HOW ICE PERIODS IN THE NORTHERN HEMISPHERE ARE BROUGHT ABOUT.
A large number of geologists are of the opinion that during the whole
of the Tertiary period the climate of the northern temperate and arctic
latitudes was uniformly warm, without a trace of intervening frigid
periods. I have before explained why the climate was made warm in the
southern hemisphere during the Tertiary epoch, and how on the closing
of that age, and subsequently, a considerable portion of the ocean
waters had moved from the northern hemisphere into the southern.
Therefore, the northern seas during Tertiary times covered a much
larger area than have obtained during periods following that mild
epoch. So, when the low lands of Europe were submerged, the Baltic,
Caspian, and other neighboring seas, now land-locked, were a portion of
an enlarged Atlantic. Consequently, the westerly winds blew over a much
wider North Atlantic than during the later periods.
Thus the high sea-level caused by such winds on its European side was
greater than has since been obtained with the Atlantic of less breadth.
This high sea-level, composed largely of drift water from the ancient
Gulf Stream, had convenient access to the enlarged Arctic Ocean, which
then covered the low plains of Northern Europe and Siberia. And owing
to the trend of elevated lands north-eastward, which then formed the
southern shores of the Arctic Ocean in those regions, the warm waters
of the high sea-level of the Eastern North Atlantic found an easy
passage into the arctic seas; for, while they moved over the European
and Siberian seas to the north-east, they had the assistance of the
westerly winds well into the arctic seas, from which position they
were attracted across the Arctic Ocean to the low sea-level abreast
Labrador and Davis Strait.
The Gulf Stream of Tertiary times comprised a much larger area than it
now obtains; for with Florida and a large portion of the Gulf States
submerged, and a wide, shallow sea covering the Mississippi valley
and the Great Lake region, the tropical waters of the enlarged Gulf
of Mexico moved from their vast high sea-level to the low sea-level
abreast British America and Labrador, without being confined to the
narrow Florida channel. Thus with an enlarged Gulf Stream in possession
of a wide and clear passage leading northward, in connection with a
mild period in the southern hemisphere, giving warmth to the southern
oceans, the resources of the ancient Gulf currents for warming the
northern regions were so ample and inexhaustive they were fully able to
maintain a mild climate on the shores of the European seas, and also on
the shores bordering the Arctic Ocean, during the Tertiary epoch.
Furthermore, the Humboldt current, which had its rise in the mild
southern seas of that age, mingled its warmth with the equatorial
current of the Pacific, which in turn gave its warmth to the Japanese
current. Therefore, the latter stream under such conditions was
competent to maintain a mild climate on the North Pacific coasts.
The origin of a cold period in the northern hemisphere was largely
owing to the changed condition of the northern oceans following the
close of the Tertiary epoch. The movement of the ocean waters into the
southern hemisphere lessened the area of the Arctic and North Atlantic
Oceans, and brought them to their present reduced limits, and also
diminished the volume of the Gulf currents.
This great geographical change, in connection with a cold period
progressing in the southern hemisphere, and so increasing the coldness
of the Japanese current, and the cold antarctic currents, previously
explained, which set northward on the bottom of the sea through the
torrid latitudes even into the North Pacific and North Atlantic
Oceans, were altogether sufficient to cause conditions favorable for
the advancement of a cold period in northern latitudes. Besides, with
reduced northern oceans and a diminished Gulf current, conditions were
favorable for an independent circulation of the arctic waters, such as
is being carried out at the present time. Hence an explanation of the
movements of the ocean waters of to-day will explain the conditions
which caused the northern ice periods in times past, as well as
those to come in a future age. Although the conditions are such that
the independent circulation of the arctic waters cannot be so well
performed as the independent circulation of the southern ocean, still
the open arctic channels are able to prevent the tropical Gulf Stream
water from largely entering the higher northern latitudes. For it is
certain that the prevailing westerly winds blow the surface waters of
the North Atlantic away from the eastern shores of North America from
Georgia to Labrador.
