SUBSEQUENT FAUTL-LINE VALLEYS 203 



the faulting, and hence should not be called fault valleys; they are the 

 work of headward erosion along the weak structures of the fault line, and 

 hence should be called subsequent fault-line valleys. 



Second, let it be supposed that the rocks that are brought together at 

 the base of the fault scarp are of strikingly different resistance, and let 

 those in the uplifted mass be the weaker, as might be the case in figure 1 , 

 if we suppose that the lava sheet which there formed the pre-faulting 

 surface lies on weak horizontal strata, such as the sands or clays of a 

 coastal plain or of an aggraded interior basin. As the fault scarp retreats, 

 the weak strata beneath the capping lava sheet will be exposed along a 

 belt, figure 1 E, between the fault-line boundary of the lava sheet in the 

 less uplifted mass and the base of the talus below the retreating lava cliff 

 in the more uplifted mass; a broad subsequent valley will be rapidly 

 excavated along this belt in the manner above indicated for subsequent 

 fault-line valleys. In all such cases the height of the retreating con- 

 sequent scarp above the valley on the uplifted side will be now measured 

 by the height of the fault plus the depth of the valley; and the lower 

 scarp opposite to it, retreating slowly in figure 1 E because the weak 

 strata are little or not at all exposed beneath the lava, will face the up- 

 lifted block. Thus two conditions, neither one very complicated, are 

 deduced which must lead to the development of valleys along fault lines ; 

 but the valleys are in both cases not consequent but subsequent valleys. 



Obsequent fault-line Scarps 



In figure 1, after the retreat of the heightened consequent fault scarp in 

 the uplifted block, accelerated by the sapping of the weak basal strata, has 

 gone so far as to transform the subsequent fault-line valley into a wide- 

 open lowland, the features of old age are soon locally developed because 

 of the weakness of the rocks on which the erosive processes there work; 

 the other scarp in the lower-lying block may still lie close to the fault 

 line because its retreat is slow. As this scarp is produced by erosion 

 along the fault line and not by the original faulting, and as it has an 

 aspect opposite to that of the initial fault scarp, it may be called an obse- 

 quent fault-line scarp. Other examples showing variations on this type 

 will be further considered below. 



Old Forms on faulted Structures 



In the later stages of a cycle introduced by a fault nil the forms of 

 relief will be subdued to mild expression, and eventually an old lowland 

 of erosion — a peneplain or plain, figures 1 F and 2 F — will stretch across 

 XV — Bull. Gbol. Soc. Am., Vol. 24, 1912 



