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Title: The Coming Ice Age
Author: Taber, C. A. M.
Language: English
As this book started as an ASCII text book there are no pictures available.
*** Start of this LibraryBlog Digital Book "The Coming Ice Age" ***
[Illustration: No. 1.
THIS MAP SHOWS THE SPREAD OF GLACIERS AND ICEBERGS, AND THE EXTENSION
OF SOUTHERN LANDS DURING ICE AGE AND ALSO THE DIRECTION OF WINDS AND
OCEAN CURRENTS.]
[Illustration: No. 2.
THIS MAP SHOWS THE SPREAD OF GLACIERS AND ICEBERGS AND THE DIRECTION
OF WINDS AND CURRENTS AT THIS DATE.]
THE
COMING ICE AGE.
BY
C. A. M. TABER.
BOSTON:
GEO. H. ELLIS, 141 FRANKLIN STREET.
1896.
COPYRIGHT, 1896.
BY C. A. M. TABER.
GEO. H. ELLIS, PRINTER, 141 FRANKLIN STREET, BOSTON.
PREFACE.
The explanations given in the following pages, in which I have sought
to show the manner in which an ice age is being brought about, is an
extension of a treatise on “The Cause of Warm and Frigid Periods,”
which I published in a small edition in 1894. And, from the small
number of copies circulated, only a few came to the hands of persons
particularly interested in such matter. Yet there were instances of
its having proved of special interest to persons celebrated for their
geological attainments, and also to instructors in physical geography.
Besides, it received considerate notice in some of the leading reviews.
Being thus somewhat encouraged, and thinking that the subject was too
important to be neglected, I have given it further study during the
last year, and meanwhile have obtained additional information from
recent discoveries which has served to corroborate my views. Hence I
have been able to be more explicit in my explanations in the present
volume than in my earlier writings. Still, while acting as a pioneer in
the matter, it will be seen that I have only attempted to expose the
main outlines, as my age and failing health will not permit me to enter
into the voluminous details necessary for a full explanation. In order
to show why my attention has been turned to the great climatic changes
which have taken place during past ages, and now threaten the future, I
will repeat the introduction of my earlier publication, wherein I wrote
that “the reason why I have undertaken to explain the causes which have
brought about the warm and cold epochs is because of my being unable
to harmonize the several theories that have been published with the
general mode of action which nature pursues to-day. Having in the early
part of my life been employed for a score of years in the whaling
service, during which time my sea voyages were passed in cruising over
the North and South Atlantic, and over the Indian Ocean, from latitudes
north of the equator to the southern shores of Kerguelen Land, and
along the seas of Southern Australia, I also, in my searching, cruised
over the Pacific Ocean from the icy seas south of Cape Horn to the
northern latitudes of Alaska, and, from New Zealand in the Western
Pacific to the numerous islands in the tropical zone. And it may be
said that among the chief things to be learned on such voyages was the
direction of the prevailing winds and surface currents of the sea. Thus
the impressions then received were in mind when, in after years, I had
my attention drawn to the several theories advanced for explaining the
causes which produced the warm and frigid epochs. But, so far as my
marine experience goes, such theories have not harmonized with nature’s
mode of operating at this age of the world. Therefore, I have conceived
views which, to my mind, are more agreeable to the simple operations of
nature of which I have long been witness. Consequently, I have written
several short essays on climatic changes since 1880, and also letters
relating to the same subject, which have been published in _Science_
and _Scientific American_. But the space allowed for the introduction of
such matter was necessarily too limited for so wide an explanation as
the subject required. The views then advanced I have again repeated,
with the addition of several facts pertaining to physical geography,
which, so far as I know, have never before been published.”
WAKEFIELD, MASS., U.S.A.
June, 1896.
CONTENTS.
CHAPTER I.
PAGE
CAUSE OF COLD AND MILD PERIODS, 9–36
Traces of ancient glaciers in temperate zones, 9;
prevailing winds the main cause of the circulation of
the ocean waters between the tropical and temperate
zones, 10; general direction of prevailing winds, and
how, in connection with continents, they circulate the
surface waters of the sea, 11; high and low sea-levels;
separation of antarctic lands from South America, 12;
Captain Larsen’s discoveries in antarctic regions,
13; how low lands south of Cape Horn were submerged,
13; how the winds move more surface water southward
than northward, 14; Dr. Croll’s views on winds and
ocean currents, 16; under-currents of the ocean, and
how caused, 16; Gulf Stream currents, 17; antarctic
under-currents, 18; why the winds were able to force
more of the ocean waters southward than northward
at the close of the Tertiary age, 19; Mr. Alfred R.
Wallace’s views on Tertiary seas, 20; how the Cape Horn
channel affects the ocean currents, 21; cause of the
increase of cold in southern latitudes, 22; how the
Cape Horn channel is closed during ice age, and its
effect on ocean currents and temperature of southern
latitudes, 24; the melting of glaciers from southern
lands, 27; a salt sea requisite for circulation during
ice age, 28; direction of surface currents in southern
seas, 29; Humboldt current, 30; Agulhas current, 32;
temperature of arctic ice, 34; movement of southern
icebergs, 35; glaciers south of Cape Horn, 36.
CHAPTER II.
HOW ICE PERIODS IN THE NORTHERN HEMISPHERE ARE BROUGHT
ABOUT, 37–54
Northern seas during Tertiary age, 37; Gulf Stream during
Tertiary times, 38; the origin of a cold period in the
northern hemisphere, 38; remarks on Gulf Stream and
arctic currents, 39; circulation of arctic waters, 40;
arctic channels during ice age, 41; how the weight of
glaciers in the northern hemisphere attracts the waters
of the southern seas during ice age, 42; Professor
Prestwich on the submergence of European lands, 43;
the great Atlantic tide rips the head-waters of the
Gulf Stream, 44; high sea-level of Atlantic calm
region, 45; tropical Atlantic currents, 46; Sargasso
Sea, 48; arctic and Gulf Stream currents, 49; Pacific
Ocean currents, 50; slow growth of an ice period, 52;
reduction of Cape Horn channel, 53; permanence of
antarctic glaciers elevated above the snow-line during
mild periods, 54.
CHAPTER III.
SPREAD OF GLACIERS DURING COLD EPOCHS, 54–61
Spread of glaciers in tropical zone, 54; Professor Agassiz
on the origin of Galapagos Islands, 55; the bowlders
of Hood’s Island and rookery of Albatross, 56; alpine
flora of Galapagos and tropical America, 57; Mr. J.
Crawford on ancient glaciers in Nicaragua, 58; Cuba and
Republic of Colombia during ice age, 58; destruction
of animal life during glacial age, 59; temperature
of North Atlantic and Mediterranean Sea during ice
age, 60; temperature of ocean during warm epochs, 61;
generative age ascribed to warm eras; Professor Wright
on pre-glacial man, 61.
CHAPTER IV.
THE GLACIERS OF THE TEMPERATE ZONE, 62–75
Professor Hitchcock on the early history of North America,
62; glacial deposits of Nantucket and Martha’s
Vineyard, 63; Professor James Geikie on the glacial
deposits of Northern Italy, 64; California coast ranges
the work of Sierra glaciers, 65; ancient glaciers on
the Pacific slope north of California, 67; Professor
Geikie’s views on the ancient glaciers in the Salt
Lake region, 68; Colorado Cañon, 69; the conglomerate
deposits in the Appalachian district, 69; remarks on
the glacial boundaries in United States during ice age,
70; sands of Florida, 71; ancient ice-sheets of the
plains west of the Mississippi River, 73; the driftless
region of Wisconsin, 74; tropical waters of North
Atlantic chilled during ice age, 75; the drifted snow
of British America and Siberia during ice age, 75.
CHAPTER V.
REMARKS ON THEORIES ADVANCED FOR EXPLAINING ICE PERIODS, 76–93
Professor Geikie on supposed causes of the glacial period,
76; change in the relative level of the land and sea
during glacial and post-glacial times, 77; submergence
of northern lands at close of ice age, 78; the main
cause of the movement of water from the northern seas
at the close of glacial age, 79; why the earth-movement
hypothesis should be rejected, 79; glaciers of Europe
and Alaska, 80; North Pacific currents, 81; why the
Pacific waters are growing cool, 82; the lowering
temperature of the northern seas, 83; the increase of
cold in Europe and Asia, 84; falling temperature of
the Andean region, 85; General Drayson’s astronomical
discoveries for explaining the cause of ice periods,
87; why the Gulf Stream was always confined to the
North Atlantic, 89; the improbability of the Indian
Ocean currents entering the arctic seas, 90; why the
increase of glaciers must continue while the Cape Horn
channel maintains its present capacity, 91; comments on
the coming ice age, 92; tropical zone the abode of man
during ice age, 93; preservation of the tropical ocean
fauna through the glacial period, 93.
CHAPTER I.
CAUSE OF COLD AND MILD PERIODS.
It is now generally conceded by those who have given the subject much
attention that the greater portion of North America above the latitude
of 39° north to the shores of the Arctic Ocean has been furrowed and
scoured by the action of ice.
Vast traces of ancient glaciers are also found in Europe; for it is
reported that ice-sheets have left unmistakable marks of having overrun
the greater part of the lands lying between the arctic seas and the
latitude of the Pyrenees.
In Asia evidences of glacial action have been noticed from Northern
Siberia to the mountains of Syria.
The great glaciers of Himalaya have in times past attained gigantic
proportions. In Northern China huge bowlders are found scattered over
the valleys, and a long distance from the mountains.
The southern hemisphere, in proportion to the extent of its land
surface, shows ample traces of former ice action. From the latitude of
38° south to the southern extremity of the western continent there is
said to be the clearest evidence of former glacial action in numerous
bowlders scattered over the land.
On the shores of the South Pacific, from the Island of Chiloe to
Cape Horn, the coast is fringed with deep fiords, which appear to be
channelled out by ice, like the fiords of Norway and Greenland. And at
this date the mountains of that southern region are covered with snow,
and the glaciers which flow down the valleys are said to reach the
tide-water as far north as the latitude of 47° south. The glaciers of
New Zealand, now of Alpine proportions, during the ice age descended
to the sea, and channelled the deep fiords on its south-western coast;
and certain traces of glacial action have been observed in Southern
Australia, and also in the province of Natal, South Africa.
Kerguelen Land is pierced with deep, narrow fiords, which have the
appearance of having been the work of ancient glaciers.
The lands south of the antarctic circle are to-day supposed to be
covered by an ice-sheet, of which the great ice barrier surrounding
that region furnishes ample proof.
While impressed with the above reports of the work of ancient
glaciers, in connection with my own observations along the shores of
the several oceans, I have been led to seek for the physical causes
which brought about the great climatic changes of past geological
ages. And, while having the subject under consideration, I have had my
attention directed to the manner in which the great prevailing winds in
connection with continental lands are able to move the heated surface
waters of the tropical oceans into the colder zones, and also transfer
the cold waters of the higher latitudes into the tropical zones.
And it is through this grand movement of the ocean waters that we are
enabled to account for the difference in the temperature of places now
lying in the same parallels of latitude.
The natural methods for conveying tropical heat into the higher
latitudes, and also for excluding it therefrom, are so simple and
efficient that on due consideration we are able to conceive how epochs
possessing mild climates have been succeeded by periods of frigidity.
It has been admitted by several writers on climatic changes that,
should the tropical surface waters of the ocean be moved into the
high latitudes in large volume, thus adding their warmth to the heat
imparted by the sun, such combined heat would cause a mild climate.
And it has been estimated that the amount of equatorial heat moved
into the temperate and polar regions of the northern hemisphere by
the Gulf Stream alone is equal to one-fourth of all the heat received
from the sun by the North Atlantic from the tropic of Cancer to the
arctic circle. Still, it appears to me, while viewing the subject from
a marine standpoint, that the explainers of climatic changes have never
fully comprehended the manner in which the surface waters of the ocean
are moved from the tropics into the high latitudes, and returned from
the high latitudes to the tropics. Consequently, they have neglected
necessary and efficient natural agents in their explanatory theories,
and with much learning and ingenuity have laboriously sought to show
how great changes of climate could be brought about through other
causes.
