60 IOWA STUDIES IN NATURAL HISTORY 



Both the south-dipping and the north-dipping monoclines 

 and to an extent also the nearly horizontal structures of 

 the axial area are interrupted in many places by low anti- 

 clines and shallow synclines. The whole composite struc- 

 ture of the Driftless Area might be said to be a low anti- 

 clinorium, plunging southwestward. For additional details 

 see Fig. 11. 



THE STAGE OF EROSION 



In the present cycle of erosion the general surface of the 

 Driftless Area is in late youth or early maturity, although 

 small portions exhibit a later stage of development. The 

 valley of the Mississippi and the valleys of its larger tribu- 

 taries, such as the Wisconsin, La Crosse, and Upper Iowa 

 rivers, appear to be mature, but if the thick deposits in 

 them were removed the valleys would have a much more 

 youthful appearance. Most of the valleys in the area are 

 young. There are considerable areas of unreduced flattish 

 land on the highest divides and still greater areas at lower 

 altitudes well above the valley bottoms. It is these unre- 

 duced upland surfaces which form the chief physiographic 

 problems of the region and which at the same time give the 

 investigator his best clue to the history of the surface. 

 Along the main drainage lines there are narrow graded 

 valley flats which are valuable for comparison with the up- 

 land flats. The topography has a relief of over 600 feet 

 within short distances, and before the valleys were partially 

 filled the relief was over 800 feet. 



EVIDENCES OF MORE THAN ONE CYCLE OF 

 EROSION 

 Of the several lines of evidence for more than one cycle 

 of erosion which have been used in determining the erosion- 

 al histories of various surfaces and which were discussed 

 in Part I, the Driftless Area shows seven, in diflferent de- 

 grees of perfection and significance. 



Even-Crested Summit Areas (the Dodgeville Plain) 

 In most portions of the Driftless Area the highest divides 

 are noticeably flat on their summits. These flat surfaces 



