EROSIONAL HISTORY OF DRIFTLESS AREA 127 



porarily by the deposition of at least two bodies of glacial 

 drift on the borders of the area. Deep valleys appear to 

 have been cut before the advance of the Kansan glacier and 

 the deposition of a thin body of Kansan drift on the west 

 border. The valleys of Pecatonica and Apple^ rivers had 

 been cut to depths below their present bottoms by the time 

 of the Illinoian (?) ice invasion from the east. 



Some time before or during or immediately after the 

 Wisconsin glacial epoch, when the valley trains were de- 

 posited in the Mississippi and Wisconsin valleys, there ap- 

 pears to have occurred a subsidence of the surface amount- 

 ing to about 180 feet and perhaps accompanied by a tilting 

 of all older graded surfaces slightly to the south. The sub- 

 sidence rendered it impossible for the Mississippi and its 

 tributaries to cut back to their original levels. If tilting 

 occurred the gentler slope of the present flood plain as com- 

 pared with all previous gradients is explained. 



After the withdrawal of the Wisconsin glacier the Mis- 

 sissippi river and its tributaries began to excavate their 

 valleys by the removal of the fluvio-glacial debris but they 

 reached grade 30 or 40 feet below the original top of the 

 deposit and 180 feet on the average above its bottom. Hav- 

 ing reached grade, the streams have all developed valley 

 flats in the soft material deposited by waters during the 

 Wisconsin epoch. 



1. Trowbrldyc, A. C. and Shaw. E. W., Bull. 111. Gcol. Surv. No. 26, pp. 95-99. 