But when we notice the simple methods employed by nature to-day for
transferring the heat of the tropics into the higher latitudes, and
also the manner of excluding such heat therefrom, they appear to afford
an explanation for the great changes of climate which have taken place
during past ages; for it appears that the natural manner of proceeding
by which heat is moved from the torrid zone into the high latitudes
sufficient to cause a mild climate is through the ocean currents which
are constantly set in motion by the great prevailing winds of the
globe. These winds, as is well known, blow mostly from the east toward
the west in the tropics, and from the west toward the east in the high
latitudes.
This counter-movement of the winds, in connection with a continent
extending both northward and southward from the equator over many
degrees of latitude, such as obtains on the western continent, is
abundantly able to create extensive depressions and elevations on
the ocean’s surface, and thus cause vast streams of water to move by
gravity from the high sea-levels to the low sea-levels; and in this
way the tropical waters have been moved during past ages, and to a
considerable extent are now moved far into the northern and southern
seas.
This transfer of the ocean waters is the main cause of a temperate
climate being enjoyed by countries situated in the high latitudes at
this age.
But, in order that the tropical currents should be able to flow into
the high latitudes, in quantities sufficient to cause all lands and
seas situated in such latitudes to enjoy a mild climate, it would be
necessary that the land should extend unbroken, or nearly so, from
the arctic to the antarctic circles. Thus, with a continent of such
vast extent, the westerly winds would blow the surface waters of the
ocean away from the eastern shores in the high latitudes, and so cause
extensive low sea-levels; while the easterly winds of the torrid zone
would heap the surface waters of the ocean against the eastern tropical
shores of the continent. Consequently, the warm waters of the tropical
high sea-level would be moved by gravity to the low sea-levels of the
high latitudes, even to the arctic and antarctic regions, and thus
afford them a mild climate. In this way we account for the mild climate
enjoyed on lands and seas within the high latitudes during the warm
epochs anterior to the glacial periods.
As the western continent is the only land that extends unbroken from
the equator to the cold latitudes of both hemispheres, thus affording
an opportunity for the prevailing winds to move the tropical waters
into the high latitudes, I will call attention to that portion of the
continent which extends far southward into the southern ocean, where
the winds and ocean currents have the greatest range and power to
affect the climate on different parts of the globe. Here we see South
America separated from the antarctic continent by a wide channel of
deep water, where the westerly winds blow with great force. The space
now covered by this interesting channel, owing to its being situated in
the high southern latitudes, must have been occupied by a channel of
comparatively small capacity, or else an isthmus of low land uniting
the southern portion of South America with the antarctic continent
during the warm epochs when the beds of the ancient seas of the
northern hemisphere contained a considerable portion of the water now
swelling the southern ocean.
Therefore, the obstructions which separated the Pacific Ocean from
the South Atlantic furnished opportunity for the westerly winds to
force the surface waters of the sea away from the leeward side of such
obstructions, causing a vast low sea-level, sufficient to attract the
tropical waters heaped against Brazil by the trade winds into the
southern seas in adequate quantity to cause a mild climate throughout
the antarctic regions through long periods of time.
Recent discoveries have proved that these high southern latitudes have
been subject to great changes of climate. According to the reports
from the Dundee whalers, while searching for seal in the icy seas that
surround the South Shetlands, they met with the Norwegian ship “Jason,”
Captain Larsen, who had traced the eastern shore of Graham Land to 68°
south latitude, noting two active volcanoes.
The same mariner brought from Seymour Island fossil shells and
coniferous wood of the Tertiary epoch.
These furnish sufficient evidence to show that a warmer climate once
prevailed there.
At the commencement of the glacial age the obstructions which separated
the South Pacific from the South Atlantic had become deeply submerged
by the sea, which may have been caused by a tendency of the ocean’s
waters to move southward or by a comparative small movement in the
earth’s crust. But, on account of the stability of the crust of the
earth during times so late as the glacial epochs, the submergence of
this southern region was probably owing to the movement of the ocean’s
waters from the northern hemisphere into the southern hemisphere,
which appears to have been brought about mostly through the agency of
the great prevailing winds; for it seems to have happened that the
prevailing winds on account of the disposition of the lands and seas
were able to move more of the ocean waters southward than they moved
northward during the age preceding the glacial periods. The waters
thus slowly and gradually forced into the high southern latitudes
must have deprived the northern hemisphere of their heaviness, and
added their weight to the southern hemisphere. Therefore, the waters
moved southward could not all be returned to the seas of the northern
hemisphere by gravity, for the reason that the earth’s centre of
attraction would change in accordance with the weight of water moved
from the northern hemisphere into the southern. It will thus be seen
that, while the northern seas were drained or became shallow, the
augmented southern oceans deeply submerged the region south of Cape
Horn, thus widely separating the western continent from the antarctic
lands.
Although the south-east trade winds on the eastern sides of the
Atlantic and Pacific Oceans extend further northward than the
north-east trade winds extend southward, owing to the heated tropical
shores north of the equator being more extensive than such lands
south of the equator, still, on account of the general weakness of
the south-east trade winds at the equator, and also because of the
obstructing northern lands, they have during remote times, and at this
age, been largely prevented from impelling the surface waters of the
sea into the northern latitudes in opposition to the brisk north-east
trades. Furthermore, on account of the widening of the oceans as they
extend southward, the surface currents setting in the latter direction
have more broad and easy passages than the great currents setting
northward.
Moreover, the great currents setting southward on the western sides
of the oceans south of the equator are also much assisted during the
southern summer months by the strong north-east monsoons which prevail
along the east coast of equatorial Africa and the east coast of South
America as far as the latitude of 30° south.
The South African current is impelled northward by the trade winds down
the south-western coast of Africa; but it is debarred from entering
the northern latitudes by the Guinea currents, and so turned away into
the south equatorial current which flows into the Brazilian stream.
The Gulf Stream is much obstructed in its northern movement by the
narrow Florida channel and the opposing arctic currents, and also
by the trend of the North American coast eastward; while its return
current on the eastern side of the Atlantic has a much less obstructed
passage in its southern movement, and, while on its way past the Azores
and Madeira Islands, is largely assisted by the prevailing winds.
The Brazil current, with the impelling force of a strong north-east
monsoon during the summer season, has no obstruction whatever in its
southern passage until it meets with an offshoot from the great drift
current of the southern ocean.
And the same favorable conditions are obtained by the great currents
setting southward on the western sides of the South Pacific while on
their way to the low sea-levels east of Southern Australia and New
Zealand. That portion of the equatorial stream of the Pacific which
continues west across the Indian Ocean finds no open passage to the
northern seas. Consequently, it turns south along the east coast of
Africa into the southern seas.
Therefore, this current, in connection with the great currents setting
southward east of Australia, offsets the great Humboldt current setting
north along the coast of Peru.
In the North Pacific the Japanese current setting northward is
obstructed by the narrowing of the ocean; while its return current
on the American side has a constantly widening ocean on its passage
southward, and also favorable winds to impel the surface waters toward
the equator. Still, with all the facilities above mentioned for the
movement of the ocean waters into the southern latitudes, it is
probable that since the shallow seas of the northern hemisphere were
drained, or much diminished, the prevailing winds have not possessed
sufficient force to further augment the southern seas, because of the
superior weight of the land in the northern hemisphere compared with
the lands south of the equator.
It will appear to those who attribute the rotation of the earth as
being the main cause of ocean currents that I am too much given
over to the wind theory. But I have reason to believe, as Dr. Croll
has asserted, that “the winds are the principal cause of the ocean
currents, and are not due to the trade winds alone, but to the general
impulse of the prevailing winds of the globe.”
Dr. Croll also declares that “all of the principal currents of the
globe are moving in the exact direction which they ought to move,
assuming the winds to be the sole impelling cause.”
Those who think that the rotation of the earth is the real cause of
the movement of the great surface currents of the sea should explain
in some reasonable way why the Agulhas current turns west into the
Atlantic from the Mozambique stream, and why the Guinea current turns
to the east from the main tropical current of the North Atlantic; for
it seems that these two great currents move in direct opposition to the
rotation theory, while at the same time many things go to show that
they receive their motion from the winds. This view of the question
will receive further attention in succeeding pages.
It is the opinion of some writers that a difference of temperature
and density between the waters of the polar latitudes and the torrid
zone is the principal cause of the movement of the surface waters of
the ocean from the equatorial latitudes toward the polar seas, and
so returned in under-currents; and this is a favorable factor for
assisting the winds on some parts of the sea, especially in aiding the
Brazil current in moving the surface waters from the high sea-levels
abreast Brazil, and the equatorial calm belt of the Atlantic into the
southern ocean, and also for favoring the surface currents setting
southward on the western sides of the South Pacific and Indian Oceans.
Yet, whatever gravitating force it may possess for assisting the
above-named currents, it would also act against the impelling force of
the trade winds, while they were drifting the surface waters northward
toward the equator on the eastern sides of the several oceans, and also
to retard the returning surface currents, while being drifted by the
winds southward on the eastern sides of the North Atlantic and North
Pacific. Therefore, while it would seem to favor the winds in their
work on the one hand, it would act as an opposing agent on other parts
of the ocean. Still, the difference of temperature between the tropical
and antarctic seas probably does act in opposition to the wide and
brisk trade winds on the eastern sides of the great oceans south of the
equator, and so prevents their impelling the surface waters northward
to a great extent; and this seems to be one great cause of there being
less surface water moved northward than southward over the greatest
oceans of the globe.
The theory that the difference of density caused by the difference
of temperature between the polar seas and the equatorial oceans made
under-currents to flow from the polar latitudes, and meet in the
equatorial seas, can only be carried on in the Atlantic Ocean, and in a
comparatively less perfect way in the Pacific Ocean, and not at all in
the Indian Ocean.
The North Atlantic being open to the Arctic Ocean, a portion of the
Gulf Stream waters that enter it from the north-west of Europe do sink
and return southward in under-currents; and the cold waters which pass
down the east and west coast of Greenland also sink under the Gulf
Stream while on their southern movement. The meeting of these arctic
currents with the cold under-currents from the antarctic seas in the
tropical zone is probably one cause of their cold waters rising near
the surface of the sea in the torrid latitudes of the Atlantic; and
the same conditions probably obtain in a somewhat less degree in the
Pacific Ocean.
Yet it appears that the cold waters of the Antarctic occupy the largest
space in the tropical zone, even in the North Atlantic. Dr. Carpenter,
in his lectures on Ocean Currents, speaks of meeting with antarctic
water so far north as the latitudes of the West India Islands; and
he also says that all of the Pacific Ocean at its depths is supplied
from the Antarctic Ocean, as are the cold under-waters of the tropical
Indian Ocean, which extend over twenty degrees north of the equator.
Thus, from what we can learn of the antarctic under-currents, they seem
to show that they are not wholly attracted northward on account of
the difference of temperature between the antarctic and the tropical
oceans, but partly because of more surface water being moved southward
by the prevailing winds than they are able to move northward.
And it appears that, if through the winds, combined with the difference
of temperature between the antarctic seas and the equatorial waters,
and also because of the oceans widening toward the south, more surface
water is being carried southward than northward, the waters of the
under-currents so caused must rise toward the surface in the latitudes
from which they were first removed. Having called attention to the
fact that the prevailing winds are not able at this date to augment
the southern ocean waters from the scanty northern seas, because of
the preponderance of northern lands, still there is reason to believe
that even now, owing to the form of continents and oceans, and the
attraction of the tropical surface waters into the Antarctic Ocean
because of the difference of density between the warm and cold seas,
the prevailing winds of this age are able to force more of the surface
waters of the sea southward than they force northward; but, owing to
the superior weight of the land in the northern hemisphere, the surplus
surface water forced into the southern seas is returned by gravity
after being cooled by the antarctic ice, and so adding to the deep
under-currents which flow with a sluggish movement over the bottom of
the sea into the tropical and northern temperate latitudes. And in this
way the northern oceans are maintained at their present sea-level.
