354 



PHYSICAL GEOLOGY 



mountains in which gentle slopes are characteristic. The form of 

 mountains of this class depends upon the nature and arrangement of 

 the material (Fig. 340) out of which they were sculptured, and to 

 some extent upon the climate. The Catskills owe their gentle slopes 

 to the fact that the rocks of which they are composed do not differ 

 greatly in hardness, and also to the smoothing effect of a moist climate. 

 The steep-sided mesas of the southwestern United States are often 

 the result of the erosion of lava plateaus, the hard lava forming flat- 

 topped mountains bounded by conspicuous, vertical cliffs. Residual 

 mountains are confined to those formed of horizontal rocks or slightly 

 inclined rocks. The external form of complexly folded mountains 

 (p. 356) is due to erosion, and they are in a sense residual moun- 

 tains. They have, however, been placed in a class by themselves be- 

 cause of the origin of the folded structure which gives them a distinct 

 character. 



Fault or Block Mountains. — It was shown in the study of fault- 

 ing (p. 267, Fig. 266) that important topographic features are 

 produced in this way, and that mountain ridges of this origin have 

 been formed either by uplift along one side of a fault, or by sinking 

 along one side, or by a combination of the two movements. Moun- 

 tains formed by the elevation of wedge-shaped blocks are called 

 horsts (p. 263, Fig. 257). In southern Utah and Oregon block or 

 faulted mountains have been carefully studied and have been found 

 to exhibit all the stages from young faulted mountains, in which 

 erosion has as yet been able to accomplish little, to ancient fault 

 mountains, in which erosion has proceeded so far that their origin 

 can merely be conjectured. In portions of these regions block moun- 

 tains 10 to 40 miles long and 1000 to 1200 feet high occur. The 

 ridges are steep or cliff-like on the fault side and have a gentle slope 

 on the opposite side. Between the faults are trough-like depressions 

 in which lakes sometimes rest. The steep eastern slope of the 

 Sierra Nevada Mountains marks the fault along which a great block, 

 500 miles in length and 70 to 100 miles broad, has been raised, the 

 escarpment thus formed rising from 5000 to 6000 feet above the 

 desert valleys to the eastward, and reaching a maximum height of 

 14,000 feet in the vicinity of Death Valley. (Russell.) Well-known 

 examples of block mountains are the Vosges and Black Forest of 

 Germany (p. 100, Pig. 81). 



Laccolith Mountains. — Under the discussion of laccoliths (p. 

 327) it was seen that in certain localities molten material has 



