190 



PHYSIOGRAPHY 



the land are slightly deformed, and in many places their positions 

 have been greatly changed. The warping may be slight (Fig. 183), 

 or it may be so great that the arches become close folds (Fig. 184). 

 Warping and folding give rise to great topographic features; 

 but in most mountains of folded rock, the present topography is 



Fig. 186. Diagram, showing the position of the beds of rock in the Appa- 

 lachian Mountains. (Rogers.) 



the result of erosion rather than of the original folding, though the 

 folded structure has, in many cases, determined or influenced the 

 topography which has resulted from erosion (Fig. 186). 



Faulting. At many times and in many places portions of the 

 earth's surface have sunk or risen along a deep crack, or plane of 



Fig. 187. 

 Fig. 187. A normal fault. 



a 6=fault-scarp. 

 Fig. 188. Fault passing into a monoclinal fold. 



Fig. 188. 

 The overhanging side (to the left) has sunk; 



fracture, as shown by Fig. 187. The slipping along such a plane 

 of fracture is a fault. One type of fault is shown in Fig. 187, where 

 the overhanging side (at the left) has sunk, relative to the other. 

 Cliffs due to faulting are called fault-scarps (a b, Fig. 187). Fault- 

 scarps, like other steep slopes, are in time destroyed by erosion. 

 Some faults, however, were so recent that their scarps are still 

 distinct, as in the plateau and basin regions west of the Rocky 

 Mountains. Many of the more striking topographic features of 

 that region, including numerous mountain ranges, are the result 