The cold under-currents are probably assisted in their northern
movement by whatever difference there may be in the density of the
antarctic waters over the bottom waters of the equatorial seas. But,
as such currents extend into the northern tropical latitudes of the
northern hemisphere, it seems that the winds are the main cause of the
under-currents which carry so much antarctic cold into the northern
tropical seas, because the winds have forced an undue proportion
of ocean surface water southward, to be attracted northward in
under-currents by the preponderating northern lands.
Yet, notwithstanding the superior weight of land in the northern
hemisphere, it appears that there have been periods when there was
somewhat more water in the oceans of the southern hemisphere than now;
for it is reported that a portion of the low lands of Australia show
traces of having been submerged during late geological times.
This may have happened through an increased weight in the antarctic
glaciers, which have in past ages, and probably may in future epochs,
cause more of the ocean waters to be attracted southward than now
obtains. But it is probable that an increase of southern ice would be
largely counterbalanced by the accumulation of ice on northern lands.
Yet it appears certain that since the Tertiary epoch the waters of
vast shallow seas have been moved from the northern hemisphere into
the southern. The dry beds of the ancient northern seas encourage this
opinion, while the comparatively small area of southern lands serves to
support such views.
Still, during the ages prior to the glacial periods, while the low
lands of the northern hemisphere were covered by the sea, the wide
shoal channels which submerged the lower portion of North America
afforded convenient passages for the surface waters of the ocean in
their northern movement, and so prevented the oceans of the southern
hemisphere from gaining undue preponderance.
Hence long geological ages passed away before the winds were able
to force more of the ocean waters southward than they could move
northward, and thus augment the southern ocean from the waters of the
northern seas. But the slow growth of such immense marine deposits
in the shallow seas as are found in the Florida Peninsula and other
portions of that region was at length sufficient to greatly obstruct
the passage of the Gulf currents in their northern movement, and thus
cause conditions which enabled the winds to force more of the ocean
waters southward than they could move northward after the close of the
Tertiary epoch.
Mr. Alfred R. Wallace says in “Island Life” that the seas in the
northern hemisphere during the Tertiary period covered a much larger
area than now, and extended across Central Europe and portions of
Western Asia, and the Arctic Ocean was enlarged.
As it is not likely that any portion of the waters of the sea have been
absorbed by the earth during the late epochs in the world’s history,
therefore the ocean waters have not diminished except during cold
periods, when the water evaporated from the sea was converted into ice,
and, eventually, again returned to the sea.
Thus it necessarily follows that, when the seas of the northern
hemisphere contained a much larger portion of the waters of the globe
than at this age, the seas of the southern hemisphere must have
contained proportionally less. Consequently, during such times a
portion of the shoal seas of the high southern latitudes must have been
dry land. Therefore, this must have been the condition of the shallow
sea basins in the region of Cape Horn.
Mr. Wallace also says that “many peculiarities in the distribution of
plants and some groups of animals in the southern hemisphere render it
almost certain that there has sometimes been a greater extension of
antarctic lands during Tertiary times.”
And he also asserts that the great ocean basins have not changed, and
that the form of continents has been permanent. It will thus be seen
that it was through the movement of the ocean’s waters southward that
the low lands south of Cape Horn were covered with water previous to
the frigid periods, and so caused the wide separation between the
western continent and the antarctic lands.
The Cape Horn channel thus enlarged, the continuous mildness of the
high southern latitudes which possessed the earlier ages came to an
end, and gave place to alternate epochs of frigid and mild weather.
For it appears that it is owing to the creation or enlargement of the
Cape Horn channel that it is possible for frigid periods to be brought
about, for the reason that its enlarged space of water prevents the
westerly winds from maintaining a great low sea-level in the higher
latitudes of the southern ocean; for, whenever the capacity of the Cape
Horn channel is enlarged, the westerly winds, instead of maintaining a
low sea-level on the South Atlantic, employ their force in impelling
the surface water of the southern seas around the globe. And this work
the strong westerly winds of the high southern latitudes have always
accomplished whenever the Cape Horn channel was widely open, and this
is what the winds are doing at this date.
Therefore, such waters of the torrid zone as are moved southward from
their high sea-level, caused by the trade winds abreast the Brazilian
coast, are largely turned away from the high southern latitudes. It is
true, even with an enlarged Cape Horn channel, they can always flow
along the South American coast to an inferior low sea-level, caused by
the westerly winds blowing the surface waters of the sea away from the
coast of Argentine and Patagonia; but on gaining that region they meet
the cold ice-bearing currents which turn away east of Cape Horn from
the great southern drift current to gain the same low sea-level which
attracts the Brazil water. Consequently, the ice-bearing currents from
the south, which branch off from the great southern drift current, are
able to largely turn away the warm Brazil current from the higher
southern latitudes; and, furthermore, the great southern drift current
which passes through the Cape Horn channel, and so onward around the
globe, also partly turns away the Mozambique current as well as the
East Australian current, and so largely prevents their waters from
warming the southern seas.
Therefore, it is evident that, whenever the Cape Horn channel obtains
sufficient capacity to give an independent circulation to the southern
ocean, the conditions are favorable for the increase of cold in the
southern latitudes. For it is because of the large exclusion of the
tropical waters from the southern seas that ice-sheets have been
able to form in early periods and in later epochs on the antarctic
lands, and store away the annual frosts for thousands of years, and
at the same time furnish icebergs sufficient to chill the waters of
the southern temperate oceans, and consequently make cold such of the
surface waters of the sea as are forced into the southern latitudes by
the winds in surface currents, and so returned to warmer seas in cold
under-currents, and thus with such frigid combinations bring about cold
periods.
Thus it appears, as I have previously shown, that it is owing partly
to there being more of the surface waters of the sea forced southward
by the prevailing winds than they impel northward that the cold
under-currents are maintained; but it also requires an independent
circulation of the southern ocean, such as I have pointed out, to cool
its surface waters before they can sink and form cold under-currents.
And there is reason to believe that such cold under-currents are more
efficient in lowering the temperature of the temperate and tropical
oceans than even the icebergs which such under-currents move into the
temperate seas. And, when it is considered that the cold antarctic
under-currents fill the depths of the Pacific and Indian Oceans in the
northern hemisphere, and also largely the tropical depths of the North
Atlantic, I am led to believe that the frigid conditions of the ice
age were concurrent in the northern and southern hemispheres. The main
reasons for such belief I will explain in the following chapter.
After the foregoing explanations, showing how frigid periods are
brought about through the independent circulation of the southern ocean
surface waters, it is evident that, whenever through a slow natural
process the Cape Horn channel is closed, a great change is wrought in
the circulation of the southern ocean.
For instead of the westerly winds blowing the surface waters of the
southern seas constantly around the globe, and so turning away and
preventing the entrance of the tropical currents into the high southern
latitudes, the strong westerly winds, whenever the Cape Horn channel
is closed or greatly obstructed, would blow the surface waters away
from the Atlantic side of the closed channel, and so cause a great low
sea-level, sufficient to attract the ocean waters of the tropical high
sea-level abreast Brazil well into the southern seas. Therefore, it is
important to trace nature’s slow methods of closing the wide Cape Horn
channel at the perfection of an ice age.
In my previous explanations on the subject I have thought that, should
the southern seas have remained at or near the same sea-level as now,
through an ice period brought about in the manner I have described,
ice-sheets would accumulate on the antarctic continent, and also on the
southern lands of South America, sufficient to flow out into the sea
and close the Cape Horn channel.
But further consideration shows the impossibility of the southern seas
having maintained their present sea-level during the growth of frigid
epochs which have left such ample traces of glaciers having extended
widely over the lands of the high latitudes of both the northern and
southern hemispheres. For it appears that the larger areas of land in
the northern latitudes, embracing wide continents and large islands,
must, during the growth of a frigid age, have increased the spread of
glaciers many times greater in extent than could be obtained on the
smaller lands of the high latitudes of the southern hemisphere.
For it is evident that the water evaporated from the sea and deposited
in snow on the large continents and islands of the high northern
latitudes during the growth of an ice period would, while thus
diminishing the ocean waters, greatly increase the weight of northern
lands. Therefore, the waters of the diminishing seas of the southern
latitudes would be attracted into the northern oceans in opposition to
the prevailing winds.
Thus it appears that the Cape Horn channel would be too much reduced at
the perfection of an ice age to afford an independent circulation for
the southern ocean, even without being filled by glaciers to the extent
I have pointed out in previous essays. Still, to whatever dimensions
the Cape Horn channel might be reduced at the perfection of a frigid
period, the enlarged shores bordering its diminished waters would be
covered by heavy glaciers that would flow into the shrunken strait, and
so close it effectually. Thus the reduction of the Cape Horn channel
during the advance of an ice age seems, on close consideration, to be a
simple operation of nature, which in the normal course of events must
have taken place.
As the closing of the Cape Horn channel has been considered by
reviewers the weak and questionable point in preventing my views from
gaining acceptance, it becomes necessary to be explicit concerning the
manner in which the Cape Horn channel has in past ages been obstructed.
According to the charts prepared by John James Wild, the middle portion
of the strait is represented as being over a thousand fathoms in depth;
but, as far as I know, its true soundings have never been determined.
The deep portion of the mid-channel is described as being narrow
when compared with its whole breadth from Cape Horn to the antarctic
continent.
And, when it is considered, with the growth of an ice age, how much of
the ocean waters would be stored in the vast ice-sheets of the northern
hemisphere, and consequently because of their weight a large portion of
the diminished southern oceans would be attracted into the northern
seas, it seems that the bottom of the shoaler waters of the Cape Horn
channel, which now comprise so large a portion of its breadth, would be
raised above the surface of the sea.
The one-hundred-fathom depth south of Cape Horn, now supposed to extend
from longitude 70° west to 55° west, and southward to the latitude of
57°, would be a land supporting heavy glaciers for six hundred miles
along the north side of the reduced channel during the advanced growth
of a frigid age; and the same conditions would be obtained in the
vicinity of the South Shetland. And when, in addition, we contemplate
the great snow-fall of that region, and the consequent gathering of
glaciers which would occur on the widened shores of the lessened
channel, and the certainty of their flowing into the diminished strait,
together with the immense icebergs of such an age grounding in the
shoaled waters, it seems that the complete obstruction of the reduced
channel would be accomplished.
While contemplating the conditions that would obtain while the Cape
Horn channel was being reduced, it will be seen that the independent
circulation of the icy southern ocean would be carried on to a
considerable extent even after the narrowing strait was no longer
able to afford space for wide drift currents, for the reason of the
strong current that would be caused on account of the high ocean-level
maintained by the westerly winds on the Pacific side of the diminishing
channel, and the great low sea-level that would take place on its
Atlantic side.
Still, as previously shown, it seems that during an advanced stage of
the frigid epoch, the heavy glaciers from the enlarged northern and
southern shores of the shrunken channel, together with the ponderous
icebergs, blocking its waters, the closing process would at last be
speedy and effective.
And on further consideration it might be said that a channel of much
less width and depth would not have been of sufficient capacity to
have caused ice periods so wide-spread as those that have left their
traces on the continents and islands of the globe, for the reason
that the independent circulation of the southern ocean would not have
been sufficiently complete and long continued to have brought such
world-wide cold periods to perfection.
With the Cape Horn channel closed, as above explained, there would be,
as I have asserted, a great change wrought in the circulation of the
southern ocean; for instead of the westerly winds blowing its surface
waters constantly around the globe, and so turning away and preventing
the entrance of tropical currents into the higher latitudes, the strong
prevailing westerly winds would blow the surface waters of the sea from
the Atlantic side of the closed Cape Horn channel, and so cause a great
low sea-level, sufficient to attract the ocean waters of the tropical
high sea-level abreast Brazil well into the southern seas.
The winds of the southern westerly wind-belt being stronger in that
region than on any other portion of the globe, consequently they are
able to do nearly as much work while drifting surface water as the belt
of westerly wind of greater width on other parts of the southern seas.
Thus a person who has had a long experience with the forcible westerly
winds of the southern ocean can well understand their ability for
disturbing the ocean waters in the latitudes of the Cape Horn channel.
