194 



W. M. DAVIS 



^NOMENCLATURE OF SURFACE FORMS 



esses that have worked on it should be announced (except that if it has 

 been worked on only by the ordinary or "normal" process of weather and 

 water this may be understood without explicit announcement), and the 

 stage reached by the action of process on structure should be indicated. 

 Although the principle here involved is simple in essence and of evident 

 value, it is very commonly neglected. An explorer may say that a coastal 

 district has suffered submergence, and yet fail to tell what the form of 

 the district was before submergence ; thus he leaves his readers in the 

 dark instead of enlightening them as to the present coastal forms. An- 

 other explorer may say that a district is traversed by a recent fault, and 

 yet fail to tell what the form of the district was before the fault took 

 place ; his readers are then left too little informed and are unable to con- 

 ceive the landscape that the explorer has traversed. 



Figure 3. — Young Faults in a Mountainous District 



If the surface of the lower mass is plain and gently inclined toward 

 the fault, the longitudinal depression thus produced may be called a fault 

 trough ; a stream consequent on the faulting will then soon erode a fault 

 valley. But if the faulted surface is hilly or mountainous, as in figures 

 2 and 3, no continuous fault trough will be formed by the faulting, and 

 the irregular relief along the scarp base will cause any streams that may 

 flow for a short distance along the fault line to depart from it sooner or 

 later and perhaps to leave it altogether. 



If the surface of the uplifted block is of small relief and is inclined 

 by the faulting, new consequent streams may be formed on it ; but this 

 can only be the case when the prefaulting slopes are weak and the fault- 

 ing is rapid enough to extinguish the preexisting streams. Xew conse- 

 quent streams might be expected on the inclined surface of the upfaulted 

 lava plain, figure 1 B, but not on the rolling or mountainous surface of 

 figures 2 and 3. In these two cases any new slant given to the upfaulted 

 masses will, instead of developing new consequent streams, merely favor 

 the headward erosion of those whose slope is increased and discourage it 





