98 



IOWA STUDIES IN NATURAL HISTORY 



to the south from the divide. In the knowledge that the 

 formation of this arch antedated the establishment of the 

 present course of the Mississippi river and that the various 

 rock formations do not thin out appreciably in approach- 

 ing the crest of the arch, it is possible to reconstruct the 

 original surface for a line following the present course of 

 the Mississippi river. This surface is found to lie at 960 

 feet at Bellevue, 1030 feet at Dubuque, 1520 feet at Prairie 

 du Chien, 1850 feet at Lansing, 2000 feet at La Crosse, 1870 

 feet at Winona, and 1410 feet at Minneapolis. A section 

 showing this original surface and the attitude of the strata 

 beneath it for the whole length of the Mississippi river 

 from Minneapolis to Bellevue is shown in Fig. 27. 



Fig. 27. A section showing the original surface of the Driftless Area and the struc- 

 ture of the strata along a line now followed by the Mississippi river. The total 

 horizontal distance is approximately 250 miles. Vertical scale: 1 inch equals 360 feet. 



Such a surface would be eroded by streams which would 

 exhibit different stages of adjustment in different stages 

 of the erosion cycle. 



In the first stage of adjustment streams would form, 

 flowing south and north from the crest of the arch. From 

 the main streams, tributaries would develop which would 

 curve headward up the slope of the plain toward the divide 

 from either side. As all the streams in this first stage were 

 flowing on the Niagara dolomite there were only slight 

 differences in resistance, and the courses of the streams 

 would be determined primarily by the topographic slopes 

 which were in turn determined by the structure. The gen- 



