76 IOWA STUDIES IN NATURAL HISTORY 



These tables show 

 that the original 

 surface was much 

 steeper than the 

 present Dodgeville 

 plain ; that the am- 

 ounts of material 

 removed from the 

 divides decrease by 

 a regular progres- 

 sion to the south ; 

 that different pro- 

 portions of resist- 

 ant and non-resist- 

 ant rocks have 

 been removed in 

 different places ; 

 and that there is a 

 certain definite or- 

 der in the rela- 

 tions of original 

 altitudes, amount 

 of rock removed, 

 and present alti- 

 tudes, irrespective 

 of the relative 

 thickness of resist- 

 ant and non-resist- 

 ant formations re- 

 moved (Fig. 18). 

 It seems improb- 

 able that degrada- 

 tion of the tops of 

 divides would' take 

 place in so orderly 

 a fashion unless 

 the streams re- 

 duced the general 

 surface to grade. 



