120 



LAKES 



water, which does not form part of the sea, but lakes are formed 

 in very different ways and have correspondingly different histories. 

 Most lakes occupy depressions below the general drainage level of 

 the country, whether these depressions be due to movements of the 

 earth's crust, to glacial excavations, to unequal decomposition by 

 the atmospheric agencies, or to some other factor. Others, again, 

 are held back by dams, such as lava streams, glacial moraines, or 



Fig. 44. — Old lake terraces, western New York. (U. S. G. S.) 



the glaciers themselves, by the debris of landslips, or by the deltas 

 of tributary streams which bring in more material than the main 

 river can dispose of. Others still are enlarged basins cut out by 

 rivers. Great lakes that persist for long periods of time are con- 

 tained in basins, often of great depth, which were formed by 

 movements of the earth's crust ; the other kinds are more evanes- 

 cent and usually of rather small size. 



Small lakes accomplish very little in the way of rock destruction, 

 but are rather places of accumulation. The waves, even in storms, 



