EROSIONAL HISTORY OF DRIFTLESS AREA 125 



the Kansan drift, where it is thin. Where either drift is 

 thin, it has been brought to its limits of weathering. 



There is then some evidence, which seems to the writer 

 to be strongly indicative, if not conclusive, that this drift, 

 in a district which has been called driftless, is pre-Kansan 

 in age, and that it was deposited while the Lancaster plain 

 was still intact and before the deep valleys were formed. 

 Otherwise, why should there be no patches of the drift in 

 the valleys? And why should the valleys show not the 

 slightest indication of having been glacially worn? The 

 Kansan drift seems clearly enough to have been deposited 

 after the valleys were formed. 



The above evidence seems to justify the interpretation, at 

 least as a working hypothesis, that the Lancaster plain was 

 intact at the time of the pre-Kansan ice invasion, but that 

 it was uplifted and partly dissected before the Kansan 

 epoch. Leverett^ objects to this interpretation and cites 

 the presence of pre-Kansan drift in the bottoms of deep 

 rock-bound valleys in Wisconsin and southeastern Iowa. So 

 far as the writer has been able to investigate the evidence, 

 he finds it inconclusive. The writer's interpretation is 

 strengthened by E. W. Shaw-, whose recent work in the 

 Ozark district seems to show that the main uplift and the 

 main development of the Mississippi valley there took place 

 during the early Pleistocene, rather than at the close of the 

 Tertiary as was previously supposed. 



Accepting the above interpretations of the ages of the 

 Dodgeville and Lancaster plains, at least as probabilities, 

 the probable dates of diastrophic events and erosion cycles 

 are easily determined and are stated in the following sum- 

 mary of events. 



SUMMARY OF EVENTS 



The first step in the history of the surface of the Drift- 

 less Area was the final emergence of the surface from the 

 sea and the formation of an anticline with its axis running 

 through La Crosse and its south limb forming a great 



1. Personal communications and oral discussions in the field. 



2. Personal communications to the writer. 



