104 WORK OF RUNNING WATER 



to consider areas of considerable size, and to eliminate the topo- 

 graphic effects of inequalities of hardness. 



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 (the Kittatinny peneplain) from the Hudson 

 River to Alabama. The old peneplain surface is indicated by 



Base Levsl 



Fig. 104. Diagram to illustrate cycles of erosion where the beds are horizontal. 



the level crests of the Appalachian ridges. The system was then 

 warped (not folded) up, 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 b, b', and b" of Fig. 103. 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 corresponding to the high 

 points of Fig. 103. The evenness of their crests testifies to the com- 

 pleteness of the first peneplanation. The evenness of the crests is, 

 however, interrupted (i) by notches cut by the streams in later 

 cycles, and (2) by occasional elevations (monadnocks) above the 

 common level. Most of the monadnocks are rather inconspic- 

 uous, but there is a notable group of them in North Carolina 

 and Tennessee, of which Mount Mitchell and Roan Mountain 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 up- 

 warp is indicated by the extent of the valley plains (Fig. 103) 

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

 oped on the weak rocks, narrow valleys (wafer-gaps) only were cut 

 in the resistant rocks which stood out as ridges. Similar valleys, 

 whether shallow or deep, from which drainage has been diverted, 

 are sometimes called wind-gaps. 



The second cycle of erosion, still incomplete, was interrupted by 



