188 ELEMENTARY GEOLOGY 
LAKES 
Cause and Condition.—In most cases, lakes are 
formed by some obstruction in the course of a stream, 
and they act as filters which strain out the sediment 
from the rivers that enter them. Sometimes lakes 
are formed between mountains where a mountain bar- 
rier rises across a river valley; but more commonly 
they are due to some less pronounced dam, one of the 
commonest being a deposit of clay or gravel left by 
glaciers (Fig. 117). 
Usually lakes are fresh and have outlets to the sea; 
but in some of the arid countries, they may be en- 
closed and cut off from the ocean, because evaporation 
is greater than the rainfall, and hence the water can- 
not rise to the rim of the basin: Lakes which have 
no outlet, like the Great Salt Lake, are usually saline 
(p. 193). 
Filling with Sediment.— Rivers that enter the 
lakes, if they only have time enough, will, by their 
sediment deposit, destroy the largest lakes, whatever 
the cause of these may be. Every particle of sediment 
brought by them settles in the quiet lake water; and 
in course of time this will fill the basin, provided the 
outlet is not cut down meanwhile. The coarser sedi- 
ment builds deltas out into the lake (Figs. 94-96); the 
finer particles are strewn over the bottom. 
