104 IOWA STUDIES IN NATURAL HISTORY 



It is not known that this holds in detail for that part of the 

 river north of La Crosse. 



Other Streams 



All the smaller tributary streams of the Driftless Area 

 appear to have been formed in the present cycle of erosion, 

 be it the first, second or third cycle which the Area has ex- 

 perienced. It would seem likely, however, that if the 

 Mississippi river existed in a previous cycle, some of its 

 larger tributaries, such as the Wisconsin, La Crosse, Upper 

 Iowa, Turkey, Root and Whitewater rivers might have 

 established their courses at the same time and have held 

 their courses to the present. 



Within the Driftless Area, Wiscoivsin river liows from 

 Prairue du Sac to the Mississippi in a general direction 

 S 73 °W, which is at an angle of 47° with the general dip of 

 the strata and what must have been the slope of the original 

 surface. The whole course of the river in this distance is 

 on Potsdam sandstone, but before it had cut quite so deeply 

 it must have flowed from the soft sandstone, across the re- 

 sistant Prairie du Chien dolomite, instead of remaining on 

 the soft sandstone as it could have done by developing a 

 course more nearly parallel with the strike of the strata. 

 As Wisconsin river is not adjusted to the structure the con- 

 clusion seems reasonable that it probably developed a course 

 in harmony with conditions which existed in old age of a 

 previous cycle and held that course during rejuvenation. 

 In the sense in which Willis, Davis, and Hayes and Camp- 

 bell used the term in connection with certain rivers of the 

 Appalachians, Wisconsin river is, then, probably antecedent 

 and an evidence of more than one cycle of erosion. 



The direction of dip of the strata in the neighborhood of 

 La Crosse river near the crest of the arch is not known 

 accurately, and it is not known whether the river, which 

 flows S 56°W is parallel with or oblique to the axis of the 

 anticline. It may, therefore, be antecedent or consequent. 



The courses of the streams in Iowa doubtless have been 

 influenced by glacial drift which extends eastward almost 

 or quite to their points of junction with the Mississippi; 



