EROSIONAL HISTORY OF DRIFTLESS AREA 21 



Antecedent Streams 



The value of antecedent streams as evidences of more 

 than one cycle of erosion in a region depends upon the 

 definition of the term antecedent. The term was originally 

 applied by PowelP to streams which hold previously estab- 

 lished courses as their beds are warped, folded, or faulted 

 The streams were supposed to degrade their beds as rapidly 

 as the beds were warped up. Such a history does not 

 necessarily involve more than one cycle of erosion, except 

 in the places where up-warping occurs. 



Later the definition of antecedent streams was greatly 

 broadened by Davis, Willis, and Hayes and Campbell, who 

 applied the term to the Potomac, Susquehanna and other 

 rivers of the Appalachian mountains. This whole region 

 was folded and then peneplained, the rivers acquired their 

 present courses on the peneplain, and then uplift of the 

 whole surface took place and the streams intrenched them- 

 selves in their old courses. According to this definition 

 antecedent streams are those which develop courses inde- 

 pendent of rock structures in old age of an erosion cycle 

 and hold those courses after uplift. Such streams are im- 

 portant evidences of more than one cycle. 



In a folded region, such as the Appalachian mountains 

 at the end of the Paleozoic era, streams adjust their courses 

 in several distinct stages during the first cycle of erosion. 

 In stage I, the main streams flow parallel with the strike 

 of the strata in the axes of the synclines and the tributaries 

 flow down the limbs of the anticlines parallel with the dip 

 and at right angles to the main streams. In this first stage 

 the slope of the land controls the courses of the streams. 

 In stage II, those streams acquire an advantage, which first 

 penetrate resistant layers and come to flow on non-resist- 

 ant layers parallel with the strike. In this stage the main 

 drainage lines shift to the limbs or axes of the anticlines. 

 (Fig„ 7) . Now it is the resistance of the rock and the rock 

 structures which control the courses of the streams. Finally 

 when old age has been reached and all or most of the rocks 



1. Powell. J. W., "Exj^lorations of the Colorado River of the West and Its Tribu- 

 taries," p. 163. 1875. 



