STRUCTURAL GEOLOGY, 



515 



many of the basin ranges of Utah, Nevada, etc., are found though much 

 modified by erosion (Fig. 419). 



Faults sometimes arise from over-intense folding (Fig. 420). A 

 deformation which at one point results merely in a bending of the beds, 



Fig. 419. — A fault-scarp; the triangular faces rising abruptly above the plain at the ends 



of the spurs. (Davis.) 



may occasion a fault at another. Faults may pass into folds either 

 vertically (Figs, 421 and 422) or horizontally (Fig. 423). In such cases, 

 thickening and thinning, and stretching and shortening of the beds is 

 often involved (see Figs. 421 and 422). Faults are often due to the 



Fig. 420. — Diagrams showing relations of faults and folds. 



greater settling of the beds on one side of a fissure than on the other, 

 without special disposition to fold. 



The rock on either side of a fault-plane is oft^en smoothed as the 



