IgoT | ° GENETIC DEVELOPMENT OF FORESTS 313 
may be made directly by the moving ice, or glacial deposits 
may dam up drainage lines. A second and subsequent class of. 
lakes is made by bars cutting off lagoons from the larger lakes, 
or by spits formed across their embayments. Both classes 
have a_ similar _life- 
history. Some find out- 
lets and ultimately pass 
out of existence through 
the normal stages in the 
life-history of a river. 
A larger number, how- 
ever, never find outlets, 
but are silted up by the 
wash of the surround- 
ing soil, and by the ac- 
cumulation of vegeta- 
tion. Thus swamps are 
formed, and since they 
may represent a stage 
in the life-history of 
the forest they deserve 
treatment here. 
The zonal distribu- 
tion of plants in swamps 
is one of the stock illus- 
trations of ecologists, so 
that development of the 
various zones need only 
be mentioned briefly. 
Most swamps start as 
ponds or lakes, in which 
water lilies and other 
pond plants dominate. 
On the borders of the 
pond sedges appear ; 
these, by their decay, 



Fic. 9.—A Norway pine growing from a crevice 
in granite ; the rock has been cracked and uplifted 
by the growth of a root. 
