LAND FORMS, THEIR DEFORMATION AND FORMATION 



63 



It can elevate a circular, oval, or elongate dome 

 mountain or depress an isolated basin. Also, it can 

 end in individual, long, folded mountains (anticlines) 

 or troughs (synclines), or in alternating anticlines and 

 svnclines. forming a system of folded mountains. 

 Finally, there is folding in which the dominant move- 

 ment compresses an arch of folds (i.e., small folds 

 superimposed upon a single, large, upward fold or 

 anticlinorium) or in a basin of folds (a synclinorium). 



Anticlines and synclines at their ends merge into 

 the surrounding flat terrain. This condition of merg- 

 ing in both cases involves abrupt changes in the dip of 

 strata from the fold to the flatland and tlie strike of 

 surface rocks assumes a U-pattern. Where such 

 anticlines and synclines come together and are eroded, 

 the resulting surface topography is a pattern of zig- 

 zag ridges (Figure 4.23). 



Jointing. Joints are caused by various forces, es- 

 pecially tension, compression, torsion, and shear, 

 acting upon any kind of rock. For this reason, joints 

 are a common feature of many landscapes. 



Perhaps the rocks most frequently jointed are the 

 igneous types. Even cooling of igneous rocks leads 

 to stresses that cause jointing. However, most other 

 rocks also joint from the application of any or all 

 forces of diastrophism and from the creation of like 

 forces by weathering. Therefore, joints are features 

 of any rocks subjected to irregular forces, especially 

 the forces produced by cooling, weathering, and 

 diastrophism. 



Faulting. Faulting consists of very minor to ex- 

 tensive displacements of the earth's crust (Figure 

 4.24). The abrupt movements are felt as moderate to 

 severe earthquakes. Pronounced elevation occurred 

 in the 50-foot uplift of a portion of the Alaskan coast 

 in 1899, and great lateral movement in the up to 

 21 -foot ofTsets along part of the San Andreas Fault 

 during the San Francisco earthquake in 1 906. 



Faults are zones of weakness in the earth's crust. 

 They often are linear features, called /aw// linei, along 

 which rock strata move as a result of diastrophism. 

 (Terms in the discussion that follows refer to Figure 

 4.24. Only those in italics are generally used.) The 

 plane of movement is called a fault plane, or fault 

 surface; and because individual planes are rarely per- 

 pendicular to the surface, a hanging wall and a foot 

 wall are defined. In relation to the plane of the fault 

 into the crust, the hanging wall lies above and the foot 

 wall below the fault plane. When the hanging wall 

 is elevated, it forms an overhanging clifT; when the 



Figure 4.24 Diastrophism: faulting and land forms due to faulting. 



foot wall is uplifted, it never forms an overhanging 

 clifF. Either of the cliflfs so elevated is called a fault 

 scarp. 



The various kinds of faults are associated with 

 vertical and horizontal displacements of the strata on 

 either side of the fault line. Vertical movements 

 produce normal and reverse faults. Lateral move- 

 ments produce horizontal faults. A normal fault is 

 one in which the fault scarp is formed by the foot wall 

 and the hanging wall appears to have moved below 

 the- foot wall. A reverse, or thrust, fault is one in 



