12 



PHYSIOGRAPHY 



but its crest is usually narrow (PI. II, p. 5). The several ridges 

 shown in Fig. 2, PI. Ill, p. 6, are examples. Numerous peaks or 

 ranges are often associated, making a mountain group or a mountain 

 chain, but even in great mountain groups there is no great unbroken 

 expanse of high land. 



Where mountains rise abruptly to great heights above their 

 surroundings, they are the most impressive and awe-inspiring 

 features of the earth. In not a few cases they rise from low, warm 

 plains to such heights that their summits are always covered with 



Fig. 8. Map showing the principal physiographic subdivisions of the United 

 States. A Adirondack Mountains. B Black Hills. (After Davis, 

 with slight modifications.) 



snow. Nowhere else are such extremes of climate found so close 

 together. Mountains are in contrast with plains and plateaus, and 

 are the third of the three topographic types of the second order, as 

 they appear on the lands of the earth. In this grouping of mountains, 

 only great groups or systems of mountains, such as the Appala- 

 chians, the Rockies, the Alps, the Himalayas, the Andes, etc., are 

 considered. Since the term " mountain" is applied to any point or 

 ridge of such steep slopes and so much above its surroundings as to 

 be conspicuous, it follows that many elevations called mountains 

 do not belong to the great physiographic type which is to be brought 

 into contrast with plains and plateaus. 