The drift currents of this region are moved by the winds and waves
from one to four miles an hour. Therefore, with the Cape Horn channel
closed, there is nothing more certain than that the westerly winds
would be able to cause a vast low sea-level on the Atlantic side of
the closed Cape Horn strait, and that the waters of the high tropical
sea-level abreast Brazil would be attracted to its wide depression, as
shown on map No. 1.
The tropical waters thus attracted far southward would be cooler
than the tropical waters of to-day, owing to the great amount of
cold imparted to the ocean by the numerous icebergs of a frigid age.
Still, they would begin the slow process of raising the temperature
of the southern ocean, and would in time carry sufficient heat into
the southern regions to melt the ice from all southern lands; for, in
addition to the Brazil currents, the waters of the high sea-level of
the tropical Indian Ocean which pass southward down the Mozambique
channel would reach a much higher latitude than during periods when the
Cape Horn channel was open.
The ice periods of the northern and southern hemispheres being
concurrent, a condition which I shall explain in another chapter, makes
it obvious that during the melting of the glaciers from the antarctic
continent and other southern lands the depleted Cape Horn channel
could not gain sufficient capacity to give an independent circulation
to the southern ocean during the melting of the southern ice-sheets,
on account of the diminishing heaviness of the antarctic ice and the
greater weight of the extensive glaciers and augmented seas of the
northern latitudes. Consequently, it seems that the southern seas would
continue in a lessened state while the glaciers were being melted from
the northern hemisphere, as was the case during the melting of the ice
from the southern hemisphere; and, furthermore, during such times the
glaciers which overrun all the low lands and shoal waters of the Cape
Horn region would, on account of their position being to the windward
of the tropical currents, be the last great mass of ice to melt from
the southern hemisphere.
Therefore, it seems that the Cape Horn channel would continue closed
or greatly obstructed while the glaciers were being melted from the
lands of both hemispheres. Thus at length a mild climate would extend
over the globe, and so remain until the prevailing winds slowly forced
the surface waters of the sea into the southern ocean in the manner
explained in previous pages, thus filling the Cape Horn channel to its
present capacity, and again restoring the independent circulation of
the southern ocean.
While contemplating the conditions that would obtain during the melting
of the ice from the antarctic lands, it will be seen that the tropical
waters attracted to the great low sea-level to the leeward of the
closed Cape Horn channel would eventually enter the great bight of
the antarctic continent to the eastward of Graham Land, where Captain
Weddell sailed to the latitude of 74° south. This deep gulf, owing to
its situation, would receive the full impact of the southern movement
of the tropical currents; and, as the warm waters spread over the
wide sea-level, the westerly winds would convert them into a drift
current, and under such conditions would be driven along the shores
of the antarctic continent, past the South Indian and Pacific Oceans,
and eventually, after undergoing a cooling process from the long icy
passage, be forced against the Pacific side of the closed Cape Horn
channel and the western Patagonian coast.
While regarding the circulation of the sea during an ice age, it may
be said that the ocean’s being composed of brine was the cause of its
waters being able to circulate in frigid latitudes where fresh water
would congeal. Consequently, this is one of the reasons why successive
periods of frigidity and mildness have been brought about; for with an
ocean of fresh water, repeated epochs of cold and warmth could not have
occurred, because a sea composed of fresh water would have congealed
while circulating in the high latitudes during a frigid age. Therefore,
it required a sea of brine to maintain a liquid state during the low
temperature of an ice period.
For, while the cold of a glacial age increased, the saltness of the sea
increased also, because of the great amount of fresh water evaporated
from the ocean, and stored in ice-sheets on the great continents and
islands of the globe. Thus the briny sea was maintained in a liquid
state, while washing vast ice-fields and glaciated shores and floating
the numerous icebergs of a freezing age. The cold which radiated
from such ice-bound seas must have been severe; but meanwhile the
evaporation from the ocean was much reduced, while the saltness and
coldness of the sea increased, and so prevented the ice of a glacial
period from gaining invincible proportions before the independent
circulation of the southern ocean was arrested. Therefore, the
remaining warmth of the tropical waters after gaining free access to
the antarctic latitudes was able to overcome the accumulated cold of
that frigid region.
At this date the observant navigators who have visited the antarctic
seas report that the surface currents above the latitude of Cape Horn,
while being drifted eastward by the prevailing westerly winds, also set
toward the antarctic ice cliffs, as shown on map No. 2.
The reason why this southerly set of the surface currents becomes
noticeable above the latitude of 55° south is because the tropical
currents which set southward from the torrid latitudes on the western
sides of the Atlantic, Indian, and Pacific Oceans, although largely
turned away from the high latitudes by the westerly winds and drift
currents, are also able to send sufficient water into the great
belt of westerly winds to furnish water for the deep under-currents
setting northward from the antarctic shores. Thus the surface waters
moving from the north in order to gain the higher latitudes, after
entering the westerly wind-belt, are moved in drift currents by the
impelling winds easterly over many degrees of longitude, and also at
the same time slowly southward among the cooling icebergs, because of
the attraction caused by the difference of temperature and density
between the northern drift waters and the icy seas of the antarctic ice
barrier. Consequently, the gradual movement of the surface waters of
the westerly wind-belt southward before entering the higher latitudes
is not generally apparent; for it is after they enter latitudes where
the globe becomes much reduced in circumference that their southern
movement in the contracted seas becomes more noticeable. The impact of
this southerly current, which finds its outlet in deep under-currents,
and retards somewhat the increase of ice on the southern continent at
this date, also largely prevents the small icebergs and field-ice from
floating northward, away from the antarctic ice barrier; for it is such
large icebergs as penetrate the deep under-currents that are the best
able to move into the more temperate latitudes.
From the above explanations it will be seen that the impact of surface
water against the antarctic ice barrier when the Cape Horn channel
was closed would greatly assist the tropical waters attracted to the
great low sea-level to the leeward of the obstructed strait to wash the
antarctic shores while being drifted eastward by the westerly winds
over the southern ocean against the Patagonian coast and the Pacific
side of the closed channel, and there causing a high sea-level. This
movement of the winds and currents encircling the antarctic continent
is shown on map No. 1.
The vast, high sea-level caused by the westerly winds drifting the
surface waters against the Patagonian coast would obtain a much higher
plain, were it not that so much of the water of the great drift current
was required to feed the antarctic under-current which constantly sets
northward from the antarctic shores; yet it would be sufficient to
greatly increase the volume of the Humboldt current, which would flow
in the same direction it now flows, down the South American coast to
the equatorial latitudes, where it would become the main source of the
great equatorial stream, and thus offset the increased southward flow
of the equatorial waters through the Brazil and Mozambique streams.
The equatorial stream, with its increased volume, would also move, as
it moves to-day, across the Pacific; and, on gaining the western side,
after sending off large streams to the northern and southern latitudes,
it would pass through the East India passages into the Indian Ocean,
where it would be drifted westward by the trade winds and cause a high
sea-level abreast the east coast of Africa, and so become the source
of the great Mozambique current, which would flow southward along the
east coast of Africa, and, with the Cape Horn channel closed, would
gain a much higher latitude than it would with the channel open. At
this age, when the continuation of this great equatorial stream gains
the latitude of the Cape of Good Hope, its waters are largely turned
eastward by the great drift current of the southern ocean.
Still, a considerable portion of its waters turns toward the west,
forming the Agulhas current, which flows around the Cape of Good Hope
into the Atlantic, where it mingles with the cooler currents which
branch off from the great southern drift current; and so, in connection
with the latter, it is attracted to the low sea-level caused by the
south-east trade winds abreast the south-western coast of Africa,
and from thence moved as a drift current by the trade winds to the
equatorial Atlantic and coast of Brazil. Thus it will be seen that the
Agulhas current, even with the Cape Horn channel in possession of its
present wide capacity, serves to retard somewhat the advance of a cold
period.
The Agulhas current at this date also partly serves to replenish the
water which is forced from the South Atlantic by strong westerly winds
into the Southern Indian and Southern Pacific Oceans. For it appears
that more water is now removed by such winds from the South Atlantic
than enters it from the South Pacific, even through the enlarged Cape
Horn channel of this date; and this fact seems to favor an impression
that a portion of this enlarged channel existed prior to the glacial
periods, but with its waters so much reduced as to be unable to give
the southern ocean an independent circulation sufficient to exclude the
tropical currents from reaching the high southern latitudes in adequate
volume to maintain a mild climate in the southern hemisphere.
For previous to the glacial age, with little or no ice gathered on
the antarctic lands, it seems that a strait possessing one-half the
capacity of the Cape Horn channel of the present age could not prevent
the Brazil current and the Agulhas stream from flowing into the
southern ocean in quantities sufficient to make it impossible for
glaciers to form on southern lands.
Thus it is probable that a reduced channel separated the western
continent from the antarctic lands even in the mild eras previous to
the glacial epochs.
The Cape Horn channel, at the present age, with a capacity sufficient
to largely maintain an independent circulation for the southern ocean,
is still only one-third of the breadth of the westerly wind-belt of the
southern seas. Therefore, the drift currents do not all pass through
it from the Pacific into the Atlantic. Consequently, a considerable
portion of the drifted water turns northward west of Cape Horn, and so
forms the Humboldt current.
The Agulhas stream, which even now assists in replenishing the South
Atlantic with tropical water, would, during the perfection of a glacial
period, with the Cape channel closed, be a much stronger stream than
it now obtains with the Cape channel possessing its present enlarged
capacity, for the reason that the South Atlantic waters would continue
as now to be forced eastward by the westerly winds, while they could
not be replenished, as they are to-day, directly from the South Pacific.
Consequently, the waters of the South Atlantic Ocean would be
correspondingly reduced.
Such conditions alone would greatly increase the volume of the Agulhas
stream at the culmination of a frigid age. Therefore, the work of
subduing a frigid period in the southern hemisphere after the Cape Horn
channel was closed would not rest on the Brazil current alone, but also
on the great equatorial stream of the Pacific and Indian Oceans.
Yet during such frigid times the sources of the equatorial stream would
be greatly chilled by its two great feeders, the Humboldt current and
the returning Japanese current, both of which flow down from the high
latitudes and meet in the equatorial latitudes on the eastern side of
the Pacific, thus cooling the source of the great equatorial current.
But this latter stream, while on its long western passage across
the Pacific and Indian Oceans, beneath a torrid sun, with only
one cold feeder from the south which approaches it along the west
side of Australia, would, on its long tropical journey, be able to
obtain considerable warmth, even during an ice period, to supply the
Mozambique and Agulhas streams, and so greatly assist the Atlantic
waters in bringing about a mild period. Still, the process of subduing
the cold of the southern latitudes would be slow, even with the Cape
Horn channel closed, because of the vast collection of ice burdening
the sea and land.
Yet there were conditions that were naturally brought about to favor
the process of returning warmth; for it appears that, when the southern
ocean was made shallow because of a considerable portion of its waters
having been moved into the northern hemisphere, it will be seen that
the conditions were more favorable for the westerly winds to create
drift currents than would be the case on deeper seas. Therefore,
the high and low sea-levels caused by such winds would be greater
on a shallow ocean than would occur on deeper waters. Thus the low
sea-levels of the shallow southern sea would have strong attraction
for tropical surface waters, and so increase the thickness of its warm
drift currents, and at the same time its lessened depths would have
less capacity for the storage of cold water to reduce the temperature
of the under-waters of the tropical zone.
And, furthermore, when the southern ocean was shallow, New Zealand
acquired a longer extension of land to the north and south.
Consequently, the enlarged low sea-level on its eastern side attracted
more tropical water into the southern latitudes than now.
So, according to the conditions I have pointed out, the ice-sheets
would at length melt away, and a long period of mildness would succeed
on account of the length of time it would require after the ice
disappeared from the earth for the prevailing winds to move the surface
waters of the augmented northern seas into the southern ocean, and
again restore its independent circulation, and so, after a considerable
lapse of time, bring about the geographical and climatic conditions
existing at the present date, which can be seen on map No. 2, which
shows that a cold period has already made considerable advance in the
southern hemisphere, the southern continent and islands being covered
with glaciers, and the prevalence of icebergs as far north as the
latitude of 35° south.
