28 IOWA STUDIES IN NATURAL HISTORY 



by the application of a combination of other evidences, that 

 the Appalachian mountains have been eroded in more than 

 one cycle and that at the accordant summits of the ridges 

 there are remnants of a surface formed in a cycle previous 

 to the present one. 



It has also been argued that mountain ridges can main- 

 tain only a certain elevation because the surrounding area 

 is not able to support the greater pressure which would 

 operate if the ridges were higher. This is a part of the 

 theory of isostasy. No discussion of this theory is in place 

 here. It need only be said that the structure of the rocks 

 in most areas where there are accordant summit levels is 

 such as to prove that the elevations were once much higher 

 than they now are and that they have been reduced by 

 streams. 



It is also possible to assume, until proven otherwise, that 

 accordant summit levels in a folded region are remnants of 

 a plain of marine denudation. The criteria for distinguish- 

 ing remnants of such a plain and remnants of a true pene- 

 plain are clear. If the sea cut its way on the land for any 

 considerable distance, portions of the wave-cut terrace 

 would become sites of marine deposition and when the sea- 

 denuded plain became land, it would be covered with marine 

 sediments, which of course might be removed later. Also 

 the border of an old plain of marine denudation would be 

 a shoreline and erosion remnants on its surface would have 

 ihe contour of islands. It has never been proven that plains 

 of wide extent are made by this method, especially if the 

 plain be inland, and no even-crested summit areas have 

 over been proven to be remnants of plains of marine de- 

 nudation. 



It should not be said that even-crested summit areas in 

 regions of folded strata considered alone, prove more than 

 one cycle of erosion, but they afford strong corroborative 

 evidence to that effect. 



In Regions of horizontal or nearly horizontal strata: Ac- 

 cordant summit levels, where strata are horizontal or near- 

 ly so, afford possibilities of interpretation not applicable in 



