EROSION OF ANTICLINES 



845 



results in the formation of a peneplain across which rivers wander 

 with little or no regard to the underlying structure. As already 

 outlined, the mapping of the outcrops of the strata on such a sur- 

 face would form a series of color bands parallel for a long dis- 

 tance, but uniting when the pitch of the anticline carried the strata 

 below the erosion surface. The central color band of the eroded 

 anticline would, of course, represent the oldest formation, while on 

 either side of this would be bands corresponding on opposite sides 

 and representing the successively younger formations from the cen- 

 ter outward (Fig. 190, a, b, p. 806; see also Figs. 217, 218.) 



Elevation of the peneplain and renewal of the erosive processes 

 will result in the revival of the topography, since the harder layers 



Fig. 217. Anticlinal fold with pitch- 

 ing axis, truncated across the 

 top. The harder beds form 

 monocHnal ridges ; the valleys 

 were cut on soft strata. (After 

 Willis.) 



Fig. 218. Synclinal fold with pitch- 

 ing axis eroded. The harder 

 beds form monoclinal ridges. 

 (After Willis.) 



will again be carved into relief by the concentration of the erosive 

 processes on the softer layers. The Appalachians furnish an in- 

 structive example of such a revived topography- — they are at pres- 

 ent in the second if not in a later cycle of erosion.* This fact is 

 well brought out by the numerous entrenched transverse streams 

 which 'cross the monoclines more or less at right angles. These 

 streams, of which the Susquehanna is a good example, came into 

 existence on the tilted peneplain, and their constant downward cut- 

 ting made possible the openings of the longitudinal valleys on the 

 softer strata, by the tributary streams. 



* This is graphically expressed by the formula n*^'^ + i cycle suggested by 

 Davis for such cases, where it is known that the region is not in the first cycle 

 of erosion, but where it is impossible to say how many cycles have been completed. 

 Thus n may stand for one or for more than one. 



