EROSIONAL HISTORY OF DRIFTLESS AREA 11 



of geologic history leads to the conclusion that the complete 

 erosion cycle is in most cases at least longer than the dias- 

 trophic period. This being the case most cycles of erosion 

 are interrupted by uplift and few if any cycles of erosion 

 have been complete. Doubtless there have been cases in 

 which lands have been so reduced by all the agencies at 

 work on their surfaces and by deposition in the sea that 

 the sea spread over them, but probably there has never 

 been a time when whole continents have been so destroyed. 

 In any case the present paper deals only with those surfaces 

 which have not been reduced to the condition of sub- 

 mergence. 



The question now arises as to what stage of degradation 

 is reached by the average surface before the cycle is in- 

 terrupted. Tarr' has argued that because there are few if 

 any low level plains to-day which have been developed by 

 streams, there never have been any and that pleneplains have 

 never existed. This conclusion is hardly warranted, for the 

 present day may be one closely following an uplift. 



The writer would agree that probably no continent-wide 

 and perfectly flat erosional plains have been developed in 

 the past, but he cannot agree that smaller areas have not 

 been brought to an earlier stage of reduction which might 

 be defined by the term peneplain. 



It seems unavoidable to suppose that erosion cycles might 

 be interrupted either in youth, maturity, or old age. But 

 interruption in old age should theoretically be most common, 

 for degradation takes place most rapidly in youth and 

 maturity, and is much slower in old age. Land is reduced 

 rather quickly to the peneplain stage but further reduction 

 to complete base level is almost infinitely slow. That is, in 

 a complete cycle the stage of old age would be longer than 

 youth and maturity. 



If a region reached old age in the first cycle and has gone 

 only to maturity of the second, some of the characteristics 

 of the first cycle will have held over into the second, and 

 the history should be ascertainable. If on the other hand, 

 a region is in youth of the first cycle when the interruption 



l7~Tarr, R. S., "The Peneplain," Am. GcoL, Vol. 21, pp. 351-370. 