Consequently, the low sea-level thus caused attracts the waters of
the Arctic Ocean southward through Baffin’s Bay and Davis Strait, and
likewise down the east coast of Greenland, thus surrounding that large
island with an arctic temperature, and so causing it to become a land
of glaciers, which are constantly launching icebergs into the sea to
cool the waters of the northern oceans. The tropical waters of the high
sea-level of the Gulf of Mexico also seek the low sea-level abreast the
American coast, thus causing the Gulf Stream. This great ocean current,
being the main conveyer of tropical heat into the high latitudes of
the North Atlantic, calls for particular notice. The great gravity
currents, of which the Gulf Stream is one of the most conspicuous, are
moved by small gradients.
Hence the gradient which causes the Gulf Stream waters to move out
of the Florida passage is small. The levellings which have been made
place the surface waters of the Gulf of Mexico as being about one
metre higher than the Atlantic abreast New York, the pressure of the
higher Gulf waters toward the low level of the Atlantic being nearly
equal in the narrow Florida channel from the surface to the bottom of
the stream. Therefore, according to descriptions given by Commander
Bartlett, the warm stream moves like a river over the hard level floor
of the channel; but to the northward of the Bahamas, abreast Cape
Hatteras, the stream spreads out in fanlike form, and flows over a bed
of cold water of great depth.
A bed of cold water is found to cover the bottom of all the deep oceans
that are accessible to the antarctic seas, through which the cold water
is mostly supplied, as I have before pointed out.
But the cold water which underruns the Gulf Stream is probably
furnished by the arctic waters which move down Davis Strait and the
east coast of Greenland. The Gulf Stream, as it widens and becomes more
shallow, is, through its exposure to the westerly winds, gradually
converted into a drift current; and in this way its surface waters are
forced over abreast the shores of Western Europe, where it imparts its
warmth to a wide region, and also causes a high sea-level. A portion
of the waters of this high sea-level turn southward to replenish the
waters which have been moved by the trade winds from the eastern
tropical North Atlantic over into the Caribbean Sea and Gulf of Mexico,
while its northern and smaller portion mingles with the Arctic Ocean
waters north of Europe. These latter waters, having escaped from the
westerly wind-belt, and acquired a high sea-level, and also made cool
on mingling with the icy arctic seas, lose a part of their bulk on
becoming chilled by sinking and returning in under-currents to the seas
from which they were forced by the south-westerly winds; while the
larger remaining surface waters set across the Arctic Ocean over to
the northern coast of Greenland, and so down the east and west coasts
of that large island to the low sea-level abreast the American coast,
where the cold waters not only crowd the Gulf Stream from the shore,
but they also sink under it, and form the vast bed of cold water over
which the Gulf currents flow. This cold underflow of water southward
probably joins the deep antarctic currents south and south-east of the
Bermuda Islands, and returns to the tropical latitudes a portion of the
water that is carried into the Arctic Ocean by the Gulf Stream.
There are times during the late summer and early fall months when the
arctic channels are considerably obstructed by icebergs, and the low
sea-level of Davis Strait and Baffin’s Bay, with the assistance of
occasional south-east winds, is able to attract the temperate waters
of the Atlantic as far north as the Arctic Circle. Also from the same
cause the icy waters which flow down the east coast of Greenland are
attracted along its southern and south-western shores into Davis Strait.
Yet at the same time the icy waters which flow from Smith’s Sound and
other arctic channels move in a counter-current down the westerly
side of Baffin’s Bay and Davis Strait, and so carry the icebergs and
field-ice past Labrador and Newfoundland well on to the borders of the
Gulf Stream. And, according to Lieutenant Maury, the westerly gales of
the winter months force the temperate waters of the Atlantic, which
pertain to the Gulf Stream, several degrees away from the south-east
coast of Greenland. Therefore, during such seasons the surface waters
of the returned arctic currents, which flow down the east coast of
Greenland and Davis Strait, are drifted past Southern Greenland and
Iceland, and so onward into the arctic seas, north of Europe. Thus the
arctic waters maintain an independent circulation sufficient to largely
exclude the Gulf Stream from the arctic seas, and surround Greenland
with an arctic temperature; and it is on this account glaciers have
formed on Greenland and other arctic shores, and such glaciers are
probably increasing, as every iceberg launched from the frigid lands
and floated to the lower latitudes lowers somewhat the temperature
of the North Atlantic, and so causes conditions favorable for larger
accumulations of ice on the arctic shores.