Moreover, when we consider that the independent circulation of the
southern ocean is caused by the westerly winds blowing its surface
waters constantly around the globe through the open Cape Horn channel,
and so largely preventing the tropical currents from entering the
high southern latitudes, and how, in consequence, the cold is slowly
on the increase through the constant accumulation of ice on the lands
and in seas of the southern latitudes, it appears that a frigid age
is slowly progressing in the southern hemisphere. For it seems that
continental ice-sheets should not only be able to retain their freezing
temperature, but also the mean of the low temperature in which they
were formed, for a considerable length of time, and so impart their
extreme coldness in the shape of icebergs into such seas as border on
the glaciated lands.
It has been proved at Point Barrow that strata of ice and gravel can
maintain a wintry temperature through the summer months. Captain
G. B. Borden, keeper of the refuge station in that region, states
that Lieutenant Ray, of the Signal Service, excavated through ice and
gravel to a depth of forty-one feet, and that the lower portion of the
excavation maintains a temperature 15° Fahrenheit above zero the year
around. Therefore, with the probability of southern glaciers obtaining
a temperature of over 15° Fahrenheit below the freezing point, we
can well realize the frigidity imparted to the southern oceans while
melting numerous immense icebergs, and consequently will conclude that
the temperature of the southern latitudes is gradually lowering.
The icebergs of the antarctic seas would not move northward into the
temperature latitudes so readily as they now do, were it not that the
general southward set of the southern ocean currents were interrupted
by the movement of northerly surface currents in the longitudes of
the low sea-levels, caused by the westerly winds drifting the surface
waters of the sea from the eastern coasts of Southern South America and
New Zealand. For it is owing to the low sea-levels thus created, in
connection with the deep under-currents which set northward from the
ice cliffs of the antarctic lands, that many icebergs are enabled to
move into the temperate latitudes, especially to seas north-east of the
Falkland Islands.
On other portions of the southern ocean above the latitude of 55° south
the surface waters, while being drifted eastward by the strong westerly
winds, also set toward the antarctic shores, and so furnish water for
the cold under-currents which set northward from that frigid region.
Thus from such parts of the coast only the largest bergs, which require
a deep sea to float them, are moved by the under-currents into the
temperate latitudes. Therefore, it happens that, while an ice period
progresses, and the antarctic icebergs increase in size, the more
readily the cold, deep under-currents force them into the temperate
zone, in opposition to the winds and surface currents.
The icebergs, after gaining the temperate latitudes, are moved more
or less eastward by the westerly winds and drift currents, and so are
scattered over the southern temperate oceans, where the melting bergs
impart whatever coldness they were able to store up while forming in
the antarctic regions.
The low sea-levels caused by the westerly winds to the leeward of New
Zealand and to the leeward of Argentine, not only cause the ice-bearing
currents to set northward, but they also cause the tropical currents to
make considerable inroads into the high southern latitudes. This is the
reason why the lands are less burdened with ice on the antarctic shores
opposite Cape Horn than on other parts of that glaciated continent.
The tropical currents which turn southward east of New Zealand largely
mingle their waters with the great southern drift current, and so
are carried through the Cape Horn channel. Owing to this cause, the
antarctic lands abreast Cape Horn are less burdened with ice than other
portions of the antarctic shores.
Thus, were it not for this penetration of warm waters southward, the
antarctic coasts south of Cape Horn, because of the great snow-fall of
that region, would obtain heavier glaciers than other portions of the
southern continent. But the time is slowly coming when, with a lower
temperature, the ice-sheets on the lands in the vicinity of the South
Shetlands will attain greater thickness than the glaciers on other
shores of the antarctic continent.
Hence it appears that, when the several agents for producing
and distributing cold in the southern latitudes are taken into
consideration, the immense and continuous storage of ice on the
southern lands, which adds to the wide-spread fleet of icebergs that
float the southern temperate seas, and also the vast movement of
cold antarctic water into the temperate and tropical oceans in deep
under-currents, combined with the increasing coldness of the westerly
winds, are now slowly bringing about in the southern hemisphere a
period of frigidity.
CHAPTER II.
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.
CHAPTER III.
THE SPREAD OF GLACIERS DURING COLD EPOCHS.
I have before explained that the conditions are such that the cold
periods of the northern and southern hemispheres were concurrent.
Through this cause, while the glacial epoch was being perfected, the
ice followed down the mountain ranges of both hemispheres; and, while
gathering on the lands of the temperate latitudes, it also spread
over a portion of the tropical zone. It is reported that traces of
ancient glaciers are found in India, and also in Central America and
in tropical South America. In fact, the denudation caused by ancient
glaciers on the elevated lands of the tropics are too well defined to
be attributed to any process of weathering, while Alpine plants of the
same species are found near the summits of mountains in the tropics as
well as in the high latitudes of both hemispheres.
This fact goes to show that a portion of the lowlands of the tropical
zone have experienced a temperature favorable for the growth of Alpine
plants. And, judging from the tropical islands I have visited, situated
in the cold currents which flow down the eastern sides of the oceans
from the high latitudes, I think they show strong traces of having
during some remote period been subject to the action of glaciers. The
island of St. Helena, situated in the southern tropical Atlantic, has
the appearance of having been heavily iced during a frigid age. Its
steep ravines, which deepen as they approach the sea, recall to the
southern voyager the ice-worn islands of the high latitudes. It seems
improbable that these deep ravines which penetrate the hard volcanic
rock, on their short course to the sea, could have been caused by their
scanty brooklets.
The bowlders scattered over the island are not in harmony with the
weathering process, while the obliteration of its craters seems to
point to a more rapid process of erosion than could be attributed to
weathering.
Professor Agassiz, in his “General Sketch of the Expedition of the
‘Albatross,’” states that the Galapagos Islands are of volcanic origin,
and that their age does not reach beyond the earliest Tertiary period;
and his report seems to favor the impression of their having undergone
denudation sufficient to slough off large portions of the rims of the
older craters, and also the eastern face of Wenman Island. On Hood’s
Island, at the time of my visit, its crater had entirely disappeared.
The highest portion of the island, which was the probable site of
its ancient crater, showed no trace of its former existence; yet at
the foot of this low mountain, on its southern side, I saw a large
collection of loose bowlders, composed of hard volcanic rock, which
were mostly free from soil and other débris, and easily moved from
their places, while the spaces afforded by the loose piles of dark
basaltic rocks afforded a secure retreat for numerous owls and lizards.
Beyond the rocky piles to the southward a horizontal area of land was
strewn with bowlders to the sea, which was some two miles distant from
the higher land. The bowlders which covered the plain were somewhat
smaller than those at the foot of the mountain, as none of the former
were more than three or four feet in their longest measurement.
They seem to have been formed from thin strata of lava, which were
broken in pieces from pressure, such as the action of ice could
perform. In fact, the crowded and angular and somewhat worn blocks
of lava presented a different appearance from stones thrown from the
crater of a volcano, while no such bowlders are found among the recent
volcanic eruptions on the islands.
The plain so thickly strewn with bowlders, and partly shaded by a tall
growth of shrubs, fell off abruptly at the seaside, forming a steep
cliff some two hundred feet in height.
The rocky floor at the foot of the cliff received such débris as fell
from the sea-washed land; yet it contained few bowlders, they having
been washed away by the waves soon after falling.
At one place a steep, dry ravine penetrated the land from the seashore,
which was dangerous to cross on account of the loose stones resting
on its sides. Two or three miles further west, on the level land
bordering the sea, a large rookery of albatross were brooding their
eggs and chicklings. The land on the south side of Albemarle, near the
sea, consists of débris from the eroded high lands; and, judging from
the crumbling cliffs by the sea, it seems that the land at one time
extended further seaward.
Besides the excessive denudation which appears to have taken place on
portions of these bowlder-strewn lands, we have other unmistakable
testimony of their having formerly possessed a frigid temperature. The
characteristic Alpine flora of these islands points to a time when they
were exposed to a cold climate. Furthermore, rookeries of seal and
albatross, which naturally belong to shores situated in cold latitudes,
still exist on these equatorial islands; and, when we consider the
favorable position of the Galapagos for the reception of cold during a
frigid period, we can well account for the lingering signs which point
to their former cold climate.
During the perfection of an ice period the western shore of South
America was covered with an ice-sheet from the summits of its mountain
range to the sea, extending northward as far as the latitude of 38°
south.
This vast ice-sheet, situated in a region of great snow-fall, was
constantly sending icebergs into the sea, where they were borne
northward by the cold Humboldt current directly toward the Galapagos
Islands; while, on the other hand, in the northern latitudes, in
regions of great snow-fall, such as Alaska and British America,
numerous icebergs were launched into the ocean, to be currented
southward to the Galapagos seas. Thus during the frigid epoch the
equatorial waters surrounding the Galapagos group was one of the
greatest gathering places for floating ice to be found on the globe.
And here the frigidity stored up in the glaciers of the higher
latitudes was set free, thus chilling the waters as well as the
atmosphere of that region. The Alpine flora of the American coast
mountains was probably carried by floating ice to the Galapagos, while
its rookeries of albatross and seal date back to a cold period. And
it seems that these cold-weather animals, with the assistance of the
cool Humboldt current, may be able to preserve their rookeries at the
equator until the advent of another ice period. In connection with the
evidences of a cold climate having possessed the Galapagos, there are
ample traces of ice-sheets having flowed over a large portion of the
high lands of tropical America, and in some places the ice may have
flowed down to the sea, especially where the large rivers now empty;
and it is said that masses of clay, mixed with sub-angular stones, have
been found in Brazil, which goes to prove the glaciation of portions of
that tropical land during a remote age. Professor Louis J. R. Agassiz,
during his research in the Amazon valley, found bowlders resting near
the summits of the low hills of that region, which he attributed to
the action of ice. The spread of glaciers on southern continents and
islands is shown on map No. 1.
In _Science_, Nov. 17, 1893, Mr. J. Crawford published a summary of
his discoveries in Nicaragua, during ten months of nearly continuous
exploration since August, 1892.
The author of this report says: “The numerous eroded mountain
ridges and lateral terminal moraines of that tropical region give
unquestionable evidences of the former existence of a glacial epoch,
which covered an area of several thousand square miles in Nicaragua
with glacial ice. The ice-sheet covered a large part of the existing
narrow divide of land (containing about 48,000 square miles) between
the Pacific and Caribbean Sea.” And it is likely that other large areas
of tropical America were glaciated at the same time, especially in
regions of great precipitation.
The island of Cuba, during a portion of the ice age, probably
supported heavy glaciers, and obtained an average temperature as low
as South-western New Zealand at this age. According to the description
given by J. W. Spencer, of the Cuban land, great valleys have been
excavated, the lower portion of which are now fiords, reaching in one
case at least to seven thousand feet in depth before gaining the sea
beyond. Thus, while keeping in view the glacial condition of Central
America during the frigid period, it seems that the great Cuban
excavations were partly the work of glaciers of the same cold epoch.*
Judging from such reliable statements, it is probable that the climate
of tropical America during the frigid age was somewhat colder than
obtained in the tropical regions of the eastern continent, owing to
the wide connection of the Atlantic with the Arctic Ocean as well as
with the antarctic seas, and because of its shores possessing a larger
area of glaciated lands in proportion to its size than the Pacific and
Indian Oceans, and also owing to the tropical Atlantic containing so
small a portion of the world’s waters which lie within the torrid zone,
and its equatorial current being separated by continental lands from
the great equatorial stream of the Pacific and Indian Oceans.
*The meeting of the British Association for the Advancement
of Science, September, 1895, was reported in _Science_ of
October 18, where mention is made of an interesting paper by
Mr. R. B. White, on “The Glacial Age of Tropical America,” in
which he described a number of apparently glacial deposits in
the Republic of Colombia, almost under the equator. He spoke
of moraines forming veritable mountains, immense thicknesses
of bowlder clay, breccias, cement beds, sand, gravels, and
clays, beds of loess, valleys scooped, grooved, and terraced,
monstrous erratics, and traces of great avalanches.
