING OF THE SANGAMON VALLEY. 



The sand and gravel filling in the Sangamon Valley has 

 already been noted. No distinct terrace is shown at the 

 dam site, but on the south side of the valley, slightly above 

 the present flood-plain, gravel underlies alluvium and 

 "wash" from the south slope. All of this gravel filling is 

 believed to be correlative with the remnants of gravel ter- 

 races which occur both upstream and down, and which ap- 

 pear to have been deposited as a valley train from the ice 

 during the building of the Cerro Gordo moraine. 



SUMMARY OF THE HISTORY OF THE SANGAMON VALLEY 

 AT DECATUR 



Restricting our discussion to that part of the Sangamon 

 Valley at Decatur, it may be stated that present data in- 

 dicate that it dates back to the Sangamon interglacial epoch, 

 and probably to the close of the Illinoian glacial epoch. Upon 

 the melting of the Illinoian ice, the area was a flattish ground 

 moraine, and during the Sangamon inten'al, the drainage 

 cut a relatively broad, shallow valley at Decatur commen- 

 surate with that portion of the valley beyond the Shelbyville 

 moraine. Contemporaneous with the valley cutting, the 

 Illinoian till was leached and oxidized and soil was develop- 

 ed, extending down some parts of the valley slope. 



The change of climate from warm to glacial resulted in 

 the Wisconsin ice invasion which reached its extreme limit 

 a few miles west of Decatur. For a time the ice edge re- 

 mained within the zone of the Shelby\'ille moraine and 

 radically changed the contour of the Illinoian surface by 

 depositing morainic materials. Some of them were de- 

 posited directly from the ice, some by the glacial waters. 

 Readvances of the ice contorted the gravels and buried 

 them with till. As a result, the moraine was formed of 

 both till and sand and gravel with complex relations. 



The amount of drift deposited across the valley was not 

 enough to fill and eradicate the valley. The concentration 

 of the glacial waters doubtless helped to keep the valley 

 free. Consequently enough of a depression was left to 

 serve as a drainage line upon the retreat of the ice-front. 

 In this inherited depression, after some modification by the 



