CENTRAL AND SOUTH AMERICA
The scarcity of data which was bewailed in dealing with Asia is still
more marked in the case of South America, and it will be necessary
to present the glacial history of that continent in the barest
outline only. This is the more unfortunate as the chain of the Andes,
extending from north of the equator to high southern latitudes, is of
enormous importance in glacial theory, and especially in the question
of simultaneity of glaciation in the two hemispheres.
The beginnings of glaciation in South America are obscure. The
distribution of animals shows that towards the close of the Tertiary
the Falkland Islands were greatly elevated and were united to Tierra
del Fuego and Patagonia, and this enlarged land area was connected
in some way with Australia and Tasmania, but the mode of this latter
connexion is not definitely known. This question will be discussed
more fully in Chapter XI; it is sufficient to say here that the
amount of elevation may have reached 12,000 feet in Tierra del Fuego.
Equatorwards the elevation diminished, and near the equator the land
probably lay somewhat lower than now.
In South Georgia the present glaciers greatly expanded, until
practically the whole island was buried in ice, and the same is
true of the Falkland Islands and Tierra del Fuego, only the highest
peaks remaining above the ice. In the latter district there is
some evidence of two glaciations separated by an interglacial, the
earlier glaciation being due to a regional ice-sheet and the later
to smaller valley glaciers. The intricate coast-line of the Falkland
Islands and Tierra del Fuego points to fiord erosion by ice which
extended well beyond the present limits of the land, and can only
have occurred during considerable elevation. As to the character of
the interglacial, little is known. In the Falklands there is a bed of
black vegetable soil full of tree-trunks, indicating the existence of
luxuriant forests and a temperate climate. This deposit is overlain
by boulder-clay, and may be either interglacial or pre-glacial, but
since it was formed when the land stood at a comparatively low level,
while we have reason to believe (see Chapter XII) that during the
close of the Tertiary period these islands were greatly elevated,
it is probably an interglacial formation, and indicates a great
amelioration of climate. In Gable Island, Tierra del Fuego, Halle
found beneath boulder-clay a Quaternary fauna of barnacles and marine
mollusca indicating a climate slightly warmer than the present, and
this probably belongs to the same period. To the concluding stages
of the Glacial period in the Falklands belong the curious “stone
rivers,” great streams of moss-grown boulders which fill the valleys,
and under the influence of temperature changes are probably still
slowly advancing.
Passing further north to the Andes, between 39° and 44° south
latitude, the glaciation was not so severe, and its records are
therefore clearer. The first result of elevation was the cutting of
deep canyons by the rivers. This was followed, possibly without much
further elevation, by a fall of temperature, which in this connexion
may be attributed to the extension of the Antarctic and Tierra del
Fuego ice-sheets. Glaciers now developed and spread down the canyons,
leaving moraines of great volume and height, associated with all
the other criteria of glaciation. The snow-fields from which these
glaciers originated lay between 5000 and 6000 feet above the sea,
and the snow-line lay at about 3000 feet instead of above 6000 as at
present.
This glaciation was followed by a long interglacial, during which the
glaciers retreated to the highest summits of the Andes. The length of
this period is indicated by the fact that the earlier moraines have
been eroded to such an extent that they no longer present distinctly
the typical features of glacial topography, while the materials of
which they are composed are decayed to somewhat the same extent as
the older moraines of North America, the granite boulders especially
being rotten and friable. This interglacial was followed by a
re-development of the glaciers, but to nothing like the same extent
as formerly; their moraines are smaller and fresh-looking, indicating
that this glaciation was comparatively recent.
Still further north, in latitude 20°-25° S., we come to a region of
very slight snowfall, where the snow-line lies higher than anywhere
else on the face of the earth. The glaciation here was comparatively
unimportant, the snow-line descending only 1600 to 2500 feet. Here
Keidel found moraines of three glacial advances, and from his
description it appears probable that the earliest and greatest was
separated by a considerable interval from the two younger, the
interglacial between which was short and not characterized by a
return to present-day climatic conditions, since during this interval
there was very little weathering. Probably we have here to do with
two glaciations, of which the second was double. In fact, some
writers have described no less than five glacial advances in the
Argentine Andes, but most of these are probably merely retreat stadia.
