154 PHYSICAL GEOLOGY 



this base-level plain is indicated by the fact that the present 

 ridges have even crests and would rise to a common level but 

 for subsequent warping of the region. At any stage preced- 

 ing extreme old age, the tops of individual ridges would not 

 have been even, and different ridges would have stood at 

 different levels (Why?). Next, the plain was elevated and 

 warped slightly (A A ), without further folding of the beds, 

 thus beginning the second recorded cycle of erosion. The 

 rejuvenated streams, together with the new tributaries which 

 worked back from them, now opened broad valleys on the 

 weaker rocks at the level B B, above which the stronger 

 beds stood as parallel ridges. Finally, the second cycle was 



FIG. 147. The even sky line to the left is the nearly level surface of a pene- 

 plain, which bevels across sedimentary and igneous rocks. To the right 

 it cuts tilted Paleozoic beds ; to the left pre-Cambrian granite. Since the 

 formation of the peneplain the region has been elevated and eroded. 

 Western Wyoming. (Baker.) 



interrupted and the third one begun by the uplift which per- 

 mitted the streams to cut the new valleys at C. The work 

 of reducing the area to base level in the present cycle remains 

 largely to be done. The few new valleys of the larger 

 streams are narrow and steep-sided. The region is accordingly 

 in the youthful stage of the third recorded cycle of erosion. 

 Figure 147 shows a nearly horizontal sky line, and below 

 it, igneous rocks and tilted sedimentary beds. From what 

 has preceded, it will be understood readily that this nearly 

 level surface was a peneplain, now uplifted and dissected. 



