EROSION AL HISTORY OF DRIFTLESS AREA 37 



than one cycle. There would be cases where it would be 

 entirely impossible to demonstrate that (5) the surface had 

 certainly not been eroded in more than one cycle. 



If the deposits laid in a near-by sea during the erosional 

 history of a land surface are available for study, evidence 

 of more than one cycle of erosion on the land might be 

 found in them. If a series of formations graded upward 

 from conglomerate at the base through sandstone, shale, 

 and limestone to another conglomerate, it would seem that 

 the lower series including coarse, medium, and fine materi- 

 als would correspond respectively with youth, maturity and 

 old age of an erosion cycle on the land and that the upper 

 conglomerate would record an uplift of the land and the 

 inauguration of a youthful stage of a second cycle. 



However, such gradations in marine sediments might be 

 due to gradually changing climates, changing depth of 

 water without affecting land and sea relations, or slightly 

 migrating shorelines which do not materially effect the 

 height of land relative to sea. 



Alternation of sediments might be used to good advantage 

 in some cases to check the topographic evidences of erosion 

 cycles. For instance, the Cretaceous and Tertiary deposits 

 of the Atlantic Coastal Plain should and do, at least rough- 

 ly, check in this way the erosional history of the Appala- 

 chian mountains as stated by Willis, Hayes and Campl^ell, 

 and Davis. Similarly it is believed that the Tertiary and 

 Quaternary deposits of the Gulf Coastal Plain of Texas will 

 help in interpreting the erosional history of the Cordillei-a. 



MORE THAN TWO CYCLES 



When it has been demonstrated that the erosional history 

 of a given surface has involved more than one cycle, the 

 question of the number of cycles arises. Theoretically the 

 number of erosion cycles in a region is limited only by the 

 length of time during which the surface has been subjected 

 to fluvial processes and the frequency of positive diastrophic 

 movements during that time. So far as geologic time and 

 the frequency of land-forming diastrophic movements may 

 be conjectured, there is no known limit to the number of 