Yet it is probable that an ice period extending over the northern
temperate zone could not be perfected by this process alone, should the
tropical and southern oceans maintain their present temperature. But,
with the assistance of a frigid period in the southern hemisphere to
cool the ocean waters, and thus lower the temperature of all tropical
currents, including the Gulf Stream and Japan currents, an ice age
could be brought about in the northern hemisphere equal in intensity to
the glacial periods of the past.
And, when we know that a considerable portion of the heat carried into
the northern latitudes by tropical streams is largely derived through
the mingling of the waters of such currents with the warm waters of
the southern tropical oceans, it is evident that the ice periods of
the northern and southern hemispheres were concurrent; although the
culmination of the northern frigid period would be somewhat later
than the perfected southern ice age, on account of the northern seas
requiring the assistance of the cold oceans of the southern hemisphere
to perfect a northern ice age.
The small area of the northern seas, compared with the southern oceans,
and the wide mingling of the ocean waters of the hemispheres, make it
evident that the comparatively scanty northern seas could not bring
about or maintain either a frigid or mild period in opposition to the
superior oceans of the southern hemisphere.
On the consummation of an ice period in the northern hemisphere heavy
glaciers covered the larger portion of its continents and islands,
which added so much weight to the northern lands as to attract the
waters of the southern oceans into the northern latitudes, as I have
before explained.
Thus, when the ice was mostly melted from the lands of the southern
hemisphere, the heavy ice-sheets that remained on the extensive
northern lands would still continue to attract the warm waters of the
southern seas into the northern oceans; and in this way the Japanese
and Gulf currents would gain a higher temperature and greater volume,
and thus add to their ability for melting the northern glaciers
wherever they were able to flow, and so hasten the growth of a mild era
in the northern hemisphere.
And it seems reasonable to suppose that there was more water in the
northern hemisphere on the ending of its ice period than at this age;
yet it appears that it was returned to the southern hemisphere during
a short period by the prevailing winds in the manner which I have
previously explained.
Therefore, there are but few traces of such flowage to be found in the
glacial drift, especially with the scarcity of marine life after the
rigor of a frigid age.
An article in _Science_, July 5, 1895, written by Agnes Crane, states
that Professor Joseph Prestwich has recently contributed a suggestive
memoir on this subject to the Philosophical Transactions of the Royal
Society. It treats of the evidence of a submergence of Western Europe
and the Mediterranean coasts at the close of the glacial period; and in
a previous paper communicated to the Geological Society of London, in
1892, the author gave evidence, deduced from personal observation, of
the submergence of the south of England not less than a thousand feet,
at the close of the glacial epoch.
Since that time the flood of water which flowed all of the low lands
of the high northern latitudes has been returned to the southern seas,
because of the force of the prevailing winds in connection with the
great oceans which open so widely toward the south, the force of the
winds being assisted through the attraction caused by the difference
of temperature in the surface waters of the vast southern temperate
oceans and the antarctic seas, and in this manner bringing about the
geographical conditions of to-day which favor the return of another ice
age.
It is said by those who attribute the great currents of the ocean to
the rotation of the earth that the winds have little to do in causing
such currents as the Gulf Stream. But my impression is that the
southern portion of the Gulf Stream waters, after being drifted by
westerly winds over abreast Europe, are attracted to the low sea-level
in the vicinity of the Canary Islands, to be moved by the trade winds
toward the equatorial calm belt and the West India Islands. And during
my many months’ cruising over these seas I have had my attention
directed to the singular action of the surface waters, while being
impelled by the trade winds toward the West India sea; for during the
first fifteen hundred miles of their passage they are moved by the
prevailing easterly winds without much apparent resistance or unusual
disturbance. But on nearing the longitude of Cape St. Roque, and
having acquired a high sea-level from which there is no easy or wide
outlet, the impelled surface waters begin to rebel against the forceful
winds, and cause a remarkable commotion in the shape of tide-rips and
white-capped ripples, which extend from the equator in a northerly
direction to the latitude of about 19° north, thus crossing the central
portion of the north-east trade-wind belt, with a breadth of over three
hundred miles, as shown on map No. 2.
