126 GEOLOGY 



Tributaries are almost sure to develop along each main \ alley 

 and their heads work back across the uplands between the main 



Fig. 91. A mountain slope thoroughly cut up by erosion of running water. 

 Looking east from Echo Rock, on Mount Wilson, California. (Ellerman. ) 



valleys, dissecting them into secondary ridges (Fig. 90). Tribu- 

 taries will develop on the tributaries, and these tertiary valleys 

 dissect the secondary ridges into ridges of a lower order. This 



Fig. 92. Appalachian mountain topography near Canton. North C.-imlina. 

 The foreground shows a plateau now well drained. (Keith, U. S. GeoL i 

 Surv.) 



