60 



EARTH FEATURES AND THEIR MEANING 



FIQ. 43. Diagrams to show how 

 an escarpment originally on the 

 upthrown side of the fault may, 

 through erosion, appear upon the 

 downthrown side. 



When we have further studied the erosional processes at the 

 earth's surface, it will be appreciated that faults tend to quickly 



bury themselves from sight, where- 

 as fold structures will long remain 

 in evidence. Many faults will thus 

 be overlooked, and too great weight 

 is likely to be ascribed to the folds 

 in accounting for the existing atti- 

 tudes and positions of the rock 

 masses. Faults must therefore be 

 sought out if mistakes of interpreta- 

 tion are to be avoided. 



The most satisfactory evidence of 

 a fault is the disaovery of a rock bed 

 which may be easily identified, and 

 which is actually seen displaced on 

 a plane of fracture which intersects 

 it (Fig. 42, p. 59). When such an 

 easily recognizable layer is not to be 

 found, the plane of displacement 



may perhaps be discovered as a narrow zone composed of angular 

 fragments of the rock cemented together by minerals which form 

 out of solution , in water. Such a fractured rock zone which 

 follows a plane of faulting is 

 a fault breccia. If the fault 

 breccia, or vein rock, is much 

 stronger than the rock on 

 either side, it may eventually 

 stand in relief at the surface 

 like a dike or wall. At other 

 times the displacement pro- 

 duces little fracture of the 

 walls, but they slide over each 

 other in such a manner as to 

 yield either a smoothly cor- 

 rugated or an evenly polished 

 surface which is described as 



"Slickensides." It may be, FlG . 44.- A fault plane exhibiting "drag." 

 however, that during the move- The opening is artificial (after Scott) . 