Therefore, the tropical Atlantic waters must have been reduced to a
lower temperature during a frigid age than the tropical waters of the
Indian Ocean or the western part of the tropical Pacific, as a large
portion of the great equatorial current of the latter oceans, during
its western movement, was exposed to the rays of a tropical sun for a
much longer time, after being replenished by the cold waters of the
high latitudes, than the tropical currents of the Atlantic; and it
is probable that, on account of tropical America possessing a colder
climate than the tropical lands of the eastern continent during the
frigid epoch, the cold of the western continent was more destructive
to its fauna and flora than was the case in the tropical regions of
the eastern continent. Professor Wright, in his valuable work on “The
Ice Age of North America,” gives a good description of the “flight
of plants and animals during the glacial epoch,” and also of the
extermination of many superior species because of the frigid climate.
The high lands of tropical Africa, above the altitude of three thousand
feet, and situated in places of great precipitation, were probably
covered with snow and ice during the glacial age. Travellers have
reported that islands composed partly of granite bowlders are found
in the lakes at the head-waters of the Nile. But the glaciers that
invaded the tropical latitudes were of short duration compared with the
ice-sheets that burdened the lands of the temperate zones. Besides,
such tropical ice as flowed to the low lands was so near a melting
condition that it made small impression on the rocks; but on steep
mountain slopes, where the movement of the ice was comparatively rapid,
it possessed considerable eroding power. The climate of the tropical
zone on both continents during the perfection of an ice period was so
cold that such animals as could not endure a low temperature retreated
into the warmest regions of the equatorial latitudes, while many
species who failed to reach such places perished. And especially was
this the case with the pre-glacial fauna of the western continent. Mr.
W. B. M. Davidson, in his treatise on Florida phosphates, says: “The
great mammal hordes of the glacial epoch were driven into Florida in
their flight southward for life and warmth, and there perished because
of the deadly cold which ever moved southward. The Florida waters
grew so icy cold, fishes, reptiles, and mammoth animals died, and
added their frames and teeth to the valley of bones now found in that
southern region.”
Such species of the tropical fauna of the ocean as survived the ice age
could have existed only in torrid seas with small connection with the
cold oceans during the frigid epochs. For, with the diminished oceans
of a cold period, it seems that the conditions were favorable for the
maintenance of such seas in the region of the East India Islands.
Such parts of Southern Europe and Northern Africa as bordered on the
Mediterranean Sea probably possessed a milder climate during the ice
age than regions in the same latitudes on the Atlantic coast, for the
reason that the North Atlantic was proportionally a greater receptacle
for icebergs which were launched into it from the numerous glaciers of
North-eastern America, Greenland, Iceland, and North-western Europe
than the great inland sea obtained from its less frigid shores. And it
may have happened that during such times the tropical waters of the
Indian Ocean had some connection with the Mediterranean through the
Red Sea and Suez, and so during portions of the year the waters of the
tropical Indian Ocean were forced by the periodical winds into the
inland sea. It is the opinion of several writers that man, along with
other species of animal life, existed previous to the glacial period;
for, since the seas and lands of the globe were chilled, the conditions
seem to have been less favorable for the spontaneous generation of
animate bodies than during the previous warm ages. Therefore, it
appears that the generative ages should be ascribed to the long genial
eras prior to the glacial epochs. For it is probable that the lower
parts of the ocean, which now possess a low temperature even in the
tropical latitudes, were, during the warm eras, wholly composed of warm
water, because the surface waters of the antarctic seas of that age,
which supply the great under-currents of the ocean, would possess a
high temperature; and it is probable that the temperature of a large
portion of the seas of the torrid zone was for a long time maintained
at blood heat. For it should be considered that the waters which moved
from the torrid seas, after making their journey through the warm
regions of the high latitudes, would on their return to the tropics
retain a large portion of the heat they acquired in the torrid zone
before making their journey to the mild polar regions.
And, when we reflect how the heat of the sun’s rays was conserved by
the ocean waters, and that their circulation during such times was
almost wholly performed by the winds, as the difference of temperature
between the polar latitudes and the equator was small, it appears
that during the eras previous to the glacial age the oceans must have
obtained a higher temperature than possessed by the warmest seas of
to-day.
According to the discoveries of Professor Wright and others, ancient
stone implements have been found beneath the glacial drift, as well
as the bones of animals whose descendants are now living, which goes
to prove that man, with other species of fauna which now inhabit the
earth, existed anterior to the glacial epoch.
And on consideration it seems unreasonable to suppose that any of the
superior species of animals could have been brought into existence
since the waters and lands of the earth were chilled by the cold of
a glacial age. And it appears that many species of animals which
are known to have survived the cold periods were indebted for such
survivals to the slow process through which a frigid period is brought
about, thus affording time for evolutionary inurement to the slow
increase of cold which at length perfects a glacial epoch.
The inurement to cold acquired by animals during the glacial age is
still an attribute possessed by many species of fauna to-day. For, when
a warm climate took possession of the tropical zone, it was deserted
by a large portion of the animals that found refuge there during the
glacial age.
Thus, while the seas and shores of the cooler latitudes swarm with
animate bodies, the torrid latitudes seem comparatively lonely to the
voyagers on the tropical oceans.
CHAPTER IV.
THE GLACIERS OF THE TEMPERATE ZONES.
Having asserted that during the culmination of a frigid period the
ice-sheets spread over a portion of the lands of the tropical zone,
I will give my views, with those of several writers, on the spread
of ice-sheets within the now temperate latitudes; and meanwhile I
will repeat a portion of my former essays on the subject. Professor
Hitchcock, in his lectures on the early history of North America,
says that “the history opens with igneous agency in the ascendant,
aqueous and organic forces become conspicuous later on, and ice has
put on the finishing touches to the terrestrial contours.” But there
appear to be various opinions held by geologists respecting the changes
brought about on the earth’s surface during the glacial period. Some
think that glaciers have never been an important geological agent,
while others assert that during the glacial epoch heavy ice-sheets
covered the elevated portions of Western North America as far south
as the thirty-sixth parallel of latitude, and Eastern North America
was overspread with ice-sheets, which attained a depth of five or six
thousand feet, and were able to move their débris over wide lands of
little declivity toward the sea, their immense deposits forming the
lands of Cape Cod, and also the islands of Nantucket and Martha’s
Vineyard.
But it is now said that this implied magnitude of the glacial deposits
on the lands skirting the New England coast is without foundation,
since the larger bulk of these islands consists of upturned Cretaceous
and Tertiary strata, which are only thinly covered with glacial débris,
such as bowlders, gravel, clay, and sand, from the eroded shores of the
mainland of New England. But it appears that the dislocated and folded
cretaceous strata which underlie the glacial drift of Nantucket and
Martha’s Vineyard were during an early period deposited on the bottom
of a shallow sea, which then covered the Vineyard Sound, Buzzard’s Bay,
and their surrounding lowlands. Thus the ice-sheets of the frigid age
which moved over New England displaced the yielding stratified deposits
of the shallow sea, and forced them southward in a disturbed condition
to the position which they now occupy.
Still, it is apparent that only a small portion of the glacial drift is
found on these islands, which, according to appearances, must have been
eroded and moved southward from the rocky lands of New England during
the ice age; but there is sufficient to show that large quantities
of such débris were carried over the islands into the Atlantic. And,
judging from the eroded rocky New England lands, there must have been
sufficient glacial drift moved over Nantucket and Martha’s Vineyard
into the ocean beyond to far exceed in bulk the deranged Tertiary and
Cretaceous deposits which now form so large a portion of the islands.
For, when we look over lands bearing traces of the ice age, where
the glaciers did not move their drift into the sea, so the terminal
moraines of such glaciers can be better estimated, we can realize the
great work that has been performed by the ice-sheet that overran New
England during a frigid age.
Professor James Geikie states, in his discussion on the glacial
deposits of Northern Italy, that the deposits from Alpine glaciers of a
frigid period “rise out of the plains of Piedmont as steep hills to a
height of fifteen hundred feet, and in one place to nearly two thousand
feet. Measured along its outer circumference, this great morainic mass
is found to have a frontage of fifty miles, while the plain which it
encloses extends some fifteen miles from Andrate southward.” And it
is reported that there are found on the southern flank of the Jura
numerous scattered bowlders, all of which have been carried from the
Alps across the intervening plains, and left where they now rest. Many
contain thousands of cubic feet, and not a few are quite as large as
cottages.
Such blocks are found on the Jura, at a height of no less than two
thousand feet above the Lake of Neuchâtel. The Jura Mountains being
formed of limestone, it is easy to distinguish the débris deposited
by Alpine glaciers; and, from what I can learn of extensive glacial
work, it appears that intervening plains, lakes, and sounds are so
often found separating the source of ancient glaciers from their
deposits that their existence becomes almost necessary to represent
the general outlines of disturbance performed during an ice period. In
consideration of such facts and the foregoing statements of reliable
observers, I am prompted to offer my views on glacial work performed on
a portion of the Pacific shores of North America, which seems to me to
be much more extensive than hitherto supposed.
Professor Whitney describes the coast mountains of California as being
made up of great disturbances, which have been brought about within
geologically recent times; and this statement I found to be so obvious
in my travels over that region that it appears to me that the coast
ranges originated in a different manner from the older Sierras. The
western sides of the latter mountains everywhere show the great eroding
power of ancient glaciers; and, when I considered their favorable
position for the accumulation of snow during a glacial period, I was
led to seek for the glacial deposits adequate to represent the great
gathering of ice which an age of frigid temperature would produce.
But it seemed to me that such deposits could not be found in the
foot-hills of the Sierras, which contain the moraine of inferior
ice-sheets that terminated at the base of the mountains.
Under these conditions I came to the conclusion that during the earlier
ice period the immense glaciers which must have formed on the western
slopes of the Sierra range moved their gigantic accumulation of débris
so far seaward as to form the range of hills now existing next the
coast line, and perhaps the islands abreast the Santa Barbara coast,
the Contra Costa, or eastern range, being formed during a subsequent
ice period, in the same manner as the hills next the coast line.
Still, it may be that neither of the coast ranges was the work of a
single cold epoch; but the western range must necessarily have been the
earliest deposit. Although the coast ranges differ from the Sierras
in their make up, yet it does not disagree with the glacial origin
of the former inferior mountains, from the fact that the ice-sheets,
while moving their bulk westward, displaced the deposits of such bays,
lakes, rivers, and marshes as lay abreast of the Sierra slopes. The
advancing ice-sheets, thousands of feet in depth, moving from a lofty
and steep incline, pressed and ploughed below the somewhat superficial
cretaceous and alluvial strata which lay in their course. The disturbed
strata, while forced along in confused heaps in front of the ice, were
amassed in ridges sufficient to form the hills of the coast ranges.
The bowlders found imbedded in several of the coast hills must have
been moved by the ice from the Sierras on account of the coast ranges
not having a rocky core of sufficient firmness to give shape to such
bowlders. Moreover, the temperature of the Pacific waters would not be
favorable for glaciers to form on the coast ranges, with the ice-sheets
of the Sierras terminating at the foot-hills.
The Sacramento and San Joaquin Valleys are now covered by recent river
deposits. Therefore, the glacial drift which should be traced from the
Sierras to the coast ranges is concealed.
Yet the abraded appearance of exposed solid rocks at the base of the
foot-hills, and also the scattered bowlders which gradually disappear
beneath the diluvial deposits of the plains, indicate that the Sierra
ice-sheets could not have ended at the foot-hills, but must have moved
further westward, while gathering immense accumulations in their front,
sufficient to form the coast hills, the débris thus amassed being able
to arrest the further movement of the ice seaward.
The coast ranges in several places have been subject to igneous action,
which may have been brought about through heat generated from pressure
exerted on the interior masses after the ice had melted away, the heat
thus produced being sufficient to cause outbursts of lava, where the
nature of the material favored combustion. The low plains, lakes, and
bays which separate the Sierras from the coast hills are in a position
similar to the shallow sounds which separate Nantucket, Martha’s
Vineyard, and Long Island from the inferior slopes of the mountains of
New England. Therefore, while agreeing with glacialists, who believe
that great geological changes have been wrought by ice-sheets in Italy
and New England, it appears to me that the ancient glaciers of the
Sierra Nevada have accomplished more extensive work, owing to the
Sierras being situated in a more favorable position to receive the
humidity of the ocean.
