74 IOWA STUDIES IN NATURAL HISTORY 



of the plain southward, on the assumption that the upland 

 surfaces are merely parts of unrelated cuestas. Estimat- 

 ing that erosion started when the whole region was covered 

 by 1000 feet of strata now gone, and knowing the amount 

 and direction of dip of the strata and the elevation of the 

 various portions of the summit plain, it is possible to es- 

 timate what was the original altitude of the surface and to 

 what altitude the upland surface was reduced at any given 

 locality. The dip of the strata is so slight that the altitude 

 of the original surface may be obtained by adding the 

 thickness of the strata removed to the present altitude of 

 the surface, without appreciable error. The results of a 

 series of such computations for a series of localities from 

 north to south and including each of the three cuestas south 

 of the structural axis are tabulated as follows: 



TABLE SHOWING THE RELATION OF THE ORIGINAL SURFACE OF THE DRIFT- 

 LESS AREA TO THE SURFACE OF THE DODGEVILLE PLAIN 



