42 IOWA STUDIES IN NATURAL HISTORY 



with the intermediate plain parallel with an undissected 

 peneplain or with graded streams, the first recorded move- 

 ment seems to have involved warping and the second up- 

 lift was uniform. If the upper plain and the intermediate 

 plain are parallel, but with irregular obliquity related to 

 present graded streams, the first movement was uniform 

 and the second one was accompanied by warping. 



This method of interpretation lacks much of being de- 

 cisive. In the first place the warping or folding of an ero- 

 sion surface destroys accordance of levels and makes it ex- 

 tremely difticult to decide whether the surface was once 

 smooth and has been warped, or whether it was never 

 smooth. In the latter case there would be little evidence 

 that there has ever been more than one erosion cycle. The 

 warping of a surface is likely to destroy evidence that there 

 has been any moveemnt at all. There would have to be some 

 evidence beyond the accordance of levels to prove that the 

 surface actually was a peneplain. However, such evidence 

 might consist in fluvial deposits on remnants of the warped 

 surface, or in antecedent streams cutting across the folds 

 of the surface. 



Another difficulty with the interpretation of warping 

 movements grows out of the fact that no erosion surfaces 

 are altogether flat and that there is therefore an irregular 

 obliquity between two consecutive surfaces whether warp- 

 ing has taken place or not. However, departures from 

 parallelism due to erosional irregularities in the surface 

 would show themselves in topographic details and those 

 due to warping would be more general ; that is, they would 

 be difl"erences between averages rather than between speci- 

 fic points. Careful study of the valleys cut in the old ero- 

 sion surface during the next cycle should also aid in de- 

 termining whether the irregular lack of parallelism is due 

 to warping or to erosional irregularities. If the valleys 

 vary in depth or width or stage of development from 

 point to point warping could be called in to explain such 

 variations. 



After all probably the best evidence of warping of pene- 

 plains is found when it is determined that the individual 



