30 IOWA STUDIES IN NATURAL HISTORY 



or escarpment, on one side and a long, gentle slope on the 

 other," and stating that "the gentle slope usually corres- 

 ponds to the inclination or dip of slightly inclined sediment- 

 ary rocks\" he contends that the upland plains in the Drift- 

 less Area are simply the gently sloping surfaces of cuestas. 

 If this is the correct interpretation of such upland surfaces, 

 (1) the slope of any individual patch of summit area should 

 correspond in direction and amount with the dip of the 

 rock formations, (2) each upland area should be formed 

 by resistant rocks, and (3) the altitude of the summit of 

 any given cuesta should depend upon the resistance of the 

 rocks forming it and the length of time it had been exposed 

 to erosion after the removal by streams of all overlying rock 

 formations. 



Any considerable areas of summit flats now poorly 

 drained and forming broad divides between present streams 

 would hardly be formed in this way in a single cycle of 

 erosion. If individual summit areas were found to bevel 

 the edges of layers or formations, if these areas are large 

 and far from present streams, if some of the rock forma- 

 tions bevelled by the surfaces are non-resistant, and es- 

 pecially if a surface reconstructed by filling the lowlands to 

 the summit areas is found to have a uniform slope in direc- 

 tion and amount, if this slope be uniformly greater or less 

 than the dip of the beds, and if irregularities in rock struc- 

 ture and rock resistance do not influence this surface, the 

 even-crested summit areas conld hardly be considered to 

 be merely a series of cuestas. 



In Regions of Igneous Rocks: If massive igneous rocks 

 solidified below the surface of the lithosphere be eroded in 

 such a way as to leave flat-topped and accordant elevations, 

 it seems that at least two cycles must have been involved 

 in the history of the topography, except in cases where the 

 massive rock had a flat surface to begin with. In the nor- 

 mal case it would require a cycle of erosion to remove over- 

 lying rocks and flatten the surface of the igneous rocks, and 



1. Op. at., p. 42. 



