164 GEOLOGY. 



above the level of its main, the former is called a hanging valley. Hang- 

 :ng valleys developed by stream erosion alone are not common except 

 just after the recession of a falls past the mouth of a tributary. Hang- 

 ing valleys, as well as the characters and relations illustrated by Figs. 

 152-154 are criteria of rejuvenation, but they must be apphed with 

 discretion. Such profiles, for example, as that shown in Fig. 154 may 

 be developed when the rock of a stream's bed is unequally resistant, 

 and hanging valleys are generally a result of glaciation (see Chapter V). 



Rejuvenated streams sometimes inherit certain peculiarities from 

 their aged ancestors. Thus a rejuvenated stream may intrench the 

 meanders possessed by the old stream which preceded (see Fig. 1, 

 PI. XIV, near Harrisburg, Pa.), and intrenched meanders are one of 

 the marks of rejuvenated streams. They are not uncommon in the 

 Appalachian Mountain reg'.ons, and are known in other parts of the 

 world. The Seine and the Moselle furnish further illustrations.^ 



The history of the new cycle of erosion inaugurated by the upHft 

 would differ from that of the preceding cycle in that the new one would 

 begin with a drainage system already developed. Other things being 

 equal, therefore, the reduction of the land would proceed more rapidly 

 in a subsequent cycle than in the first. 



The recognition of different cycles of erosion, separated by up- 

 Hfts, is often easy. The principles involved are illustrated by Fig. 

 155 which represents an ideal profile of considerable length (say 50 



G 



















a" 



w. 



m 





C 



^^ 



^ 



% 



^ 



^M 



^ 



Fig. 155. — Diagram to illustrate cj^cles of erosion where the beds are tilted. 



miles). The points a, a' , and a'^ reach a common level. Below them 

 there are areas h, V , and h'' which have a nearly common elevation, 

 below which are the sharp valleys d, d' , and d" . The points a, a', and a" 

 represent the cross-sections 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 



1 Davis. The Seine, the Mouse and the Moselle. Nat'l Geog. Mag., Vol. VIT, pp. 

 181-202. and 228-238. An article which throws much hght on the behavior of rivers. 



