34 IOWA STUDIES IN NATURAL HISTORY 



It is possible for a stream not at grade to deposit, to 

 shift its course slightly, and to sink its channel deeper, 

 leaving its deposits in small areas as pockets or patches on 

 upper slopes. Small patches of fluvial gravel or sand found 

 on slopes above present drainage might be explained in this 

 way, but thick fluvial deposits spread widely over upland 

 flats could not be so explained. 



The application of this point is also limited by the diffi- 

 culty in distinguishing fluvial deposits from marine or 

 eolian or glacial or lacustrine deposits, especially after long 

 exposure. 



Undoubted fluvial deposits spread widely over upland sur- 

 faces would disprove that those surfaces represent plains of 

 marine deposition or of marine erosion or are merely the 

 tops of cuestas. They would almost or quite disprove the 

 structural hypothesis for the origin of the upland plain on 

 which they lie. Therefore, they constitute strong evidence 

 of more than one cycle of erosion. 



Coynhinations 



If the analysis of each of the evidences of more than one 

 cycle of erosion has been followed up to this point, it is 

 clear that no one of these evidences, in the abstract and 

 taken alone, can be said to prove more than one cycle in 

 the erosional history of a region. However, the study of 

 concrete cases of individual points may yield such proof. 

 Some of the evidences usually assigned merely suggest more 

 than one cycle of erosion if taken abstractly, but become 

 strong evidence when properly restricted by the elimination 

 of other possible interpretations. Others are strong evi- 

 dence in the abstract, and amount to proof if properly ap- 

 plied and limited. The relative values of these evidences 

 are summed up in the accompanying table. 



Interrupted profile and stream terraces could hardly 

 amount to more than a suggestion of more than one cycle 

 of erosion unless all other possible interpretations had been 

 eliminated by careful study in the field. 



So nearly impossible is it to distinguish the meanders of 

 old age from other crooks in streams that intrenched mean- 



