io6 



PHYSICAL GEOLOGY 



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Fig. 89. — Black Mesa. (U. S. Geol. Surv.) 



overlying softer beds will produce such hills, the name " mesa " 

 being used to designate the shape of the hill, not the kind of rock. 

 In the western United States the word butte is used for any steep- 

 sided hill and is also loosely used for any conspicuous elevation. 



Outliers. — When a part of a formation is separated from the main 

 body by erosion (or, occasionally, by faulting), it is called an 

 outlier (Fig. 90). It is, therefore, simply a remnant of a more ex- 

 tensive bed or series 

 of beds. Outliers are 

 usually short-lived, 

 since they are objects 

 of attack on all sides 

 by erosion. Outliers 

 often occur scattered 

 along the front of 

 prominent escarpments; as, for example, near the border of the 

 High Plains in the Middle West (Fig. 86). 



Rock Terraces. — In a region underlain by alternate hard and 

 soft beds, such as in the Colorado Plateau, the resistant rocks may 

 form* rock terraces and the softer rocks slopes in the river valley 

 or canyon. The ''steps" or "benches" of the walls of the Grand 

 Canyon of the Colorado (Fig. 73, p. 95) are among the most striking 

 features of this remarkable valley. Rock terraces may also result 



Fig. 90. — In the diagram outlier A was formerly united 

 to By but was separated from it by erosion. 



