36 IOWA STUDIES IN NATURAL HISTORY 



But the investigator must depend upon certain combina- 

 tions of the various evidences, rather than upon single 

 points, if he is to prove or disprove that given regions have 

 suffered more than one cycle of erosion. It is unlikely that 

 a surface which has been peneplained and then uplifted 

 relative to the sea would show only one of the evidences of 

 having had such a history. The sequence of events which 

 involves two erosion cycles, and which gives rise to one of 

 these lines of evidence, may give rise to all. If a region 

 shows only one of these evidences the value of that one 

 should be discounted because none of the others is shown. 

 On the other hand a combination of several of these evi- 

 dences in a ]'egion furnishes a progression toward definite 

 conclusions which is geometrical rather than arithmetical. 



Referring again to the table above, Nos. 5, 8 and 9, all 

 found together in a region, even without special analysis, 

 would come near to proving more than one erosion cycle in 

 the history of the surface, and if properly analyzed the com- 

 bination might prove such a history beyond the possibility 

 of a doubt. Similarly, proof might be obtained through the 

 combinations of Nos. 4, 7 and 9; or Nos. 5, 6 and 8; or 

 Nos. 4, 5 and 7 ; or perhaps by a combination of 8 and 9, 

 or 5 and 7. Combinations of 4 and 8, 7 and 9, 4 and 6, or 

 1, 4 and 7 might under certain topographic conditions af- 

 ford strong evidence or even proof. Even Nos. 1 and 2 

 combined might under certain circumstances be strong 

 evidence of more than one cycle. 



It therefore seems clear that by distinguishing these 

 various features in the topographies of regions, by the 

 proper analysis of their possibilities and limitations under 

 existing conditions, and by certain combinations, it is pos- 

 sible to determine that (1) regions have certainly suffered 

 more than one cycle of erosion, or that (2) they have prob- 

 ably suffered more than one cycle, or that (3) they have 

 possibly suffered more than one cycle. If careful study of 

 a topography reveals no one of these evidences, or if it re- 

 veals only one or two whose origin after analysis is found 

 not to involve more than one cycle, it could be concluded 

 that (4) the surface has probably not been eroded in more 



