LAND FORMS, THEIR DEFORMATION AND FORMATION 



65 



Folding or uplift may take place after or before 

 peneplain formation and may cause rejuvenation. 

 If rejuvenation involves new folding, arches and 

 troughs form, but, because the land was eroded prior 

 to new folding, the rock strata no longer follow the 

 folds. Erosion leads to this confusing picture because 

 differential removal of the once continuous rock layers 

 causes new folding to result in complexly overturned 

 and contorted layers. On the other hand, if reju- 

 venation involves uplift, the landscape still empha- 

 sizes the old pattern of erosion, but is modified by 

 the differential hardness of the various strata. There- 

 fore, many cases of rejuvenation produce land forms 

 that cannot be distinguished from complex moun- 

 tains. In fact, rejuvenation is sufficient to character- 

 ize the new elevations as complex mountains. 



Block Mouniains. Initially, block mountains pre- 

 sent a steep cliff that rises above the fault line and 

 a gentle back slope that retreats from the summit 

 and away from the fault line. During the youth of 

 block mountains, erosion slightly modifies the cliff 

 and back slope, but the original outline of the block 

 is not greatly changed (Figure 4.26). Maturity occurs 

 when both cliff and back slope are dissected by V- 

 shaped valleys. The streams that excavate these 

 valleys also form alluvial fans in front of the cliff, and 

 those streams that drain the back slope may form a 

 lake, especially in situations between the back slope 

 of one block mountain and the cliff of another. In 

 spite of this erosion, mature block mountains still 

 maintain their general outline and the position of 

 the fault line is not obliterated. In the early stages 

 of old age, the once continuous block mountain be- 

 comes a row of rolling hills. This is the final picture 

 before complete peneplaination. 



Grabens and Horsts. Grabens and horsts are fea- 

 tures of complexly faulted areas, so both tend to be 

 complex rectangular blocks displaced between paral- 

 lel fault lines. For example, grabens and horsts may 

 occur among a confusion of fault lines or be irregular 

 because fault lines are not linear or parallel to one 

 another. 



In many respects, horsts are complex block moun- 

 tains. For this reason, the life cycle of horsts and 

 blocks follow the same general pattern of initial clar- 

 ity of features, mature modification but with the same 

 general outline, and the old peneplain. The erosion 

 of horsts and blocks usually produces residual forms 

 that are characteristic of these elevations. The forms 

 are trapezoidal and triangular facets (Figure 4.26) 



YOUTH 



MATURITY 



mM^m 



OLD AGE 





Figure 4.26 The life cycle of fault block mountains. 



upon the cliffs, and each facet is produced between 

 adjacent streams coursing down the cliff. The 

 streams, by progressive erosion, leave, first, a trape- 

 zoidal, residual land form and, later, a triangular one. 

 These residues are different from those of folded 

 mountains, and the two fault mountain types often 

 can be distinguished by observation of both sides of 

 a fault mountain. The block has only a single fault 

 line, hence one faceted cliff; the horst has both sides 

 faceted. 



Complex Mountains. Complex mountains can form 

 from any kind of rock. One might call them combina- 

 tion mountains because, by definition, each consists of 

 any single mountain or group of mountains elevated 

 as a result of more than one major process. For ex- 

 ample, the processes of folding, faulting, and vulcan- 



