102 IOWA STUDIES IN NATURAL HISTORY 



the slope of the original surface. The original surface had 

 the theoretical altitude of 1410 feet at Minneapolis, 1870 

 feet at Winona and 2000 feet at La Crosse, and the present 

 river flows from Minneapolis, past Winona, to La Crosse. 

 South of the axis of the arch and the main divide on the 

 original surface, where the strata have an average dip in 

 the direction S 26°W, which also must have been the aver- 

 age direction of slope of the original surface, the river 

 follows a curved course from La Crosse to Bellevue in the 

 general direction S 18°E, forming an angle of 44° with the 

 dip of the strata and the slope of the original surface. On 

 neither side of the arch is there any evidence that the minor 

 curves of the river really are controlled by minor structural 

 features such as anticlines and synclines. 



It should also be borne in mind that the Mississippi river, 

 by taking a course more nearly directly south at some point 

 south of St. Paul, would have flowed around the southwest 

 end of the plunging anticline, avoiding the crest of the 

 arch and the high portion of the original surface entirely. 

 Such a course also would have been little if any longer to 

 Dubuque than the course which was actually established. 



There is a further indication that the Mississippi river 

 has had such a history as outlined above in the fact that 

 certain of the larger streams north of the axis of the anti- 

 cline, for instance, Whitewater river, join the master 

 stream with an acute angle down-stream. This suggests 

 that the Mississippi river was flowing in the opposite 

 direction while Whitewater river was being developed, and 

 that its direction of flow was later reversed. 



This lack of harmony between rock structures and 

 original slopes on the one hand and the present course of 

 the Mississippi river on the other, cannot be explained on 

 the basis of the ordinary superimposition. Such an inter- 

 pretation could be correct only in case some post-Paleozoic 

 formation on which the stream could have established its 

 present course, had been deposited over the strata of the dis- 

 trict, after they were deformed, so as to let the stream 

 down on the stratigraphic structures and topographies as 



