248 DISLOCATIONS OF STRATA 



pound faults, and all hading in the same direction, are called step 

 faults. If two parallel dislocations hade toward each other, they 

 form a trough fault and include a wedge- shaped fault block between 

 them, which is on the downthrow side of both dislocations. 



However long it may be, a fault sooner or later dies away, the 

 throw constantly diminishing toward the ends, until it vanishes. 

 This implies that the fault is due to the bending of the beds up- 

 ward or downward along the plane of dislocation ; only three 

 intersecting or two curved faults can actually isolate a fault block. 

 Intersecting or cross faults may be of very different dates, and 

 then the more ancient ones can be determined by the fact that 

 they are themselves dislocated. 



It is comparatively seldom that the upthrow side of a fault is 

 left standing as a line of cliffs ; when such is the case, the cliffs 



"^^ S^^JM^m^ 



Fig. 104. — Abert Lake, Oregon. The line of cliffs is a fault scarp. ^ Russell.) 



form a fault scarp, many of which may be found in the more arid 

 districts of the West, where atmospheric erosion has been slow. In 

 the great majority of instances, the upthrow side suffers more rapid 

 denudation than the downthrow, and the two are weathered down 

 to approximately the same level, or the same general slope (see 

 Figs. 102, 105). Under these circumstances, faults are rarely visi- 

 ble on the surface, and their presence must be inferred indirectly 

 from their effects upon the outcrops of the strata involved. These 



