198 W. M. DAVIS NOMENCLATURE OF SURFACE FORMS 



when a stream valley is found to follow a fault line, it probably has some 

 other than a consequent origin. 



In examples of faults of moderate slip that traverse uniform or 

 massive structures at such a stage of the pre-faulting cycle as permit- 

 medium or strong relief, the post-faulting erosion may obliterate the 

 fault scarp long before the relief is subdued or reduced to small measure ; 

 but where rocks of different kinds are brought together, the fault will 

 long be marked by a change of form following the fault line long after 

 the pre-faulting surface is worn away. If this change of form is a slope, 

 more or less perfectly graded, descending to the relatively depressed side, 

 it may be called a resequent scarp : if it descends toward the relatively 

 uplifted side, as must be the case when weaker rocks occur on that side, 

 it may be called an obsequent scarp, these terms being further explained 

 below. If the fault has produced a belt of breccia, it may be followed 

 by a subsequent ravine, which will often have a peculiar appearance from 

 pitching obliquely down the slope that it dissects : this topic also will b3 

 further discussed below. 



Physiographic Evidexce of Faultixg 



Let a special case now be considered, in which the lower side of a 

 faulted mass suffers depression, while the upper mass suffers elevation. 

 As a result the waste washed down from the uplifted mass will tend to 

 accumulate heavily on the depressed mass : at first only near the base of 

 the fault scarp: later on. advancing farther and farther from it and, if 

 the relief of the depressed block be small, burying its surface more and 

 more completely. In such cases the evidence of faulting will not be 

 found in the relative displacement of corresponding structures on the 

 two sides of the fault line, for all outcrops on the lower side will be 

 buried, possibly for miles away. The existence of the fault at this early 

 stage is inferred simply from the form of the scarp by geologists and 

 geographers alike. As long as the scarp which limits the higher mass is 

 strong and little dissected, the existence of a fault would probably be ac- 

 cepted on this evidence by geologists as well as by physiographers: but 

 as dissection progresses and the continuity of the scarp is more maturely 

 interrupted, the evidence of faulting is less immediately apparent: the 

 occurrence of a fault may indeed then be doubted by geologists who give 

 little attention to the evidence presented by land forms and who depend 

 instead wholly on displaced structures: hence the physiographic features 

 which demonstrate faulting in a dissected fault scar]) that rises from n 

 waste-covered lowland deserve special attention. 



