MOUNTAINS AND PLATEAUS 



355 



been injected into the earth's crust in such quantity that the cover 

 of sedimentary strata has been lifted into dome-like forms. After 

 prolonged erosion the softer strata are partially or wholly removed, 

 and the hard, igneous core is left as a mountain or hill. In moun- 

 tains of this origin, the strata dip away in all directions from the 

 center, and not uncommonly "hogbacks," with steep cliffs facing 

 towards the center, form one or more broken rings about the moun- 

 tain. Although mountains of this type are not abundant, a large 



Fig. 341. — Little Sundance Dome, Sundance, Wyoming. This is a laccolith 

 from which the overlying strata have been eroded. 



number are known to exist, of which those in Utah, California, Wyo- 

 ming, South Dakota, British Columbia, and Canada might be men- 

 tioned (Fig. 341). 



Domed Mountains. — The Black Hills of South Dakota may 

 be taken as a type of domed mountains. They rise 2000 to 3000 

 feet above the surrounding plains and about 7000 feet above sea 

 level, and are carved from a dome-shaped uplift of the earth's crust. 

 The length of the dome is about 100 miles and the width about 50 

 miles, — about the size of Connecticut. As will be seen from the 

 diagram (Fig. 342), the eroded central part is composed of crystalline 

 rocks from which the strata dip in all directions. As a result of the 

 more rapid erosion of a stratum of shale, a trench called the Red 

 Valley, in many places two miles wide, entirely surrounds the 

 center, except where it is cut through by streams. The Red Valley 

 is separated from the flat plains surrounding the central mountain 

 mass by a rim or " hogback," which presents a steep face towards 

 the valley and rises several hundred feet above it. 



