EROSIONAL HISTORY OF DRIFTLESS AREA 25 



vantage of the antecedent streams oblique to the strike and 

 will behead the antecedent streams. This leaves windgaps 

 where the antecedent streams flowed across hard forma- 



-""'"' '■'■^■■' ""^^ 



■~^ 



-x^^ 

 ->?^ 



Fig. 9. Pai't of the Harrisburg, Pa. topographic 

 niap, showing the antecedent course of the Susque- 

 hanna river. Such a relation between stream 

 course, topography, and rock structure proves that 

 the surface has suffered more than one eyc'.e of 

 erosion. 



tions, provided these streams intrenched themselves in their 

 old courses before piracy took place. 



In regions of folded strata there is possibilitj^ of piracy 

 and the formation of windgaps in the first cycle of erosion. 

 In passing from Stage I to Stage II of adjustment the main 

 streams are diverted from courses on the synclines to cours- 

 es on the limbs or axes of the anticlines, giving rise to 

 drainage systems some portions of which are parallel with 



