THE WORK OF STREAMS 



153 



extend along valleys and are above the reach of ordinary 

 flood waters (Fig. 143). Many terraces are not due to uplift 

 (p. 183). A rejuvenated 



B 



C 



river responds to the ele- 

 vation first in its lower 

 course, and the new val- 

 ley formed there extends 

 itself upstream by head- 

 ward erosion. Until this 

 extension is completed, 

 the upper river, as yet 

 unaffected by the uplift, 

 flows on the relatively 

 broad and gently slop- 

 ing bottom of the old 

 valley (A to B, Figs. 144 

 and 145), while the lower 

 river flows in the narrow, 

 steep-floored new valley 

 (B to C, Figs. 144 and 

 145). Such rivers are 

 said to have interrupted 



FIG. 144. Diagram of an interrupted 

 profile. 



FIG. 145. Diagram of a rejuvenated area. 

 In what stage of erosion was the re- 

 gion before it was rejuvenated ? When 

 did the two lower tributary streams be- 

 gin to cut new valleys ? What are all 

 the things which may have helped to de- 

 termine the fact that the new valley of 

 the tributary stream "D" is longer than 

 that of " E " ? What changes will occur in 

 the character of the topography in 

 the future ? 



profiles. 



Figure 146 shows in principle the topography and structure 

 of a portion of the Appalachian Mountains. Water-laid beds 

 (What indicates this origin?) were folded into a series of 





FIG. 146. Diagram of folded mountains in the youthful stage of the third 

 cycle of erosion. (Modified after Salisbury.) 



anticlines and synclines, the former constituting parallel 

 mountain ridges, the latter structural valleys. The area was 

 then worn down to base level, a broad, nearly level plain re- 

 placing the mountain topography. The former existence of 



