172 GEOLOGY 



rock becomes deeply notched does it acquire an even crest, until it is 

 base-leveled. 1 After the cycle represented by the remnants a, a', 

 and a" was completed, the region suffered uplift. A new cycle 

 represented by the plain 6, b r , and b" was well advanced, though 

 not completed, when the region was again elevated, and the reju- 

 venated streams began to cut their valleys d, d', and d" in the 

 plain of the previous incomplete cycle. The elevations, c and c' 

 (intermediate in elevation between a, a', and a", and b } b' , and ?/'), 

 may represent either remnants of the first base-level plain, lowered, 

 but not obliterated, while the plain b, b' , b" was developing; or 



Fig. 141. Diagram to illustrate cycles of erosion where the beds are tilted 



they may represent a cycle intermediate between that during which 



a, a', a" and b } b 1 ' , b" were developed. 



If the strata involved are horizontal, the determination of 

 cycles is sometimes less easy. Thus in Fig. 142, it is not possible 

 to say whether a and a' represent remnants of an old base-level, 

 or whether they represent the original surface from -which degra- 

 dation started. So, too, the various benches below a, such MS 



b, b' ' , and b", may readily be the result of the superior hardness of 

 beds at this level. For the determination of successive cycles in 

 the field, it is necessary to consider areas of considerable size, and 

 to eliminate the topographic effects of inequalities of hardness. 



It is by the application of the preceding principles that it is 

 known that the Appalachian Mountains, after being folded, were 

 reduced to a peneplain (the Kittatinny peneplain) from the 

 Hudson River to Alabama. The old peneplain surface is indi- 

 cated by the level crests of the Appalachian ridges. The system 

 was then warped (not folded) up, and in the cycle of erosion which 

 followed, broad plains were developed at a new and lower level, 

 corresponding in a general way to the plains b, b', and b" of Fig. 



1 Other views have been entertained. See Tarr, Am. Geol., Vol. XXI. 

 pp. 351-370, and Daly, Jour, of Geol., Vol. XIII, pp. 105-125. 



