THE GLACIATION OF ANTARCTICA
The great Antarctic continent offers a unique problem to the glacial
climatologist, for here we have a land area with the theoretical
snow-line already at sea-level, and accordingly covered with a thick
ice-sheet that leaves only a few mountain ranges and nunataks exposed
above its surface, and yet in the past these ice-sheets and glaciers
have attained a thickness several thousand feet greater, and have
extended further north. Various suggestions have been made to account
for this former extension, perhaps the most remarkable being that it
coincided with a milder and therefore snowier climate. This, however,
is untenable, for the Glacial period of Graham Land and the South
Orkneys is obviously a southward extension of the Glacial period of
Tierra del Fuego, which was obviously due to a colder climate, and
can be traced northward along the Andes into tropical regions. A more
fruitful suggestion is that as one of the most potent factors in
preventing the accumulation of snow is at present the wind, it was a
decrease in the strength of the wind which enabled the ice to reach a
greater thickness. This is probably true in a sense, the decrease of
wind force being due to a great increase in the area of the Antarctic
continent during the Quaternary.
We have seen that in the early Quaternary there was great elevation
in the south of South America and also in Australia and New Zealand.
The amount of this elevation increased southward and was very great
near the polar circle. This is borne out by considerations based
on the distribution of living and fossil animals, which point very
definitely to a land connexion between Australia and South America
in Tertiary and early Quaternary times, most probably by way of
Antarctica.
The first line of evidence is the distribution of the marsupials,
living and extinct. As is well known the chief home of this type of
mammal is now in Australia and New Guinea, but in Tertiary deposits
in Patagonia remains of extinct forms known as Dasyurids have been
found, which are allied to Australian forms, and can only have
come from Australia, probably via Tasmania. Secondly, there are
two peculiar families of fresh-water fishes, the _Haplochitonidæ_
and _Galaxiidæ_, the first common to Australia and South America,
while one species of the second is found in New Zealand, Tasmania,
the Falkland Islands and Patagonia. Thirdly, Beddard has found an
intimate relation between the earthworms of New Zealand, Eastern
Australia and Patagonia. Finally there is a curious similarity
between the slugs of Patagonia and those of Polynesia.
What is the explanation of these relationships? Assuming that
there has been a land connexion, it can have been either by way
of Antarctica or Polynesia. The earthworms cannot endure a very
severe climate, but on the other hand there is a total absence of
any tropical forms common to Australia and South America, and the
general dissimilarity of the faunas shows that the connexion cannot
have been available for a very long period. A study of the oceanic
depths suggests that the Antarctic connexion is the more probable.
A comparatively slight elevation would connect Patagonia and the
Falkland Islands with the South Shetlands and Graham Land, and an
elevation of 12,000 feet would give a large land connexion between
Australia and the opposite coast of Antarctica via Kerguelen. Forbes
even postulates an immense Tertiary Antarctica in which several forms
of animals and plants were able to evolve, but except possibly in
the case of the edentates this supposition is not necessary.
The course of events may provisionally be taken as follows: In late
Tertiary times an elevation of at least 12,000 feet in the South
Polar regions caused a great increase in the area of Antarctica,
which was united to South America on the one hand and Australia on
the other. The northern shores of this continent were far to the
north of their present position, and though the interior was very
cold the coast lands had at first a moderate temperature, and for a
short time allowed animals to migrate from Australia to South America
or vice versa. But the high mountains of the interior were already
glaciated, and ice-sheets gradually crept down their slopes. Owing
to the small precipitation the advance of the ice-sheets was slow,
but ultimately, probably in late Tertiary times, they approached
the coast, and the track along which migrations had taken place
was closed. The distribution of animals and plants shows quite
clearly that the land connexion was maintained into the period of
refrigeration. The shores of the continent being further north, the
pressure gradient between the pole and the present coast was less,
and consequently the winds were lighter. This and the diminished loss
by calving into glaciers allowed the ice to become thicker than it is
now.
Hedley apparently considers that the migrations referred to
above took place in an interglacial period, but the Patagonian
beds in which the fossil marsupials are found are Tertiary and
not Quaternary. No direct evidence of an interglacial period has
been found in Antarctica, nor, considering the intensity of the
glaciation which the country is even now undergoing, is any such
to be expected, and we can only infer from the bipartition of the
Glacial period in Australia, New Zealand and South America—which,
in New Zealand at least, was associated with submergence—that there
was probably a similar bipartition in Antarctica. Nordenskjold
states that the submarine relief showing river erosion which, in
Tierra del Fuego, was developed partly at least in the interglacial
period, is also developed in West Antarctica. It is improbable that
the ice ever entirely vanished from the continent. We shall see in