OCEANS AND LAKES 



along many ocean shores, back of bars and barriers (Fig. 280). 

 They occur in irregular belts or groups in certain interior 

 basins and plateaus, 

 as in central Asia and 

 parts of Africa. Some 

 of the larger lakes of 

 these areas are in basins 

 formed by the down 

 warping or down fault- 

 ing of portions of the 

 surface (Fig. 281) ; some 

 have formed behind 

 dams of stream-swept 

 waste ; some have gath- 

 ered in basins of other 

 origin. Lakes are common features of many broad flood plains, 

 representing abandoned sections of shifting rivers (p. 180). 

 They are characteristic, too, of many large deltas (p. 186). 

 A few lakes occupy basins that have originated in still other 

 ways. Avalanches and lava flows may dam rivers. The 

 surfaces of recently emerged coastal plains may contain shal- 



1 Miles 



FIG. 280. Coastal lakes shut off from the 

 sea by sand bars. South coast of Long 

 Island. 



f f f f 



FIG. 281. Section across the Dead Sea. //, faults. 



low depressions. Certain areas underlain by limestone have 

 many sink holes (p. 118). These and other depressions may 

 contain lakes. 



The general conditions necessary for the formation of a per- 

 manent lake will be inferred readily from what has already 

 been said. They are (1) a depression below the surface of the 

 surrounding country, (2) whose bottom is beneath the lowest 

 level of the water table. 



