EROSIONAL HISTORY OF DRIFTLESS AREA 115 



proof exists in the Driftless Area. It is impossible that a 

 surface could have been developed in a single cycle of ero- 

 sion, which has (1) even-crested summit areas which, after 

 analysis, represent a peneplain, (2) an intermediate plain 

 which can be interpreted only as a partial peneplain, (3) 

 antecedent streams which could have developed their pres- 

 ent courses only in old age of an erosion cycle,, (4) in- 

 trenched meanders for which no other explanation than that 

 they record more than one cycle have been worked out, (5) 

 associated sets of crooked and straight streams which are 

 valuable corroborative evidence of plural cycles, and (6) 

 undoubted fluvial deposits widely distributed on flat sur- 

 faces far above present drainage, 



THE NUMBER OF EROSIOXAL CYCLES 

 The next question which confronts the interpreter of the 

 erosional history of the Driftless Area has to do with the 

 number of cycles of erosion which have been involved in 

 the formation of the surface. The question can be answered 

 when it has been determined how many distinct sets of 

 evidences of more than one cycle are included among the 

 five evidences outlined above. 



In the first place, there is no way to ascertain how many 

 cycles, if any, intervened between the time of withdrawal 

 of the last Paleozoic sea and the cycle in which the Dodge- 

 ville plain was formed. There may have been time for 

 several cycles of erosion between these dates. But of any 

 such cycles all evidence was obliterated in the making of 

 the Dodgeville plain. If it be assumed that the cycle which 

 was inaugurated by the final withdrawal of the sea was 

 the same cycle as that in old age of which the Dodgeville 

 plain resulted, there would be no way to prove the assump- 

 tion to be incorrect. 



Considering the evidences of more than one cycle of ero- 

 sion it is clear that the Dodgeville plain and the Lancaster 

 plain could not have been formed in the same cycle. From 

 foregoing discussions it is clear that the antecedent sti'eams 

 and upland fluvial deposits go with the Dodgeville plain, 

 and the intrenched meanders and associated sets of crooked 



