LAND WATERS STREAMS 173 



141. The plains were located, for the most part, where the less 

 resistant strata come to the surface. Above them rise even- 

 crested ridges, the outcrops of the resistant layers, isolated by the 

 degradation of the weaker beds between. It is the outcrops of 

 these layers which constitute many of the present mountain ridges 

 (the high points of Fig 141). The evenness of their crests testifies 

 to the completeness of the first peneplanation. The evenness of 

 the crests is, however, interrupted by (1) notches cut by the streams 

 in later cycles, and (2) by occasional elevations (monadnocks) 

 above the common level. The monadnocks are generally rather 



Fig. 142. Diagram to illustrate cycles of erosion where the beds are hori- 

 zontal. 



inconspicuous, but there is a notable group of them in North 

 Carolina and Tennessee, of which Mount Mitchell and Roan Moun- 

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

 crests depart somewhat from horizontality. This is believed to be 

 due, in part at least, to deformation of the old peneplain during 

 the uplift which inaugurated the second cycle of erosion. 



The extent to which the second cycle of erosion recorded in 

 the present topography had proceeded before its interruption by 

 upwarp, is indicated by the extent of the valley plains (Fig. 141) 

 below the mountain ridges. While these plains were being devel- 

 oped on the weak rocks, narrow valleys only were cut in the resistant 

 rocks which stood out as ridges. Deep narrows of this sort are 

 often called water-gaps. Similar valleys, whether shallow or deep, 

 from which drainage has been diverted, are sometimes called 

 wind-gaps, 



The second cycle of erosion, while still far from complete, was 

 interrupted by uplift (relative or absolute), and a new cycle in- 

 augurated. This event was so recent that the new (third) cycle 

 has not yet advanced far. 



Some of the features just described are illustrated by Fig. 118. 

 The even mountain crest in the background is the Kittatinny 



