9 6 



WORK OF RUNNING WATER 



If the axis of an eroded anticline were horizontal, a given hard 

 layer, the arch of which has been cut off, would outcrop on both 

 sides of the axis. When the topography has become mature, these 

 outcrops will constitute parallel ridges, or parallel lines of hills. 

 When the region had been base-leveled, the outcrop will be in 

 parallel belts, though no longer ridges or hills. The lower the 

 plane of truncation, the farther apart the outcrops will be in the 



Fig. 86. Diagram showing the outcrops of hard layers (shaded) on the flanks 

 of truncated folds; cd, present surface; ab, an earlier erosion surface. 



anticline, and the nearer together in the syncline (compare outcrop 

 of H, along ab and cd, Fig. 86). 



If, on the other hand, the axis of the anticline or syncline is not 

 horizontal, that is, if it plunges (dips), the topographic result will 

 be different. In this case the outcrops of a given layer on opposite 

 sides of an anticline will converge in the direction of plunge, and 

 come together. At a stage of erosion antedating planation (say 

 late maturity) there will be a ridge or a succession of hills, in the 

 position corresponding to the outcrop of a hard layer, with a canoe- 

 shaped valley within. If two hard layers are involved, instead 

 of one, there will be two encircling ridges, with a curved valley 

 between them, and a canoe-shaped valley within the innermost 

 (Fig. 85). A succession of plunging anticlines and synclines might 

 give rise to a very complex series of ridges and valleys. Illustra- 

 tions of the above phenomena are found at various points in the 

 Appalachian Mountains. 1 



Fig. 87. Cross-section of a portion of the Appalachian Mountains to illustrate 

 the relations of mountain ridges to anticlines and synclines, and the phenomena of 

 erosion cycles. (Rogers.) 



1 Willis, The Northern Appalachians, in Physiography of the United States. 



