282 



MATERIALS AND THEIR ARRANGEMENT 



which the fault-plane departs from the vertical is the hade (bac, Fig. 

 271). The vertical displacement (ac) is the throw, and the hori- 

 zontal displacement (be) the heave. The displacement is the amount 

 of movement along the fault-plane, ab. The cliff above the edge 

 of the downthrow side is a fault-scarp. In many cases the scarp 

 has been destroyed by erosion; but a few fault-scarps of mountain- 

 ous heights are known, as along 

 some of the basin ranges of 

 Utah and Nevada. Most fault- 

 scarps which persist are much 

 modified by erosion. 



Faults involving vertical dis- 

 placement along joints are of 

 two general classes, normal (or 

 gravity] and reversed (or thrust}. 

 In the normal fault (Fig. 271) 

 the overhanging side is the 

 downthrow side, i. e., the down- 

 throw is on the side towards which the fault-plane declines. Nor- 

 mal faults, as a rule, indicate an extension of strata, this being 

 necessary to permit one of the dissevered blocks to settle. In the 

 thrust fault (Fig. 272), the overhanging beds appear to have moved 

 up the slope of the fault-plane, as though the displacement took 



Fig. 272. A thrust-fault. The dotted 

 lines at the left show the portion which 

 has been removed by erosion. The pres- 

 ent surface is shown by the line to the left 

 of a. 



Fig. 273. Diagrams showing relations of faults and folds. 



place under lateral pressure. This is clearly shown to be the case 

 where an overfold passes into a thrust fault. Another type of 

 thrust-fault is shown in Fig. 273. 



In thrust-faults, the heave may be great. The eastern face of 

 the Rocky Mountains near the boundary-line between the United 

 States and Canada has been pushed over the strata of the bordering 