This disturbed region where the winds and waters conflict is the
probable fountain-head of the Gulf Stream. The reason why the surface
waters of this disturbed portion of the Atlantic do not flow peacefully
along through the West India passages into the Caribbean Sea and Gulf
of Mexico is because of their narrow outlet at the Florida channel. For
it is mainly through this narrow channel that the vast waters of the
tropical high sea-level are attracted to the low ocean-level of the
Western North Atlantic.
Thus it seems that the great fountain-head of the Gulf Stream is
situated between the wide tide-rips and the Caribbean Islands. The
waters from this high ocean-level enter the Caribbean Sea mainly
through the several passages south of Guadeloupe; while the northern
portion of the raised waters set mostly toward the north-west, and so
unite with the eastern portion of the Gulf currents after they enter
the Atlantic. Still, the great high sea-level which presses against
the Windward Islands, being somewhat higher than the Caribbean Sea,
forces its waters through the island passages in quantities sufficient
to supply the Gulf Stream; and there are times when the winds are so
strong and favorable that all of the passages east of Cuba conduct
water into the Caribbean Sea, the cold under-waters entering the deeper
channels as well as the warm surface waters. Yet the currents setting
through these numerous channels are subject to fluctuations, and so
also is the Gulf Stream which they supply.
That portion of the high sea-level south of Guadeloupe receives
considerable assistance as a feeder for the Gulf Stream through being
connected on the south by the great high sea-level abreast Brazil and
the great high sea-level of the equatorial calm belt. The latter high
level is caused by the trade winds, which generally blow briskly down
the coast of Sahara, and also further off shore, and ending south of
the Cape Verde Islands somewhat abruptly in the equatorial calm belt.
The south-east trades which blow over the Eastern and Middle South
Atlantic terminate on the southern side of the calm region. Therefore,
the two trade winds impel the surface waters of the tropical Atlantic
from opposite directions directly toward the calm belt, and so raise
its waters above the common level of the sea.
This is the opinion of the writers of the South Atlantic Directory.
Still, it is probable that the high ocean-level of the calm belt is
but slightly raised above the common level of the sea, on account of
the trade winds having to contend against the tendency of the warm
tropical surface waters to move toward the polar latitudes. The calm
belt expanse which extends from Africa, where it attains its greatest
width, gradually narrows as it extends westward to the longitude of
Cape St. Roque, where it attains its highest sea-level, on account of
the borders of its narrowing space being impelled westward by the trade
winds.
The movement of the waters of this high ocean-level is mostly toward
the west, forming a portion of the equatorial current of the Atlantic.
The reason of its western movement is on account of its raised waters
being able to supply a portion of the Gulf Stream with water which is
sent off in a westerly current along the South American coast, west of
Cape St. Roque into the Caribbean Sea; while, on the other hand, it
joins with the great high sea-level abreast Brazil, and so unites with
its great southern current. The gradient of the high sea-level of the
calm belt on its southern side probably extends south of the equator,
on account of the south-east trades being weak in latitudes near the
equator; while on the north side the north-east trades generally blow
brisk and end more abruptly, so producing a gradient of less width than
that of the South Atlantic side.
It does not appear that the seas of the high northern latitudes gain
an undue proportion of the tropical Atlantic waters, because of the
south-east trades extending north of the equator, on account of such
winds being weak, and the waters of the high sea-level of the Western
North Atlantic having narrow and otherwise obstructed passages leading
to its northern seas. Yet the high sea-level of the equatorial calm
belt is always ready, whenever a favorable grade is formed by a monsoon
or otherwise, to run off its surplus water obtained by winds and rain;
and I have noticed, while cruising in these seas, that it happens at
times during the northern winter months when the north-westerly gales
drive the surface waters of the North-western Atlantic toward the
tropical zone, and at the same time a strong north-east monsoon is
prevailing along the southern coast of Brazil, the westerly currents
setting past the Amazon River are reversed, and set to the south-east,
while such conditions last.