In Peru, W. Sievers reports the existence of two glaciations
separated by a considerable interval. The present limit of the
glaciers is about 15,200 feet; during the first glaciation they
descended to about 11,000 feet, and during the second to 12,800
feet. The evidence is very complete. In Ecuador, H. Meyer records
a similar bipartition. The oldest glaciation is represented by
trough-like valleys, enormous gravel terraces, and old moraines
much weathered; the limits are far below the present limits of
glaciers, but have been much obscured by subsequent erosion. This
glaciation was followed by a long period of steppe climate resembling
the present, during which the loess-like Cangagua formation was
deposited. This in turn was followed by a readvance of the glaciers
to a level about 2700 feet below the present limit. This glaciation
is associated with crescent-shaped moraines, corrie lakes,
hanging valleys and gravel terraces, covered with vegetation, but
otherwise fresh-looking. The snow-line lay about 1600 feet below
the present. Probably during the first glaciation the Andes were
invaded by numerous mountain plants and animals related to North
American forms—a valuable piece of evidence which indicates that
the glaciation was contemporaneous with that in North America. In
Columbia and Venezuela there are traces of Glacial periods, but these
have not yet been studied in detail. The most northerly evidence of a
Glacial period comes from the Sierra Nevada de Santa Maria, near the
north coast of Venezuela in 11° N.
Except in Tierra del Fuego and Patagonia the ice did not extend far
from the mountains. But in the eastern Argentine there is a great
series of Quaternary deposits known as the Pampean. This formation
covers 200,000 square miles, and consists of at least ninety feet of
fine loam without a single pebble (except for a few thin calcareous
layers), but containing large numbers of complete skeletons of
mammals. It raises several interesting problems. Apparently it
represents the whole course of the Glacial period. By some geologists
it is considered to be a delta deposit of the combined Parana and
Paraguay rivers, but the absence of mollusca, except in a marine
intercalation near its summit, is against this view, and Steinmann
attributes it to æolian agencies and compares it to the loess of
Europe and North America. If this view is correct the Pampean
represents steppe conditions prevailing on the equatorial side of the
Patagonia-Falkland Islands ice-sheet. Apparently before the incoming
of the greatest cold the Pampas were in part at least forest-clad,
for in the older beds are found peculiar forms of ground-sloths
which were adapted for forest life and have been found also in
cave-deposits of Brazil. At the maximum of glacial conditions the
Pampas probably had a steppe climate, but the disappearance of the
forests is to be attributed rather to drought than to cold. Elevated
glacier-bearing Andes to the west and ice-sheets to the south would
render the Argentine extremely arid, and this accounts for the
gradual extinction of so many giant forms whose remains are found
in the Pampean deposits. Conditions ultimately became too severe
even for the horse, which died out in South America. The marine
transgression which left its mark near the top of the Pampean is
probably post-glacial.
In Brazil, on the other hand, there is no evidence that the climate
has ever been drier than the present, and in the semi-arid regions of
the north-east it is even probable that during the Glacial period the
climate was moister, presumably owing to the greater strength of the
rain-bearing east and north-east winds. Further west in the Andes the
existence of this wet period is borne out by the former greater size
of Lake Titicaca, and there seems to be additional evidence to the
same effect in the Chilian deserts.
BIBLIOGRAPHY
Steinmann, J. “Diluvium in Südamerika.” _Zs. d. D. Geol.
Gesellsch._, 58, 1906, p. 215.
Meyer, H. “In den Hochanden von Ekuador.” Berlin, 1907.
Sievers, W. “Reise in Peru und Ecuador, ausgeführt 1909.” München
und Leipzig, 1914.
Keidel, H. “Ueber den Anteil der Quartaren Klimaschwankungen an
der Gestaltung der Gebirgsoberfläche in dem Trockengebiete der
mittleren und nördlichen Argentinischen Anden.” _Congr. Geol.
Internat._, 12, Canada, 1913, p. 757.
Willis, Bailey. “Physiography of the Cordillera de los Andes
between latitudes 39° and 44° S.” _Congr. Geol. Internat._, 12,
Canada, 1913, P. 733.
Halle, T. G. “On Quaternary deposits and changes of level in
Patagonia and Tierra del Fuego.” _Bull. Geol. Inst._, Upsala, 9,
1908-9, p. 93.