OCEANS AND LAKES 267 



Extinct lakes. Beds deposited in lakes now extinct cover 

 extensive areas in various regions. Their origin is indicated 

 by some or all of the following characteristics: (1) In many 

 cases the beds show a concentric arrangement, the clays that 

 gathered in the deeper and quieter waters in mid-lake being 

 inclosed by the sand and gravel that accumulated about the 

 shallower lake borders. (2) Except about the borders, where 

 beach and delta structures may occur, beds laid in lakes are 

 horizontal and often essentially uniform in texture over con- 

 siderable areas (Fig. 285). This is in contrast with stream- 



FIG. 285. Lake-laid clays and fine sands. Near Devils Lake, Wis. 



(Trowbridge.) 



laid beds (p. 40). (3) The beds may be bordered by shore 

 features, such as spits, bars, cliffs, and beach ridges, although, 

 save in arid regions, such features are soon destroyed by stream 

 erosion. (4) Where fossils occur, they often indicate the 

 character of the water body in which the associated sediments 

 were deposited, since the life inhabiting fresh, brackish, and 

 salt water differs. (5) Broad flats and mountain meadows 

 may be so related to the inclosing slopes as to indicate clearly 

 that tney are the beds of former lakes. 



Certain noted extinct lakes are referred to later (pp. 453, 

 458). 



