ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 59 



the fault is called its throw. Though not illustrated by the model, 

 horizontal displacements may likewise occur, and these will be 

 more fully discussed when the subject of earthquakes is considered 

 in the following chapter. An actual example of blocks displaced 

 by vertical adjustment is represented in Fig. 41, a simple type of 

 faulting which has taken place in rocks but slightly disturbed from 

 their original attitude, but intersected by a relatively simple sys- 

 tem of master joints. In those regions where the beds have been 

 folded and perhaps overthrust before their elevation into the zone 

 of fracture, and which are further intersected by disorderly fissure 

 planes, the results are far more complex. In such cases the 

 planes of individual displacement may not be vertical, though 

 they are generally steeper than 45. For their description it is 

 necessary to make use of addi- 

 tional technical terms (Fig. 42). 

 The inclination of a sloping fault 

 plane measured against the ver- 

 tical is called the hade of the fault. 

 The total displacement is measured 

 along the plane of the fault from a 

 point upon one limb to the point 



from which it Was Separated in FIG. 42. A fault in previously dis- 



the other. The additional terms 

 are made sufficiently clear by the 

 diagram. 



Methods of detecting faults. The first effect of a fault is usually 

 to produce a crack at the surface of the earth ; and, provided there 

 is a vertical displacement or throw, an escarpment which rises 

 upon the upthrown side of the fault. In general it may be said 

 that escarpments which appear at the earth's surface as plane 

 surfaces probably represent planes of fracture, though not neces- 

 sarily planes of faulting. In many cases the actual displacements 

 lie buried under loose rock debris near to and paralleling the es- 

 carpment, and in some cases as a result of the erosional processes 

 working upon alternately hard and soft layers of rock, the escarp- 

 ment may later appear upon the downthrown side or limb of the 

 fault (Fig. 43). As an illustration of a fault escarpment, the 

 fagade of El Capitan and many other rock faces of the Yosemite 

 valley may be instanced. 



turbed strata. AB, displacement ; 

 AC, throw ; B&, stratigraphic throw ; 

 BC, heave ; angle CAB, hade. 



