166 GEOLOGY, 



might not be apparent. In this case, the main streams might be flow- 

 ing in new valleys, d, d\ etc., while the heads of their tributaries are 

 still flowing in the older valleys of the preceding cycle (compare Fig. 

 154 and Fig. 1, PI. XIII). 



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 (p. 76), throughout their whole extent from the 

 Hudson River to Alabama. The peneplain level is indicated by the 

 level crests of the Appalachian ridges, shown in cross profile by the 

 high points of Fig. 157. The system was then uplifted, 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 h, 5', 

 and h" of Fig. 155. The plains were located, for the most part, where 

 the less resistant strata come to the surface. Above them rose even- 

 crested ridges, the outcrops of the resistant layers, which had been 

 isolated by the degradation of the softer beds between. They con- 

 stitute the present mountain ridges (the high points of Fig. 157). The 



Fig. 157.^Cross-section of a portion of the Appalachian Mountains to ilhistrate the 

 phenomena of erosion cycles. (After Rogers.) 



evenness of their crests, testifying to the completeness of the first 

 peneplanation, is shown in Fig. 158, which represents, diagrammati- 

 cally, a longitudinal profile of an Appalachian Mountain ridge. The 

 evenness of the crest is interrupted by (1) notches (5, c, etc.. Fig. 158) 

 cut by the streams in later cycles, and (2) by occasional elevations above 

 the common level (monadnocks, a, a' , Fig. 158). The monadnocks are 

 generally rather inconspicuous, but there is a notable group of them 

 in North CaroHna and Tennessee. Mount Mitchell and Roane Moim- 

 tain are examples. When long distances are considered, the ridge 



Fig. 158. — A diagrammatic longitudinal profile of an Appalachian Mountain ridge. 



crests depart somewhat from horizontality. This is beheved to be 

 diie, in part at least, to deformations of the old peneplain during 

 the uplift which inaugurated the second cycle of erosion. 



