206 W. M. DAVIS NOMENCLATURE OF SURFACE FORMS 



words, the weak parts grow old rapidly, while the resistant parts pass 

 through their cycle of development slowly; hence in a carefully stated 

 description terms indicative of age must be separately applied to areas 

 of different resistance. This is the converse of another principle, namely, 

 that strong agents of erosion, like large rivers and strong sea waves, run 

 through their cycle of form development rapidly, while weak agents, like 

 weathering, advance slowly; hence the valleys of large rivers will reach 

 late maturity or old age while the surface of neighboring internuves are 

 still young or submature. 



If it happens that the rocks on one side of the fault are resistant and 

 on the other side weak, a scarp will be developed along the fault line on 

 the side where the resistant rocks retain young or submature forms, and 

 this scarp will face the worn-down area, where the weak rocks are al- 

 ready subdued or old. The aspect of such a scarp is evidently inde- 

 pendent of whether the resistant rocks are in the block of relative uplift 

 or depression. If the newly developed scarp faces the originally uplifted 

 mass, as in figure 1 G-, it may be called an obsequent fault-line scarp, as 

 already defined ; if it faces the originally depressed mass it may be called 

 a resequent fault-line scarp, because it again faces in the direction of 

 the original scarp. Both kinds of fault-line scarps are drawn in figure 

 2 G. In the case of a faulted monocline, as drawn in figure 4 B, obse- 

 quent and resequent ridge-end scarps will alternate, right and left, along 

 the fault line. 



The distinction between fault scarps and fault-line scarps, in the man- 

 ner here proposed, is not simply an academic question of systematic 

 nomenclature. If a fault-line scarp is taken for a fault scarp, various 

 erroneous inferences will follow; for example, the date of faulting will 

 in such a case be inferred to be only long enough ago to permit the ob- 

 served dissection of the scarp; while if properly interpreted the date of 

 faulting would be at any period prior to the erosion of the preceding 

 cycle, and hence possibly much more ancient than at first supposed. Fur- 

 ther, the height of a young or submature fault scarp is a fair measure of 

 the faulting, while the height of a fault-line scarp has no close relation 

 to the faulting; it is dependent simply on the depth to which renewed 

 erosion has brought about a different altitude on the two sides of the 

 fault line; hence if a fault-line scarp is mistaken for a fault scarp, the 

 height of the scarp .may be mistaken for the vertical separation produced 

 by faulting, thus giving incorrect measure of the original separation. 

 In an early stage of the new cycle the fault-line scarp will be highest 

 near the incised valleys of transverse streams, and it may remain for a 

 time undeveloped on the internuves; a variation of scarp height thus 



