276 PHYSICAL GEOLOGY 



edges of the smaller and lighter continental plateaus. The 

 matter is, however, an unsolved problem. 



The leading types of mountains are noted below. 



Faulted (block) mountains. Figure 288 shows several 

 mountain ridges, and suggests their origin. A plateau or 

 plain was divided by fissures into a series of great blocks, 

 which were displaced by faulting, the relatively elevated 

 edges forming mountain ridges. The mountain ridges may 

 owe their relief to their having been uplifted, or to the sink- 

 ing of the lower land, or to both. Such mountains are called 

 faulted or block mountains. The mountains shown are still 

 young, for their crests are without notches, and streams have 

 not carved valleys in their even slopes; little talus has ac- 

 cumulated at the base of the great fault scarps. The smooth 



FIG. 288. Diagram of block mountains. 



slopes show also that the surface from which the mountains 

 were formed was essentially level, and therefore topographic- 

 ally either young or old. The ridges diminish gradually in 

 elevation from the points where the vertical displacement of 

 the beds was greatest, and die out where the faults end. The 

 beds dip away from the fault scarps. 



As time passes, streams will dissect the now smooth 

 slopes. Later, the larger valley bottoms and finally even the 

 strongest inter-valley spurs will be worn down to base 

 level, unless the mountains are maintained by further dias- 

 trophism. Buried beneath the waste-mantled surface of 

 the resulting plain, the fault planes and tilted beds will 

 record the former existence of the mountains. 



Young block mountains 1000 or 1200 feet high and 10 

 to 30 or 40 miles long occur in southern Oregon and the 

 adjacent states. In Nevada there are block mountains now 



