VALLEY OF THE AMAZONS. 
209 
by moraines. In the ice-period these depres¬ 
sions were filled with glaciers, which, in the 
course of time, accumulated at their lower end 
a wall of loose materials. These walls still 
remain, and serve as dams to prevent the es¬ 
cape of the waters. But for their moraines, 
all these lakes would be open valleys. In the 
Hoads of Glen Boy, in Scotland, we have an 
instance of a fresh-water lake, which has now 
wholly disappeared, formed in the same man¬ 
ner, and reduced successively to lower and 
lower levels by the breaking down or wearing 
away of the moraines which originally pre¬ 
vented its waters from flowing out. Assum¬ 
ing then, that, under the low temperature of 
the ice-period, the climatic conditions neces¬ 
sary for the formation of land-ice existed in 
the Valley of the Amazons, and that it was 
actually filled with an immense glacier, it fol¬ 
lows that, when these fields of ice yielded to 
a gradual change of climate, and slowly melted 
away, the whole basin, then closed against the 
sea by a huge wall of debris , was transformed 
into a vast fresh-water lake. The first effect 
of the thawing process must have been to sep¬ 
arate the glacier from its foundation, raising 
N 
