68 IOWA STUDIES IN NATURAL HISTORY 



These computations bring out certain facts. The plain 

 constructed by connecting areas of summit plains across 

 intervening areas is by no means a geometrical plane, for 

 it dips in different directions and by different amounts in 

 different places. In small districts, widely differing re- 

 sults can be obtained by taking different sets of points as 

 bases for computation. Local irregularities obscure the 

 general slope. If the results of all the available computa- 

 tions are considered, the average slope of' the plain is in 

 the direction S 75° E to an amount of 4.7 feet per mile. If 

 the local estimates be eliminated and only those which in- 

 volve long distances be included, the effect of local irregular- 

 ities is minimized and the general slope of the plain is 

 found to be S 23° E, 3.3 feet to the mile. The direction and 

 amount of dip of the strata are much more nearly constant 

 and average S 28° W and 14.2 feet per mile recpectively. 

 Nowhere are the plain and the strata parallel. The angle 

 between their respective average directions of dip is 51° 

 and the strata dip more than three times as steeply as the 

 plain slopes. Even in small districts where upland plains 

 are broad and cover a considerable distance in directions 

 at right angles to the strike of the strata, the slope of the 

 plain and the beds are not parallel. The lack of parallelism 

 between the Dodgeville plain and the strata which under- 

 lie it is expressed in the fact that progressively younger 

 beds are bevelled by the plain from north to south on the 

 south limb of the arch. 



As outlined in Part I such a plain as the Dodgeville plain 

 is open to several possible interpretations : It might be the 

 original marine plain of deposition, a plain of marine ero- 

 sion, or a structural plain on a single hard stratum. The 

 apparent accordance of levels might be due to the erosion 

 in a single cycle of a surface underlain by gently south- 

 dipping and unequally resistant formations, developing a 

 series of somewhat even-topped cuestas whose summits 

 were never parts of a plain now dissected. Or, the Dodge- 

 ville plain might be a true peneplain. • 



The Dodgeville plain cannot be the sea bottom uncovered 

 by the withdrawal of the Paleozoic sea, for it is known that 