For, when the summer solstice is in the south, and the north-east
monsoon moves southward along the coast of Brazil, much equatorial
water moves off in that direction; and during the same season the
cooled Sahara has an outward flow of air toward the south, which
moves more or less water from the coast of Guinea, which is easily
accomplished, because the warm surface waters of that coast are
inclined to join with the south equatorial stream. Consequently, the
waters move from their high sea-level north of Cape Palmas, and so form
the Guinea current.
The high sea-level of the equatorial calm belt of the Atlantic contains
a large portion of the conserved heat of the tropical Atlantic, which
at this age sends off a somewhat limited supply of warm water to
the Gulf Stream, and also to the Brazil current. But, whenever the
Cape Horn channel is closed or much obstructed, so causing a great
low sea-level in the Southern Atlantic, the tropical waters heaped
against Brazil, and the raised waters of the great calm region being
one continuous high sea-level, would mostly be attracted to the vast
low sea-level of the southern ocean. Hence it will be seen how large a
portion of the conserved heat of the tropical Atlantic would be used to
warm the high southern latitudes during a warm period in the southern
hemisphere, and at the same time the head-waters of the Gulf Stream
would obtain the same height as now. For we now see much of the force
of the north-east trade winds lost, while maintaining so large a high
sea-level to the windward of the West India Islands, which is probably
capable of supplying a stream of double the capacity of the gulf
current which passes through the Florida channel.
And it appears, while viewing the vast reservoirs of warm water
apparently gathered by trade winds to subdue the cold of the high
latitudes, that much of the energy of such winds is now lost to the
world, while maintaining a vast and pent-up high sea-level which has
a difficult outlet to the northern seas, and no strongly attractive
low sea-level to move its waters into the oceans of the high southern
latitudes. The wide waters which are banked up to the windward of
the West India Islands, and cause the wide tide-rips, set mostly
to the westward into the Caribbean Sea through the passages south
of Guadeloupe, while the northern portion of the raised waters set
mostly toward the north, and thus form the eastern boundary of the
Gulf Stream, and comprise the inner circle of the great current that
encircles the Sargasso Sea.
I have been informed by an old Barbuda fisherman that “the weeds which
float on the surface of the Sargasso Sea grow in large quantities
on the bottom of the shoal waters to the north and eastward of that
island and Antigua.” Consequently, the currents of that region carry
such weeds as become detached from their places of growth into the
higher latitudes, where the westerly winds in the winter season drift
them eastward south of Bermuda, until finally the central area of
their gathering, where the most dense collection of weeds is found, is
situated near the tropic of Cancer, and about 55° west longitude, as
shown on map No. 2.
This position is also the centre of the great circular currents which
encompass the Sargasso Sea. The comparatively few weeds which enter
the Gulf Stream abreast Florida are currented to the northward of
the Bermuda Islands, and from thence drifted by the westerly winds
to the south-west of the Azores before entering the trade-wind belt.
The weeds, on their long drift from their native shoals, hold their
freshness, and continue to grow while floating on the sea for a
considerable time, but at length lose their renovating properties, and
in certain areas of the sea acquire an appearance of age and decay.
The Gulf Stream, and such other tropical waters as are attracted
northward to the low sea-level abreast the North American coast, pass
into the westerly wind-belt, and so gradually become drift currents,
while being forced by the winds over to the European side of the ocean,
as we have previously shown.
The vast movement of the North Atlantic waters encircling the great
Sargasso Sea has often been pointed out by writers on the subject. But
the central and most dense portion of the vast sea of weeds has always
been placed on the charts several degrees of longitude east of its true
position.
It is fifteen years since I wrote of the Gulf Stream and arctic
currents as being attracted to a low sea-level caused by the westerly
winds. But, as far as I know, writers on the Atlantic currents have
had nothing to say of the great low sea-level caused by the westerly
winds blowing the surface waters of the North Atlantic away from the
eastern coast of North America, from Georgia to Newfoundland, and thus
attracting the arctic and Gulf Stream waters in opposite directions,
fifteen hundred miles along the North American coast. For, were it
not for this low sea-level, the Gulf Stream would not be able to move
so far northward as it now flows, but would spread out, were there no
unevenness in the sea-level of the Atlantic, and become a drift current
far south of its present northern limits. The United States government
has caused surveys to be made of the Gulf Stream, and the interesting
discoveries thus obtained have all been laid before the public. Still,
such surveys cover but a portion of the whole round of the vast
movement of the Gulf Stream water, and do not refer to the vast high
sea-level of the calm belt as being one of its feeders, or to the wide
disturbance of the surface waters of the tropical North Atlantic in
their conflict with the trade winds, while being forced to the vast
high sea-level of the Caribbean Sea and Gulf of Mexico, and so giving
head to the Gulf Stream.
