EROSION AL HISTORY OF DRIFTLESS AREA 103 



the covering formation was penetrated. No such covering- 

 formation is known to have existed in the Driftless Area; 

 and it seems extremely unhkely that a deposit so thick did 

 exist and has been so thoroughly removed that no remnants 

 of it are left. There are a few patches of stream gravel 

 on the uplands, which some have considered to be Creta- 

 ceous and others Tertiary in age, but these deposits are 

 local in their distribution and are believed by no one to 

 have covered the entire Driftless Area. Certainly they did 

 not cover it to such depths as to mask the present struc- 

 tures. Neither is the glacial drift competent to cause such 

 superposition, for there are considerable areas on both sides 

 of the ^Mississippi which are driftless. Furthermore, the 

 drift must have had a thickness of more than 600 feet at 

 Minneapolis to have carried the river over the crest of the 

 arch at La Crosse. 



Because the Mississippi river is generally independent of 

 structure, because it flows for 135 miles in a direction which 

 is up the slope of the original surface, because it cuts across 

 the axis of a fold whose dips are considerable, because the 

 surface of the Driftless Area is in maturity and shows no 

 signs of old age, and because the river has not been super- 

 imposed, the Mississippi river is believed to be antecedent 

 and to record more than one cycle in the erosional history 

 of the surface. 



The relations between the course of the ^lississippi river 

 and the Dodgeville and Lancaster plains north of La Crosse 

 are not definitely ascertainable. On the south limb of the 

 anticline, however, the general course of the river comes 

 within b of being parallel in direction with the Dodgeville 

 plain and within 6' of parallelism with the Lancaster plain. 

 It seems likely therefore, that the river established its pres- 

 ent course in old age of the cycle of erosion in wliich the 

 Dodgeville peneplain was formed and held that course 

 without great change during the dissection of the Dodge- 

 ville plain and the formation of the Lancaster plain and 

 while the Lancaster plain was being uplifted r.nd eroded. 



