The Lake Systems of Southcni Patagonia. — Hatcher. 169 
Many of these lakes, like Argentine, Viedma, San Martin, 
Pueyrredon, and Buenos Aires, are of large size, fifty to icxi 
miles in length, or even longer. None of them have as yet been 
fully explored and accurately charted. All of them are, except 
on their eastern shores, surrounded by lofty, precipitous moun- 
tains. The summits of the latter are covered with immense 
fields of snow^ and ice, from which descend glaciers that occa- 
sionally extend quite down the mountain slopes into the waters 
of the lakes. Huge blocks of ice are frequently detached from 
the front of such glaciers and float off into the lake as icebergs 
of no inconsiderable proportions. 
The basins occupied by these lakes are largely of tectonic 
origin and they are chiefly due to the unequal folding of the 
strata that took place during the elevation of the southern 
Andes in late Tertiary times.* 
With the exception of lakes Viedma and Argentine, this 
great series of lakes all discharge their waters into the Pacific, 
notwithstanding the fact that they lie entirely to the eastward 
of the main range of the Andes, and that the eastern extrem- 
ities of most of them project even into the great plain of east- 
ern Patagonia. 
Just to the eastward of this series of lakes of tectonic origin 
and situated on the plains, entirely without the foot-hills of the 
Andes, there is a second series of lakes evidently of glacial or- 
igin. For the most part these lakes are of small size and of 
minor importance, though some of them, like Lagoona Blanca, 
lake Cardiel and lakes Colhue and Musters (the two latter 
are not shown on the accompanying map, since they lie some- 
what beyond the forty-sixth parallel) are of considerable di- 
mensions. These lakes have for the most part originated from 
the damming of preglacial drainage systems with glacial de- 
tritus during the recession of the glaciers that occupied these 
valleys at the close of the glacial period. Like the lakes just 
mentioned they contain fresh water. Although for the most 
part they have no surface outlet, the circulation permitted bv 
the confining glacial drift is usually sufficient to keep the 
* For a further discussion of the origin of these lakes, see "Some Geographic 
Features of Southern Patagonia, with a Discussion of their Origin," by J. B. 
Hatcher: Nat. Geogr. Mag., vol. xi, pp. -l-l-.^S. 