Thus from the foregoing explanations it will be seen that the ability
of the prevailing winds to move the surface waters of the ocean away
from the weather shores of continents over against the opposite leeward
shores in the different wind-belts of the globe, and so cause both high
and low sea-levels, is the main reason why there is an interchange of
surface water between the tropical and colder zones sufficient to carry
heat from the tropics to the cooler regions, and thus largely affect
the temperature of the higher latitudes.
The unmistakable traces of cold periods having occurred in both
hemispheres have given rise to an ingenious astronomical theory to
account for their origin. According to this theory the ice periods
in the two hemispheres were consecutive; and it is admitted by its
supporters that, should it be shown that the frigid periods in the
northern and southern hemispheres were concurrent, the astronomical
doctrine would have to be abandoned.
It is impossible for a person who is acquainted with the great surface
currents of the several oceans to conceive how a mild period could be
maintained in the northern hemisphere with a frigid period existing
in the southern hemisphere. A frigid period in the latter hemisphere
necessitates a cold temperature for the superior oceans of the globe
south of the equator. With this vast area of water reduced to a
chilling temperature, it seems impossible for the inferior waters of
the northern latitudes to maintain sufficient warmth to favor a mild
period in the northern hemisphere, especially with both hemispheres
receiving an equal annual amount of the sun’s rays. The great Humboldt
current, having its rise in the southern ocean west of Cape Horn,
would during a southern frigid period greatly lower the temperature
of the vast equatorial stream in the Pacific Ocean. Consequently, the
Japanese stream, which branches off from the equatorial current into
the North Pacific, would be cooled to such a degree that it would be
unable to maintain the mild climate on the shores of the North Pacific
which extensive lands now enjoy. Furthermore, during a cold period in
the southern hemisphere the temperature of the Gulf Stream would also
be greatly lowered by the great South-eastern Atlantic return current,
which is caused by the south-east trade winds impelling the surface
waters of that region into the equatorial latitudes, such waters
being replenished from the common level of the southern ocean, and so
mingling the cool waters of that sea with the equatorial waters of the
Atlantic during a frigid period in the southern latitudes. And it may
be said that during such times the frigid Antarctic Ocean would send
its cold under-currents to cool the inferior northern oceans. Even
to-day the northern and southern hemispheres, through the intermingling
of the waters of the northern and southern oceans, largely maintain
a like temperature in their temperate zones. Therefore, when we
consider the certain traces of ice-sheets having formed on South
Africa and Southern Australia, and to have overrun South America above
the latitude of 40° south, thus strewing the oceans of the southern
temperate zone with ice that are now largely free from it, it seems
that the maintenance of warm oceans in the northern hemisphere during
the time of a frigid period in the southern hemisphere would be
impossible.
In order to make this statement more plain, I will again refer to the
importance of the great Humboldt current for cooling the waters of
the North Pacific during the perfection of a southern ice age. For
during such times the ocean strewed with ice west of Cape Horn, where
the Humboldt current takes its rise, would impart its coldness to the
Humboldt stream, while it was floating icebergs toward the equator. The
equatorial current of the Pacific being a continuation of the Humboldt
stream, its waters would partake of its coldness. The Japanese current,
being a large offshoot from the equatorial stream, would also possess a
lower temperature than it obtains at this age. Yet at this date, with
the southern ice-sheets confined to the antarctic lands, it does not
possess heat sufficient to prevent glaciers from flowing down to the
tide-water from mountains in Alaska.
