10 PHYSIOGRAPHY 



about 91 per cent of the population of the United States lived on 

 lands less than 1,500 feet above sea-level. Not all plains, however, 

 support an abundant population. The northern plains of Eurasia 

 and North America, for example, are too cold to be hospitable to 

 varied industries, and their populations are likely to remain small. 

 Origin. Plains are formed in various ways. Some coastal 

 plains were worn down from higher lands; others are parts of 

 former continental shelves built up above the water by sediment 

 laid down upon them; while still others represent former conti- 

 nental shelves, from which the sea-water has been drawn off. Some 

 interior plains were once coastal plains. They are now inland 

 because other land has been formed between them and the sea. 



Plateaus 



Plateaus are highlands with considerable summit areas, but no 

 great elevation is necessary to make a flattish area of land a plateau. 

 In general, a plateau is so situated as to appear high from at least 



Level 



Fig. 5. Diagram to illustrate the relations of mountain, plateau, plain, 

 ocean basin, ocean deep, etc. 



Caucasus Mtt. Himalaya Hits. Nan-Shan Mts. 



Fig. 6. Section across Asia, showing plateau between the Himalayas and 

 the Nan-Shan Mountains. 



Fig. 7. Section across North America, showing plateaus between the 

 mountains in the western part. 



one side. Thus if a coastal plain rises gradually from the sea to a 

 height of 200 feet, and then rises by a steeper slope to another broad 

 tract of land which rises 200 feet higher (Fig. 5), the upper tract 

 would be called a plateau, not so much because of its altitude above 



