THE WORK OF RUNNING WATER. 



155 



is likely to begin, and the watercourses, and later the valley plains, 

 come to be located on the outcrops of the less resistant layers, while the 

 outcrops of the harder beds become ridges. 



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

 layer, the arch of which has been cut off, would, after erosion, out- 

 crop on both sides of the axis. When the topography was mature 

 these outcrops would constitute parallel ridges, or parallel Hues of 

 hills; when the region had been base-leveled, the outcrops would be in 

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



zf -T-f -r ^ i : > : ^ ^_i< 



i .VTIi^ 



7TT 



^=^=^ 



i--. i '-- J Ji.-Xi? ^ ri 



e£i 



Si 



Fig. 142. — A natural bridge in process of development; longitudinal section at 

 the left; transverse section, looking toward e, at the right. 



of truncation, the farther aparf would the outcrops be in the anticline, 



and the nearer together in the syncline (compare ah and cd, Fig. 133). 



If, on the other hand, the axis of the antichne or synchne to be 



eroded was not horizontal, that is, if it phinged, the topographic result 



? 7 



x^^ 



r^ 



T~~r: } {ZIL 



i± ZiIIIiZ zz=av 



^ 



T— [ 



r~r~T 



y" 



i_i 



dx 



.^ 



Fig. 143. — The same as Fig. 142 at a later stage of development. 



would be somewhat different. Suppose a plunging antichne to be 

 truncated at base-level. If either end of the fold plunged below the 

 plane of truncation, the outcrops of a given layer on opposite sides of 

 the axis would converge in the cUrection of plunge, and come together 

 at the end. At a stage of erosion antedating planation (say late ma- 

 tmity) there would have been a ridge, or a succession of hills, in the 

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

 shaped valley witliin. If two hard layers were involved, instead of 

 one, there would be two encircling ridges, with a curved valley between 

 them, and a canoe-shaped valley "s^ithin the innermost (Fig. 146). If 



