CHANGES OF LEVEL 



103 



have a nearly common elevation, below which are the sharp 

 valleys </, d', and d" '. The points a, a', and a" represent the tops of 

 ridges formed by the outcrops of layers of hard rock. If the crests 

 of the ridges are level, the points 

 a, a', and a" must represent remnants 

 of an old base-level, since at no time 

 after a ridge of hard rock becomes 

 deeply notched does it acquire an even 

 crest, until it is base-leveled. 1 After 

 the cycle represented by the rem- 

 nants a, a', and a" was completed, 

 the region suffered uplift. A new. 

 cycle represented by the plain b, b', 

 and b" was well advanced, though 

 not completed, when the region was 

 again elevated, and the rejuvenated 

 streams began to cut their valleys d, 

 d', and d" in the plain of the previous 

 incomplete cycle. The elevations, c 

 and c' (intermediate between a, a', 

 and a", and b, b', b"), may represent 

 either remnants of the first base-level 

 plain, lowered but not completely 

 removed while the plain b, b', b" was 



Fig. 102. Diagram to illustrate 

 an ideal case of rejuvenation as the 

 result of uplift. The black area 

 at the bottom represents the sea. 



developing; or they may represent a cycle intermediate between 

 that during which a, a', a" and b, b', b" were developed. 



Fig. 103. Diagram to illustrate cycles of erosion where the beds are tilted. 



If the strata involved are horizontal, the determination of 

 cycles may be less easy. Thus in Fig. 104, 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 degradation started. 

 So, too, the various benches below a, such as b, b', and b", might 

 well be the result of the superior hardness of beds at this level. 

 For the determination of successive cycles in the field, it is necessary 



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. 



