INITIAL AND YOUNG FORMS ON FAULTED STRUCTURES 195 



in the others, as Campbell has pointed out in his discussion of "Drainage 

 Modifications and their Interpretation" ('96). 



The face of an initial scarp is rapidly changed into a young consequent 

 scarp by the loss of waste, which slips, creeps, and washes down to the 

 scarp base, there accumulating in landslides, talus slopes, and al hi via I 

 fans, which bury the fault line. Even if a stream is initially formed 

 along the fault line, as in special cases above indicated, it is liable soon 

 to be pushed away by the abundant down-wash of waste from the scarp; 

 hence if such a stream afterward cuts a valley, the valley is not likely to 

 follow closely along the fault line. 



If faulting is soon renewed, the scarp will be refreshed and its base; 

 kept close to the fault line, as in figures 1 ( 1 and 2 C, although its earlier 

 exposed upper slope will have retreated somewhat from its initial posi- 

 tion. The details shown in figure 1 (! are highly suggestive of a second 

 uplift, which took place some time after the first, for here the face of the 

 covering lava sheet is worn hack to a slope, while the face of the under- 

 lying weaker strata is steep. During the retreat of a scarp it must take 

 on details of form appropriate to its structure. Details of this kind may 

 be called subconsequent if they do not depart greatly from the pattern 

 of the initial scarp, or subsequent if they include altogether new features 

 developed by the action of erosion on inclined structures of strongly dif- 

 ferent resistance. These details are, however, not limited to retreating 

 fault scarps; they would be equally well developed on the two retreating 

 walls of a young river-cut valley. 



The essential Principle in the physiographic Description of 



Faults 



It thus appears that a physiographic description of a fault in an early 

 stage of the cycle that it has introduced should include an account of the 

 prefaulting surface and of the fanlt scarp, as before stated, and to these 

 two headings a third should now he added, under which the features de- 

 veloped after faulting — that is, the youthful features of the new cycle 

 are described in their systematic modification of the prefaulting features. 

 Here again we find application of the same general principle of explana- 

 tory description that was mentioned above, hut now extended so as to 

 include three terms, namely, the post-movement features as well as the 

 pre-movement features and those directly due to the movement itself. 



Simple as this extension of the principle is and manifest as is its value, 

 it is repeatedly overlooked. 'The explorer who describes a coastal district 



as having suffered submergence too commonly fails to tell what changes 