Consequently, the Japanese stream could not maintain a mild climate on
the North Pacific coasts while a cold period was being completed in the
southern hemispheres. Therefore, under the conditions above set forth
the support of a mild period in the northern hemisphere during the
existence of a frigid period in the southern hemisphere could not be
carried out.
From what has been explained, it will be seen that the growth of an
ice period is necessarily slow, especially in its early stage, and also
that the storage of ice is carried on in both hemispheres at the same
time; but I will call further attention to the southern hemisphere,
because it possesses greater resources than the northern for the
production of an ice age.
The independent circulation of the southern ocean waters, as before
shown, turns away the tropical currents, and thus largely prevents
their warm waters from entering the high southern latitudes.
Consequently, the heat from the sun’s rays, and all other sources of
heat included, are not sufficient to prevent ice from gathering on
lands within the antarctic circle. This increasing storage of ice is
only another name for the accumulation and spreading of cold, and so
the increasing chillness goes on. The snow falls, and thus adds to the
extension and thickness of the ice-sheets; and at the same time the
spreading snow-fields reflect the heat received from the sun’s rays
into space, while the cold is retained and increased in the growing
glaciers.
The spreading ice-sheets having covered the land are able to flow into
the surrounding seas, where their outer edges become detached and form
icebergs, which float out to sea, and so scatter over the adjoining
oceans. Thus their coldness is mingled with and largely preserved by
the sea, while the surface water, which is carried into the southern
latitudes from the northern oceans by the prevailing winds, and also
such surface waters as are attracted into the antarctic seas because
of the difference of temperature of the antarctic waters and the
more northern seas, are on gaining the frigid latitudes made cool,
and returned to the more northern seas in cold under-currents, and
so chilling the vast under-waters of the great oceans of the globe,
and eventually their wide surface waters also; and so the coldness
increases until the ice-sheets which at first formed on polar lands are
enabled to spread slowly toward the equatorial regions so long as the
independent circulation of the southern ocean is maintained.
But at length the depth of the great southern ocean is diminished
because of the water evaporated from its surface, and precipitated in
the shape of hail and snow over the vast continents and islands of the
high northern latitudes, thus adding sufficient weight to the northern
lands to attract the waters of the southern seas and still further
lessen their depth. Thus during such times the Cape Horn channel is so
reduced as to be obstructed by the heavy glaciers and icebergs of an
ice age.
Consequently, a great change is wrought in the circulation of the
southern seas. For, when the Cape Horn channel is closed, the westerly
winds employ their strength to force the ocean’s surface waters away
from the glaciers which have filled the diminished channel. This
potent action of the winds necessarily creates a great low sea-level
on the Atlantic side of the obstructed strait, sufficient to attract
the tropical waters heaped against Brazil by the trade winds, and the
waters of the high sea-level of the equatorial calm belt, and also the
equatorial waters which set along the east coast of Africa, well into
the southern seas.
It will thus be seen that the conditions for the circulation of the
tropical ocean waters have met with a great change.
But the temperature of the waters has been lowered by the coldness of a
frigid period; and, consequently, their capability for conveying heat
to the high latitudes has largely diminished. Therefore, their first
inroads in the higher latitudes make small impression on the icy seas,
so the early process for melting ice is exceedingly slow. But the icy
southern ocean, deprived of its independent circulation, in the course
of time yields to the warming invasion of the tropical waters, whose
wide and increasing spread is eventually able to bring about a mild
period, according to the natural methods which I have explained in the
preceding pages.
And it may be said that a mild period succeeding a glacial age gained
sufficient warmth to melt the ice-sheets from all lands excepting the
highest mountains. For it is probable that there are lands situated in
the antarctic circle sufficiently elevated even during late Tertiary
times to have been above the snow-line. Therefore, the glaciers on
such lands could not have melted away during mild periods succeeding
an ice age. For, as has been explained, a portion of the waters of the
southern seas had moved into the northern hemisphere. Consequently, the
antarctic lands were raised higher above the sea-level than at this
age. Hence the area of lofty land was increased above the snow-line.
And, according to Dr. James Croll’s estimate, the ice-sheet at the
south pole is at this age several miles in thickness. Therefore, its
upper surface is above the line of perpetual snow, and could not be
melted away during the warm eras succeeding glacial periods.