204 W. M. DAVIS NOMENCLATURE OF SURFACE FORMS 



the whole district. If subsequent streams were developed along the fault 

 line in the maturity of the cycle, they may now wander away from it in 

 unrestrained meanders. The fault will then have lost all its topographic 

 value, though it may still be visible in a difference of soils on its two 

 sides. Hence the scarp of a physiographically "old" fault has not only 

 retreated from the fault line, but has been topographically obliterated; 

 yet the fault, geologically considered, still remains unchanged. Its slip, 

 strike, hade, and other elements are not altered, although its physio- 

 graphic expression has passed from its initial stage through all its se- 

 quential stages in the cycle of erosion introduced by the faulting to its 

 ultimate disappearance. Evidently, then, the physiographic treatment 

 of faults must be very different from their geological treatment. 



During all the late stages of the cycle, after the pre-faulting forms 

 have been destroyed, there is no longer any need or indeed any possi- 

 bility of saying anything about them; they have vanished. Nor is the 

 slip of the fault or the height of the initial scarp at this time of physio- 

 graphic importance. Here and there some gentle fault-line slopes, main- 

 tained on resistant rocks, bordering or inclosing a broadly opened fault- 

 line depression, may survive ; but eventually only the location and direc- 

 tion of the fault line as a limit between unlike soils need be noted in i 

 physiographic description. 



Here we find a further application of the general principles of physio- 

 graphic description, to which attention has already been drawn. At the 

 beginning of a cycle introduced by faulting, the pre-faulting forms and 

 the fault forms must be described ; in the early and mature stages of the 

 cycle the post-faulting forms must be added; now, in the late stages of 

 the cycle, the two first elements may be passed in silence, for nothing is 

 known of them; the post-faulting forms everywhere prevail and these 

 alone need mention. The fault is topographically obliterated, and is of 

 value physiographically only in delimiting different soil areas, yet it is 

 geologically of the same value as when it was first produced. 



Transverse Faults in monoclinal Structures 



A case of special interest is furnished by monoclinal structures of alter- 

 nating strong and weak strata, which before faulting have been reduced 

 to rather small relief, so as to present a series of low, rectilinear, subse- 

 quent ridges and valleys. Assume that the monoclinal dip is 20° to the 

 northwest, and that the district is then broken by a transverse fault, with 

 vertical uplift on the northeast, as in figure 4 A. The initial position of 

 the ridges and valleys in the uplifted block will be changed only in alti- 





