LAKES AND SHORES 



151 



Reference has already been made (p. 99) to the more important 

 types of lakes formed by rivers. Waves and shore currents give rise 

 to lakes by making deposits of gravel and sand across the lower ends 

 of drowned valleys or other narrow bays. Numerous examples 

 are found along many coasts (Fig. 1, PL XXXVIII, p. 133 and Fig. 

 148). Lake basins produced by glaciers are very numerous, as we 

 have seen (p. 134), and some of the ways in which the ice gave rise 

 to them have been explained in preceding pages. Slumping (p. 42) 

 sometimes makes lake basins, especially when the material sliding 

 down obstructs a valley. Such a lake, five miles long and more 

 than seven hundred feet deep, was formed on the upper Ganges, 

 in 1892. Two years later the dam which held back the water broke, 

 and the resulting flood wrought great destruction in the valley 

 below. 



By vulcanism. A few lake basins were formed by vulcanism. 

 Some of them are in the craters of volcanoes which are no longer 



Fig. 152. The crater lake of the volcano Poas, Costa Rica. 



active (Fig. 152). In other cases flows of lava have obstructed 

 valleys, ponding the water above. 



By movements of the earth's crust. Other lake basins have 

 come into existence by movements of the earth's crust. These 



