64 



GEOMORPHOLOGY: 



which the fault scarp is formed by the hanging wall, 

 so the foot wall seems to have moved below the hang- 

 ing wall. Also, when not modified by erosion, only 

 a reverse fault produces an overhanging fault scarp. 

 Normal and reverse faults display one of a pair of 

 contrasting movements. Normal faults are extensions 

 of the earth's crust, and reverse faults are contrac- 

 tions. Horizontal, or lateral, displacement creates 

 a horizontal or lateral, fault in which there is no 

 vertical change in strata relationships on either side 

 of the fault line. 



Placement of fault lines determines the appearance 

 of faulted landscapes. Adjacent parallel fault lines 

 may create a series of steps, called a step fault. When 

 one fault line runs into another, the interior segment 

 of land is a fault splinter. Multiple faults can be 

 normal or reverse, the multiple normal faults being 

 called step faults and the multiple reverse faults, 

 simply "multiple reverse faults." Long fault lines 

 often are sites of volcanoes. 



Pure fault mountains are called block mountains 

 and horsts. A block mountain has a steep cliff on one 

 side and a slight slope on the other. The steep clifT 

 is located between the block mountain summit and 

 the fault line; and in the case of a normal fault, the 

 cliff may represent the unaltered fault scarp. A horsl 

 is a mountain formed between two fault lines that dip 

 toward or away from one another, but adjacent faults 

 also allow depression of the terrain. In the case of 

 depression, the valley formed is called a graben. 



Grabens and horsts can be simple structures as 

 described. They can also be complex. Complexity 

 is the result of many intersecting fault lines and, for 

 this reason, normally creates an irregular pattern of 

 elevations and depressions, including many fault 

 steps and fault splinters. 



DIASTROPHISM MOUNTAIN TYPES 



Folded Mounfains. If formed rapidly and with 

 little erosion, folded mountains consist of sedimentary 

 beds that are bent into isolated domes or into a paral- 

 lel series of arches and troughs. This landscape is 

 reshaped by the forces of erosion (Figure 4.25); in 

 most cases streams are the greatest erosion factor. 

 Further stages of the life cycle are as presented (but 

 oversimplified) in the discussion of erosion. The 

 simplified picture occurs only if valley streams are 

 the sole efTective agent of erosion, so that youth con- 

 sists of V-shaped valleys and sharp ridges. However, 



Figure 4.25 The life cycle of folded mountains. 



in most cases both isolated folds and series of folds 

 follow the dome mountain type of erosion pattern; 

 main streams drain the folds, proceed to the summit, 

 cut into the summit, and eventually give rise to tribu- 

 tary streams running the length of the summit. 

 Therefore, maturity features reduced elevations that 

 are characterized by hogbacks. In old age, the land- 

 scape is a peneplain. 



