CONDITIONS BEFORE THE QUATERNARY ICE AGE
The third of the great periods into which the geological record is
divided is known as the Tertiary. Throughout most of its length it
appears to have been characterized by remarkably mild and equable
climatic conditions extending into comparatively high latitudes,
so that the west coast of Greenland, for instance, had a flora of
almost sub-tropical aspect. Since the plants in question—chiefly
palms and cycads—are not of identical species with their present-day
representatives, it is unsafe to base numerical estimates of the
temperature upon them, but it is at least obvious that these regions
were warmer than they are at present.
Let us glance for a moment at the geography of the Tertiary
period. The most noticeable point is a great expanse of sea over
south-eastern Europe, including the Mediterranean countries,
extending away over the Black Sea and Caspian, and stretching in a
great arm to the Arctic Ocean, south of Novaya Zemlya. The geology of
the archipelago north of Canada is not yet well known, but it seems
probable that there was a considerable area of Tertiary ocean there
also. The sea further encroached on the present boundaries of North
America, both east and west, and on the north-eastern coast of Asia.
Bearing in mind the principles set out in the first chapter, we can
infer from these changes a great increase in the winter temperature
of the regions along the Arctic circle. The increase reached a
maximum on the west of Greenland and in western Siberia, but the
west coast of Alaska also had a decidedly warm climate in the late
Miocene and Pliocene.
The basin of the Arctic Ocean, which already existed at that stage,
was raised to a temperature considerably higher than the present
by three great streams of warm water flowing into it. If, as seems
probable, the Bering Strait was deeper, and the submarine ridge
across the North Atlantic less pronounced, the obstacles to the
outflow of cold water along the ocean floor were much less than now.
Finally, the winter temperatures of the land-masses to the south,
and especially Siberia, being already very much less severe owing
to the sea over Europe, the temperature of the water of the great
rivers flowing into the Arctic was not so low. For these reasons the
development of ice in the Arctic Ocean was very much diminished,
and possibly entirely absent, allowing a great amelioration of the
climate of Greenland, the rigor of which is at present much enhanced
by the ice which flows down the Greenland Sea and round Cape Farewell.
The cumulative effect of all these changes—greater water area,
greater inflow of warm surface water, less inflow of cold river
water, less ice-development—must have been a mild equable rainy
climate, entirely suitable to a rich vegetation. The sub-tropical
aspect of that vegetation should not be stressed, for it was probably
as much an expression of the geological age of the period in question
as of its climate.
The objection may be raised that at the present time the
sub-antarctic islands in the great Southern Ocean have the most
maritime climate in the world, but are not by any means places of
opulent vegetation. The difference is entirely accounted for by the
presence of the great ice-bearing Antarctic continent. Its effect
is twofold. Firstly, the glaciers shed into the Southern Ocean an
immense quantity of ice and ice-cold water annually, which must have
an appreciable effect on temperature. Secondly, the presence of this
ice-covered continent and the floating ice in its neighbourhood
extending as far as the sixtieth parallel, by forming a marked
contrast with the warmer waters further north, greatly intensifies
the strength of the atmospheric circulation in these regions,
resulting in the development of a great succession of severe storms
which sweep the sub-antarctic islands. There are no great land-masses
to break the force of the wind, and these latitudes are among the
stormiest, windiest regions of the earth—gale succeeding gale, winter
and summer alike; and it is largely to the extraordinary power of the
wind that we must attribute the desolate appearance of the islands.
The picture we have drawn of the high northern latitudes in early
Tertiary times is vastly different. A great warm ocean occupied the
Arctic regions, fed by three ocean currents analogous to the Gulf
Drift, and the fall of temperature was gradual from the tropic to the
pole. The return colder currents were mainly along the ocean floor
and with little ice-formation the storms were few and not severe. On
the western shores of the continents mild rain-bearing south-west
winds prevailed, and a quiet moist warm atmosphere existed which was
especially favourable to plant life. This favourable state of affairs
lasted until well on in the Miocene, and then changes set in. The
land and sea distribution underwent essential modifications. The
great Tertiary continent or archipelago which is believed to have
existed in the western Pacific, and whose last remaining summits
now form the scattered islands of that ocean, gradually subsided,
and in its place elevation began in higher latitudes. Bering Strait
became narrow and shallow, and was probably for a time entirely
closed, while the connexion between the Arctic and Indian Oceans was
closed permanently, leaving in its lowest areas a chain of great
inland seas and lakes, of which the Caspian and Aral Seas are now
the greatest representatives. The Canadian Archipelago was probably
raised above its present level, and formed a great northern extension
of the American continental area. The changes in the Atlantic also
were very extensive. The West Indies were the site of a large and
lofty Antillean continent; further north a considerable land-mass
existed east of Newfoundland; Greenland was joined on the west to
the extended American continent, and considerably enlarged to the
south-east. Iceland, though it remained an island, was elevated and
probably nearly doubled in area, and between Iceland and the north of
Scotland was developed a great submarine ridge, which may or may not
have risen above the sea in places. The British Isles became a solid
block of land, united with continental Europe across the English
Channel and the great plain which is now the North Sea. Scandinavia
was elevated by more than a thousand feet, and the elevation extended
at least as far as Spitzbergen. The Murman area had a considerable
extension. In eastern Asia the Sea of Okhotsk was land and Japan was
united to the mainland.
In the southern hemisphere our knowledge is not nearly so detailed.
The presence of marine Middle-Tertiary beds with temperate mollusca
in Graham Land and of plant-bearing beds in Seymour Island point to
a smaller Antarctic continent and very much warmer conditions at
this time in the South as well as in the North Polar regions. For
the close of the Tertiary, however, we have strong grounds in the
distribution of animals and plants for assuming that the Antarctic
continent was greatly increased in size, with promontories uniting
it to Australia on the one hand and to South America on the other.
New Zealand was largely increased in area, and South Africa probably
extended further polewards. The sub-antarctic islands attained a much
greater area. Conditions were ripe for the Ice Age in the southern as
well as the northern hemisphere.
BIBLIOGRAPHY
Kerner von Marilaun, F. “Synthese der morphogenen Winterklimate
Europas zur Tertiärzeit.” Wien, _SitzBer. K. Akad. Wiss_, 122,
1913, pp. 233-98.
Osborn, H. F. “The age of mammals in Europe, Asia and North
America.” 8vo. New York, 1910.
Nathorst, A. G. “On the fossil floras of the Arctic regions as
evidence of geological climates.” London, _Bot. J._ 2, 1913, pp.
197-202; and Washington, _Report Smithsonian Inst._, 1911.
Dall, W. H. “On climatic conditions at Nome, Alaska, during the
Pliocene.” _Amer. J. Sci._, ser. 4, Vol. 23, 1907, p. 457.
Nansen, F. “The bathymetrical features of the North Polar seas,
with a discussion of the continental shelves and previous
oscillations of the shore-line.” _Norwegian N. Polar Exped._,
1893-96. _Scientific Results_, Vol. 4.
Spencer, J. W. “Reconstruction of the Antillean continent.” _Bull.
Amer. Geol. Soc._, 6, 1895, pp. 103-40.
Wilckens, D. “Die Mollusken der antarktischen Tertiärformation.”
_Wiss. Ergebn. der Schwed. Sudpolar Exped._, 1901-3, Bd. 3, 1911.
Hedley, C. “The palæographical relations of Antarctica.” London,
_Proc. Linnæan Soc._, 124, 1911-2, p. 80.