MOUNTAINS AND PLATEAUS 



3 6 3 



Fig. 353. — The slopes of the sides of the mountains are 

 determined largely by the dip of the rock forming them. 



(Fig. 344, p. 357), but in the Appalachians anticlinal valleys are 

 perhaps more common than synclinal ones. This lack of coincidence 

 is due to the fact that when strata are folded the crests of the anti- 

 clines are stretched and consequently weakened, while the synclines 

 are correspondingly compressed and strengthened. Moreover, when 

 the land surface 

 emerges from the sea, 

 the crests of the anti- 

 clines are first attacked 

 by erosion, and their 

 strata may be worn 

 through while the 

 synclines are receiving 

 sediment and are thus 

 being protected. It is conceivable that a syncline may never have 

 contained a stream, since before its surface was elevated above the 

 sea, valleys had already been established in the anticlines. 



If we imagine a number of folds, the anticlines and synclines of 

 which are exposed to erosion at the same time, it will readily be seen 

 that erosion will develop valleys in the anticlines, as it is now doing 



in the Jura Mountains. In in- 

 tensely folded mountains where 

 overturned folds occur, as is so 

 frequently seen in the Alps, the 

 variable character of the strata 

 determines the cliffs and escarp- 

 ments of the mountains (Figs. 

 353, 354). The gentle slopes of 

 mountains of this structure are 

 most likely to be found along 

 the dip of the strata, the cliffs 

 along the strike. 



The effect of a resistant stratum 

 in determining the topography 

 of a region is well shown in por- 

 tions of the Appalachian Mountains, where a single quartzite stratum 

 forms long mountain ridges wherever it outcrops at the surface. 

 The canoe valleys of the Appalachians and other folded mountains 

 are formed by the erosion of the strata of a plunging anticline 

 (Figs. 244, 245, p. 255). 



MILE 



Fig. 354. — Section showing the effect 

 of the dip of a resistant stratum upon 

 the topography of a mountain. 



