THE WORK OF RUNNING WATER. 



159 



justments by which valleys come to be located on antichnes are some- 

 what as follows : ^ Fig. 148 represents two doubly-plunging antichnes 

 with a syncline between, the relative elevations being shown by con- 

 tour lines. At the outset, the drainage of such a region must have 

 followed the structural valley, and its initial course, consequent on 

 the slope, must have been down the axial trough. Drainage from the 



Figs. 148-51. — Diagrams to illustrate the shifting of rivers from a synclinal to an 

 anticlinal position. (After Davis.) 



antichnes into the synclines would have promptly developed valleys, 

 and the valleys would soon have acquired streams. 



The anticlines and synclines under consideration are assumed 

 to have a thick hard layer at the surface, and softer beds below. This 

 is shown in the cross-section introduced in the figure, the upper hard 

 stratum (m) being indicated by the dots, while the softer one (n) 

 is white. The hne oo represents base-level, which is below the h,ard 

 layer both in the syncline and antichne, but much farther below in 

 the latter position than in the former. Because of their higher gradients, 

 and because of the greater fracturing to which the region they drain 



* This process of adjustment has been well described by Davis in The Rivers and 

 VaUeys of Pennsylvania, Natl. Geog. Mag., Vol. I, p. 211 et seq. 



