THE GREAT RELIEF FEATURES OF THE LAND 285 



higher than plains, they may be lower. Though the ideal 

 plateau has a level surface, many are deeply trenched by 

 valleys and surmounted by ranges of high hills or mountains. 



Distribution. Extensive plateaus are confined largely to 

 three classes of situations. (1) They may intervene between 

 a lower plain on one side, and higher mountains on the 

 other. The Piedmont Plateau, separating the Atlantic 

 Coastal Plain from the Appalachian Mountains, is an exam- 

 ple. (2) They may be surrounded more or less completely 

 by mountains, as in the case of the vast plateaus of Central 

 Asia and the Great Basin of western United States. (3) In 

 some cases they rise abruptly from the sea, or from narrow 

 coastal plains. The Iberian Peninsula and southern India 

 are plateaus of this type. 



Origin of plateaus. Plateaus may originate in various 

 ways. They may be built by successive lava flows, like the 

 Columbian Plateau of the Northwest and the Deccan Plateau 

 of India. The adjacent country may have been worn low 

 or warped down, leaving a table-land. Or, the plateau may 

 have been warped or faulted above its surroundings. 



The erosion of plateaus. Like mountains, all plateaus 

 will be worn down to lowlands if not renewed. Mature 

 plateaus are table-lands completely dissected by streams; 

 the original flattish surface has been carved into hills and 

 valleys, and slope and relief are at a maximum. It is at 

 this stage that the arrangement of strong and weak rocks 

 expresses itself most completely in the details of the topog- 

 raphy. Such regions have so largely lost their plateau 

 character, that, as in the case of the Catskills (p. 281), they 

 are sometimes called mountains. In one sense there are 

 no old plateaus, for, when worn low, they constitute plains. 



PLAINS 



Tracts of comparatively level and low land are plains. 

 The term is used loosely, however, for there are hilly and 

 B. & B. GEOL. 16 



