THE WORK OF STREAMS 



105 



Elevations Due to Unequal Hardness. — The term hogback is 

 given to narrow ridges which stand above the general level of a 

 region, because of the greater resistance of a steeply dipping layer 

 of rock and of the greater erosion of the softer rock (Fig. 88 A, B). 

 They are especially conspicuous on the flanks of mountains. When 

 regions in which the rocks are folded have been subjected to erosion 

 the harder beds stand up as mountain ridges. In this way the Appa- 



Fig. 88. — Photograph and section of a hogback near Canon City, Colorado. The 

 ridge is due to the superior strength of one main and two subordinate strata. SS are 

 strong and WW weak rocks. (After Brigham.) 



lachian Mountains (p. 477) were formed. The difference between 

 a hogback and such mountains is largely one of height, width, and 

 extent. 



Where sheets of lava cover softer beds, as is not uncommon in 

 the southwestern portion of the United States, flat-topped, isolated 

 hills, called mesas or tables (Fig. 89), are formed by the headward 

 cutting of tributary streams. Any harder bed of horizontal rock 