Hence, with a low temperature, vast quantities of snow must have
collected on their lofty sides; and at the same time their great
height and declivity would cause the ice to move down their steeps
with greater force than the glaciers which passed over New England.
Writers who have given the subject considerable study think that the
deep valleys of the Sierra Nevada were produced by disruptive rather
than erosive agencies. This conclusion has been formed from the lack
of large accumulations of débris about their lower extremities, which
would not be the case if such valleys were the result of glacial
erosion. But, should the coast ranges be attributed to glacial action,
as has been stated, we can well account for the débris that should
accumulate from the erosion of the deep valleys.
The only thing that could prevent the ice from gathering on the Sierra
Nevada range during an ice period in greater masses than on any
mountains in the northern hemisphere would be the lack of cold; for,
with a low temperature, the fall of snow would be enormous. This is
shown by the great snow-fall during the short mild winters of to-day.
Therefore, with ice-sheets covering a large portion of the lands of
the high northern latitudes, and with the Japanese current which
tempers the north Pacific waters made cold in the manner described
in the foregoing pages, and while the sea along the north-west coast
of America was strewn with icebergs launched from Alaska and British
Columbia, it seems that California must also have obtained a frigid
climate during the ice age. Therefore, on account of its exposure to
the ocean winds, and the consequent heavy snow-fall, the accumulation
of ice on its lands must have been immense. For, when it is considered
that the glaciers of North America extended southward even into the
torrid zone sufficient to cover a large portion of Central America,
it is unreasonable to suppose that any portion of California could
escape being covered by heavy ice-sheets during the glacial epoch. The
comparatively scant fall of rain and snow over Greenland is known to
form ice-sheets hundreds of feet in thickness.
Therefore, what must have been the depth of ice over the high lands of
the Pacific coast north of California at the culmination of a frigid
period? The descriptions given by Dr. Dawson and others, of glacial
phenomena along that coast, favor the impression that an immense
ice-sheet at one time deeply covered the whole region from the top of
the mountain range to the ocean.
Thus all the deep channels were filled and all the islands deeply
overrun with ice, while the immense bergs launched from the shore
and carried by the winds and currents southward were probably not
melted until they reached the tropical latitudes. Thus, when the
whole circulation of the Pacific waters are taken into account, it
will be seen that their temperature during the ice age must have been
considerably lowered. The movement of ice-sheets on the Pacific slope
was probably local in character, and not connected with the movement of
ice on the eastern sides of the mountains.
From what I have seen of the vast territory lying between the Sierra
Nevada and the Rocky Mountains it appears that it obtained much heavier
ice-fields than generally supposed. Professor Geikie in his lectures
says of this region that during the glacial age, “in the Second
Colorado Canyon, the sides were completely glaciated from bottom to
top. These walls are from 800 to 1,000 feet high, and at the thickest
point the glacier was 1,700 feet thick”; and he says that “the country
around Salt Lake was covered with ice, for the rocks about there show
the action of ice, and that the bones of the musk-ox are found there.”
This vast area of ancient ice, although subject to little movement in
its interior basin, still, in whatever movement it may have had, must
have found its main outlet through the Grand Canyon of the Colorado.
For in no other way can we account for the erosive forces necessary to
excavate that immense chasm. Not even the mighty torrent that carried
off the waters of the melting ice-sheets that covered the interior
portion of the continent could accomplish work of such magnitude.
According to Professor Geikie’s observations the Second Colorado Canyon
was filled with glaciers during the ice age. Therefore, it seems that
these glaciers must have flowed down into the Grand Canyon, and there
united with glaciers flowing from more northern regions.
An account of a collecting expedition to Lower California by G. Eison,
in 1895, describes ancient moraines at the extremity of the peninsula
as being prominent, large, and steep. This region lies under the tropic
of Cancer, and 8° south of the mouth of the Colorado River where
it empties into the Gulf of California. Hence it appears that the
temperature of that portion of North America during the ice age was
favorable for the great glacier of the Colorado Canyon to have flowed
into the Gulf of California.
The wide, shallow basins of Utah and Nevada were filled with the water
from the melting ice-sheet on the breaking up of the ice period,
and the lakes so caused remained for a considerable time after the
disappearance of the ice. But, owing to the great evaporation and light
rain-fall of that region, the lakes gradually shrank away, the filling
and emptying of the lake basins being governed by the cold and mild
epochs.
The conglomerate deposits in the Appalachian district of North America
are known as occurring on a large scale. Professor Shaler is inclined
to attribute them to glacial action, because he knows of no other force
that could bring together such masses of pebbles from a wide-spread
surface. In Eastern Kentucky and East Tennessee these deposits are
found to be several hundred feet in thickness. Such accumulations of
apparent glacial origin are to be found from New Brunswick to Alabama.
Hence it seems that the ice during a frigid period followed down the
Alleghany range even so far south as Georgia and Alabama; and for a
time, when the ice attained its greatest spread, it flowed over the
central portion of the Gulf States. For how else can we account for the
clay mixed with gravel and pebbles and stony fragments being spread
broadcast over that region?
I know that such statements do not agree with the views of glacialists
who have written on the subject, and have drawn the glacial boundary
from seven to ten degrees further north, where a line of bowlders with
other glacial débris is plainly traced. Still, it appears to me that a
line of bowlders deposited by an ice-sheet spreading over a continent
and across many degrees of latitude cannot be compared to the moraines
of inferior mountain glaciers of the temperate latitudes of the present
age.
An ice-sheet moving from a high latitude to a lower would, while in the
colder latitude, freeze firmly to the rocky ledges, and hold them so
strong in its frigid grasp as to break off the weaker portions of the
rocks, and drag them toward a milder region, as far as the freezing
grip of the ice-sheet would permit; but, on gaining lower and milder
latitudes, the holding and dragging power of the ice would be lost on
account of the increased warmth of the earth over which the glacier
must pass, and also because of the ice-sheet having lost a portion of
the low temperature acquired in the higher latitudes. Therefore, on
such lines the bowlders would be released, while the ice-sheet would
still move on, although largely deprived of its eroding power.
This is the probable reason why a line of glacial débris, largely
composed of bowlders, is found to extend across the Middle and Western
States, and so generally supposed to be the glacial boundary of a
frigid period. But there is no reason to suppose that an ice-sheet,
although deprived of its eroding power, was arrested in its southern
movement on the line of its stony débris, because there could be no
sudden change of temperature in a particular latitude on the eastern
lands of North America to cause an abrupt ending of the ice-sheets.
And there appears to be nothing to hinder the ice from gathering and
flowing over lands warm enough to loosen its implements of erosion;
for there is much to show that the ice-sheets flowed much further
southward, even into the middle portion of the Gulf States, and there
spread the clay mixed with gravel and pebbles, with now and then a
bowlder, over the land. The scattered bowlders, found in numerous
instances many miles south of the bowlder line, were so deeply imbedded
in the ice-sheet that they could not be dropped on the usual releasing
ground. The ice-sheet, when deprived of its rocky, eroding implements,
would, while flowing over the land, leave few or no imprints on the
rocks; but it would probably move and spread a large amount of clay,
gravel, pebbles, and sand over its wide course, especially if the ice
moved from a region abounding with such material.
Should we place the glacial boundary on the line of the rocky débris,
how could we account for the glaciated stones found on the hills and
plains situated far southward of the bowlder-strewn regions of the
Middle and Western States? The clay mixed with gravel and sand, and
spread so broadcast over a large portion of Georgia and even into
Northern Florida, makes it appear that the ice of a cold period must
have covered that southern region.
Moreover, it seems to have been through the great abrasion which only
ice-sheets could perform that the sands of the Florida peninsula were
produced; for on examination they seem to have resulted from the
abrasion and weathering of crystalline rocks.
The worn remnants of such rocks are now found in the southern
Appalachian range. In fact, the hills and mountains of that region at
the present time are supposed to be a small remnant of the ancient
highlands. Thus, on consideration, it appears that the sands caused by
the action of glaciers were, on the disappearance of ice-sheets, blown
by the strong north-west winds toward the Florida peninsula as fast
as the receding waters of the ocean which flowed the lowlands on the
breaking up of the ice age would permit; and in this way the sand was
spread over the lowland region, which was largely composed of coral
sea shells and other marine matter. And it seems that the sand must
have been blown over large areas in Florida soon after the ending of
the frigid period, because the sand, in order to be moved by the winds,
must have spread over a country nearly destitute of vegetation; and
such would be the condition of that region during times which succeeded
the ice period and the subsequent brief flowage of the lowlands on the
ending of the frigid age, which would not be the case if such sands
resulted entirely from water erosion and weathering, because with
such a state of things the country would be covered with forests and
grasses, which would prevent the sand from being moved by the winds to
any great extent.
This goes to show that the region of the Gulf States was so much
affected by the cold of the glacial period, together with the
submergence of the lowlands at its close, its flora and also its
animals were exterminated; for how else can we account for the abundant
fossil remains of animals now found buried in the Florida sands? It
appears also that, when Florida was being covered with drifting sands,
many of the lake basins now formed did not exist, as the wind-blown
sand could not have crossed a continuous chain of lakes like the St.
John’s River; and it is an easy matter to-day to trace the beds of the
ancient lakes that prevented the sands from drifting over certain lands
now nearly destitute of it. And it is probable that the sea flowed the
lowest lands during the period when the winds were drifting the greater
portion of the sands over the peninsula. Therefore, regions which
embrace the Everglades and portions of the Indian River territory are
quite free from heavy sand deposits, and so also are the extensive flat
woods of the peninsula.
Since the sands blew over the ancient desert of Florida, many lake
basins have been formed because of the sinking of the ground. This
sinking of the ground is a common occurrence in limestone regions,
where a great amount of material is moved in solution, leaving caverns
whose roofs often fall in. The great amount of sand blown upon Florida
caused the marine strata to give way in the weaker places under its
burden. The sinks thus formed, probably of frequent occurrence at one
time, have now nearly ceased. Still, there are depressions to be seen
to-day where the tops of large pine-trees, which grew on dry, sandy
land, are barely above the surface of the water which partly fills
the basins so recently formed. Yet I would not assert that all of the
depressions where Florida lakes exist were caused by the sinking of the
ground; for the winds may have caused shallow basins in the sand, where
the decayed vegetation has formed mud sufficient to hold the water
which now partly fills such basins.
The mobility of Florida sands can be seen to good advantage when
exposed to a strong, dry north-west wind, where the ground happens
to be destitute of vegetation. An observer can then realize what the
result would be, should the whole land be deprived of vegetation and
laid bare to the action of the winds.
Under such conditions, not only would the winds be much stronger than
now, but the air near the ground would be filled with sand, moving like
drifting snow in a Dakota blizzard. And, furthermore, it is probable
that the rainfall was very light while Florida was void of vegetation;
and, even if shallow basins were formed, there would be a lack of rain
to supply them with water.
The wide plains west of the Mississippi River, extending southward into
Texas, during the frigid period must have been covered with a sheet
of ice and snow. And it is probable that it was not wholly a product
of more northern latitudes, but was mostly produced by the snow which
fell on the plains during the long winters of that period, which could
not be melted away during the cold summers of an ice age, when it is
considered that an ice-sheet, with a temperature sufficiently low as to
carry glacial drift, covered the lands of Missouri as far as latitude
38° south; and it may have been through the pressure from an ice-sheet
in its south-eastern movement that we are to account for the numerous
ore-bearing faulting fissures traversing the limestone strata.
The ice-sheet was also the probable cause of the erosion of the
horizontal bedded stones, yet it appears that the ice did not greatly
change the contour of the ground; for it is well known that glaciers
do move over lands that are not frozen to the ice without causing much
disturbance, especially where the gradient is small, and this was the
probable condition of the Western plains during the ice age. Thus it
seems that whatever disturbance this region has undergone could be
partly attributed to ice-sheets without the presence of bowlder drift,
because the temperature and texture of the ground in the limestone
region were unfavorable for such accumulations; yet it may be owing to
the action of ice that minerals once diffused are now found collected
in fissures. The deep valleys through which the large rivers now pass
on their way toward the sea were once filled with glaciers which flowed
into them from their tributaries. Thus the deep trenches of the plains
are largely the work of glaciers. It is generally supposed that the
driftless region of Wisconsin was free from ice during the frigid
period. But it seems impossible for this region to have escaped being
covered by ice and snow, with the great lakes filled with glaciers, and
the regions on all sides of the driftless area covered with ice.
