72 IOWA STUDIES IN NATURAL HISTORY 



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cuesta east of Prairie du Chien. 

 This divide includes much flat 

 land on its summit. Its sum- 

 mit area has a relief of less 

 than 100 feet, and yet it bevels 

 the edges of the Prairie du 

 Chien, St. Peter, Platteville, 

 Decorah, and Galena forma- 

 tions. In its extent of over 50 

 miles its surface falls from 

 1300 feet at its north end, to 

 1200 feet near Prairie du 

 Chien, although a stratigraphic 

 horizon which has an altitude 

 of 1200 feet in the Sparta 

 quadrangle is found at 620 feet 

 at the south terminus of the 

 divide. So independent are the 

 surface and the strata that the 

 change from one formation to 

 another is not expressed in the 

 surface. (Fig. 17) It would 

 be impossible to explain the de- 

 tails of such a ridge on the 

 basis of a single cycle of 

 erosion. 



(4) In areas where the sum- 

 mit plains are broad and flat, 

 and such conditions exist in 

 many places in the Driftless 

 Area, it is difficult to conceive 

 a way by which the material 

 from above was removed to 

 make the flats, under the 

 theory that there has been but 

 one cycle, of erosion. In the 

 Sparta quadrangle, 200 feet 

 of Niagara dolomite, 100 feet 

 of Maquoketa shale, 320 feet of 

 Galena dolomite and Platte- 



