202 W. M. DAVIS NOMENCLATURE OF SURFACE FORMS 



to become rigid and controlling; it is simply a means to an end and must 

 be modified and enlarged as experience demands. 



Obsequent Eavine Heads 



In the special cases of small pre-faulting relief and a tilting of the 

 uplifted block surface away from the fault, the initial crest of the fault 

 scarp will form a new initial divide, as in figure 1 B, between the short, 

 steep face of the scarp and the longer back slope of the tilted block, 

 except. where antecedent streams maintain their courses across the fault 

 line. But the divide must soon shift away from its initial position by 

 reason of the weathering and retreat of the fault face ; the initial divide 

 thus becomes a consequent divide, figure 1 C. The rapid headward ero- 

 sion of a ravine in a retreating consequent scarp face of this kind will 

 lengthen the ravine stream and correspondingly shorten the opposing 

 back-slope stream; hence the headward part of such a retrogressive ra- 

 vine, figure 1 D, is then of obsequent origin. Whether any observer cares 

 to go as far as this in applying an explanatory terminology to forms thus 

 developed is not known; but ravines of the kind here indicated are de- 

 scribed in Abyssinia by Eathjens ('11, 71). 



Subsequent fault-line Valleys 



If an example be now considered in which regional uplift accompanies 

 faulting, then the whole surface may be more deeply dissected by its 

 streams and more worn down by weathering and washing than was pos- 

 sible in the cases thus far considered. In such an example the alluvium 

 that at first tends to accumulate rapidly along the scarp base will be re- 

 moved as the scarp slowly retreats, and the fault line may then be more 

 easily traced, at least by difference of soils even if there is no difference 

 of form. If the rocks on the two sides of the fault line are of similar 

 resistance and the fault is a smooth and simple fissure, no valley need be 

 developed along it even when it is thus laid bare by the removal of the 

 alluvium that for a time covered it ; but under other conditions two new 

 features may be developed as follows : 



First, let it be supposed that the fault is a complex fissure, along which 

 the rocks are sheared and shattered ; then a number of subsequent valleys 

 will be gradually excavated along the sheared zone by the headward ero- 

 sion of subsequent branches of transverse streams; in time an almost 

 continuous depression may thus come to follow the fault line, and much 

 of its length may by capture come to be drained by the subsequent 

 branches of a few large transverse streams. These valleys are not due to 