The reason why this territory escaped the drift from the north was on
account of the hindrance which the drift-bearing ice-sheet encountered
in the deep basin of Lake Superior. In this great depression the
ice-sheet from the north was relieved of bowlders and other glacial
drift, as well as obstructed in its southern movement.
Therefore, the snow and ice which gathered on the driftless region had
little movement in any direction, while the temperature and consistency
of the ground under the ice were not favorable for the production
of bowlder drift; and, when we consider that the Mississippi valley
was deprived of great sources of warmth during the culmination of a
glacial period, we are forced to the conclusion that its wide lands
were also covered with snow and ice.
The tropical waters of the North Atlantic were so much chilled by the
floating icebergs of North-eastern America, Greenland, Iceland, and
Northern Europe that the Caribbean Sea, its warmest reservoir, was
reduced to a temperature so low that the easterly winds which blew over
its waters were unable to prevent ice-sheets from gathering on Eastern
Nicaragua.
Therefore, during such frigid times it appears that, with the waters of
the Caribbean Sea and Gulf of Mexico reduced to a low temperature, it
was impossible for the great Mississippi valley to escape glaciation,
while being surrounded by cold seas and glaciated lands which extended
even into the tropical latitudes. The broad, level lands of British
America and Siberia during the ice age must have been thickly covered
by the snow which fell on the deeply frozen plains, besides the large
amount of snow that the cold westerly winds must have drifted over
their icy surface from lands of greater snow-fall on their western
borders. This snow during such freezing times could not be melted away.
The great ice-sheets thus formed over wide, level lands could have
but little motion in any direction, certainly not sufficient to cause
glacial drift of much magnitude; yet the ice-sheet, at one stage of
its existence, probably served to widen and deepen the channels of the
great rivers which empty into the Arctic Ocean from these vast regions,
and the glacial débris from such erosion was deposited in the arctic
seas.
CHAPTER V.
REMARKS ON THEORIES ADVANCED FOR EXPLAINING ICE PERIODS.
On Nov. 12, 1891, Professor Geikie made his presidential address before
the Edinburgh Geological Society, the subject being “Supposed Causes of
the Glacial Period.”
Many of his views advanced in this lecture were so much in accordance
with my own that I am induced to repeat them. He said that the glacial
period was a general phenomenon due to some widely acting cause, and
that where we now have the greatest rain-fall the greatest snow-fall
took place, and that the Pleistocene period was characterized by great
oscillations of climate, extremely cold and very genial conditions
alternating. He also said that in glacial and post-glacial times
changes in the relative level of the land and sea had taken place, and
any suggested explanation which did not fully account for these various
climatic and geographical conditions could not be satisfactory. And,
while examining the earth-movement hypothesis, he pointed out that in
the first place there was not the least evidence of great continental
elevations and depressions in the northern hemisphere, such as the
hypothesis postulated. Next he showed that, even if the diserrated
earth-movements were admitted, they would not account for the phenomena.
Such changes, no doubt, would profoundly affect the maritime regions of
North America and Europe; but they would not bring about the conditions
that obtained at the climax of the ice age.
Another objection to the earth-movement hypothesis was this: it did not
account for interglacial conditions. The advocates of that hypothesis
imagined that these conditions would supervene when the highly elevated
northern regions were depressed to their present level. But these
were the conditions that obtained at the present time; and yet in
spite of them the climate was neither so equable nor so genial as that
which obtained in interglacial times and during the mild stage of the
necessary post-glacial period.
Therefore, he said that the earth-movement hypothesis should be
rejected, not only because it was highly improbable that such
wonderfully rhythmic elevations and depressions of northern lands could
have taken place, but chiefly because it did not explain the conditions
of the glacial periods and interglacial times.
Still, Professor Geikie says that in glacial and in post-glacial times
changes in the relative level of the land and sea had taken place; and
it is reasonable to suppose that such changes were obtained in the high
latitudes of both hemispheres during the breaking up of the last ice
age.
We have previously pointed out that much of the ice of the glacial
period in the southern hemisphere was melted away, and its waters
warmed sufficiently to assist the Gulf Stream and Japanese current to
bring about a mild period in the northern hemisphere; for without such
assistance they would be unable to disperse the vast ice-sheets of the
northern latitudes.
Still, the attraction of the southern ocean waters into the northern
seas must have commenced as soon as the growing ice-sheets of the large
continents and islands of the high northern latitudes surpassed the
growth and weight of the glaciers on the smaller lands of the southern
hemisphere.
Hence the attraction of the ocean waters northward overcomes the force
of the prevailing winds from moving an undue portion of the ocean’s
surface waters southward. Consequently, the movement of water from the
southern seas into the northern latitudes continued so long as the
vast northern ice-sheets increased in weight greater than the glaciers
of the southern hemisphere. Therefore, at the perfection of a frigid
age straits and channels situated so far southward as the Magellan and
Cape Horn channels were much diminished in width and depth or entirely
deprived of their waters. Through this cause such reduced channels were
readily filled with glaciers in a region of great snow-fall. The depth
of water on the submerged northern lands at the close of the glacial
period is not known.
According to Professor Dawson, in the township of Montague in Ontario
the skeleton of a whale was found in post-glacial deposits 440 feet
above tide-water, and marine shells are known to occur on Montreal
mountain at an elevation of 520 feet above the ocean; and it is said
that there are traces of submergence of over one thousand feet in the
higher latitudes, including the islands of Great Britain.
According to the researches of Dr. J. W. Spencer, one great sheet of
water covered most of the great lake region about the close of the
ice age; and the lower strands of these inland seas are known to be
connected with old marine shore lines. The probable reason why so few
sea-shells collected on the glacial drift during such times was because
of so much marine life having been exterminated in the high northern
latitudes during the frigid age. Therefore, the sea, in the short
period of northern submergence, left but few traces on the glacial
drift it once flowed.
Thus it will be seen that, if the ocean waters were attracted northward
through the preponderance of northern ice-sheets, they not only
assisted in melting the northern ice, but also served to greatly reduce
the waters in the Cape Horn channel, and so largely prevented the
independent circulation of the southern ocean, thus furthering a mild
climate in the southern hemisphere until the prevailing winds, after
the northern ice-sheets were melted, were able to move more of the
ocean waters southward than they could move northward, owing to the
ocean currents setting southward being less obstructed than the lesser
currents setting northward. This tendency of the ocean waters to move
southward I have before explained in the preceding pages.
But I will say in addition that, on further consideration, it seems
that one of the main causes of the waters of the augmented northern
oceans moving southward so soon after the melting of the ice from
the northern lands was on account of so much water being attracted
southward to the great low sea-level east of Cape Horn. This vast low
sea-level remained a great area of attraction for the northern seas
until so much northern water was moved into the southern ocean as to
reduce the seas of the northern hemisphere and augment the southern
ocean sufficiently to enlarge the Cape Horn channel, thus causing
the extinction of the vast low sea-level that furnished such great
attraction for the waters of the more northern latitudes.
If the earth-movement hypothesis, so wholly rejected by Professor
Geikie, fails to explain the cause or causes of a northern ice age, it
seems to be still more inadequate for explaining the occurrence of ice
periods extending over both hemispheres. For it is not probable that
portions of continents and large islands rose above the snow-line in
both temperate zones during the same period of time, and then again
obtained their present level with the occurrence of a mild era.
Those who maintain that the continents of North America and Europe
rose to great elevations during the ice age, in order to prove their
assertions, point to the fiords which indent the eastern and western
coasts of North America, and also to the fiords of Norway, as having
been eroded by streams of ice that flowed along the bottom of such
gorges when they were above the sea.
But it appears that such erosion could be performed by heavy glaciers
with the lands at their present level. A glacier three thousand feet
thick would fill and press heavily on the bottom of a gorge fifteen
hundred feet in depth. Therefore, should the bottom of a fiord sink
hundreds of feet below the sea-level, a glacier several thousand feet
thick flowing through and over it into a sea of much greater depth, the
erosion at the bottom of the sunken channel would be greater than on
the land above the sea, where the ice possessed less weight.
Therefore, it is not necessary that lands pierced by deep fiords
should have acquired a higher level during the ice age than they now
maintain. And it is probable that on the antarctic continent ice
erosion may be going on at much greater depths below the sea-level than
the deepest channels in the high northern latitudes. For it is likely
that the temperature of a glacier is so low in such frigid regions
that it holds firmly in its freezing grasp such bowlders as may become
detached from the rocks, thus giving it great erosive power.
But this great eroding ability could not be maintained by glaciers in
the lower latitudes, where a higher temperature would largely deprive
the ice of its abrading properties except on the steep slopes of
mountainous lands.
There are deposits of ice on the North American coast bordering the
arctic shores, and also on Northern Siberia, that are supposed to have
existed since the last frigid period, and are likely to be preserved
into a future cold age, which now appears to have made considerable
progress on Greenland and other ice-clad arctic shores on account of
the independent circulation of the Arctic Ocean waters, which largely
excludes the Gulf Stream from the polar seas; and it is for this reason
that the glaciers on the elevated lands of Iceland are being enlarged
and rapidly advancing. Yet, notwithstanding the gathering of ice
and increasing coldness of lands largely removed from the warm Gulf
currents, there are still mountain regions where glaciers may have been
preserved through post-glacial times, although directly to the leeward
and under the influence of the Gulf Stream and Japanese currents.
These glaciers are situated in the Alpine districts of Europe and on
the mountain ranges of Alaska. It would appear that, were the climate
growing gradually colder in the northern temperate zones, such glaciers
should be increasing in size.
Yet it is said that such is not always the case. This is probably
owing to their being subject to the genial influence of the tropical
currents. For, although the climate of Europe and Alaska may have been
slowly growing colder for centuries, still the slow shrinkage of these
once immense glaciers may still be going on, although at a much slower
rate than formerly, even if the tender plants of these latitudes,
because of the growing coldness, have gradually moved southward.
As to the Alpine glaciers, M. Forel reports from data he has collected
that there have been several enlargements and diminutions during the
last century. And since 1875 enlargements have taken place, their
shrinkage being caused by warm and dry weather, while their enlargement
was brought about during cold and rainy seasons. The glaciers of
Alaska cannot attain much extension until the waters of the great
Japanese stream acquire a lower temperature. There is at this date a
small current setting down through the eastern side of Bering Strait,
bearing field-ice in the spring season down to Anadyr Gulf. The Okhotsk
Sea in the spring season furnishes considerable field-ice to cool the
north Pacific waters, and the wintry winds which sweep down from the
high lands of Northern Asia also serve to chill the Pacific seas; but
all such sources of cold combined at this age have but little general
effect on the vast Japanese current, which still has warmth sufficient
to prevent the increase of glaciers on Alaska.
This great ocean stream in its impact against the shores of Oregon
causes a high sea-level, which is mostly turned southward by the
prevailing north-west winds. Still, a comparatively small stream
sets along the shore of the Alaska Gulf, and also through the island
passages toward a slight low sea-level, to the leeward of the Alaska
peninsula; and it is probable that this current which warms these
in-shore waters is favored by the difference of temperature and density
between the waters abreast Oregon and the Gulf of Alaska, and it may be
owing to the same cause that a small stream is sent along the eastern
shore of Bering Strait into the deep portions of the Arctic Ocean. Thus
because of the warm waters that proceed from the great Japanese current
the glaciers of Alaska are prevented from increasing their bulk.
The only way to furnish the Japanese stream with colder water, and
so cause glaciers to increase on the north-west coast of America, is
through the great Humboldt current, which has its rise in the southern
ocean west of Patagonia and the Cape Horn channel, where a moderate but
vast high sea-level is formed on account of the great drift current of
the southern ocean being somewhat obstructed on its passage through the
Cape Horn channel, which is about one-third the breadth of the westerly
wind-belt.
Therefore, the northern portion of the waters of the high sea-level
so caused are attracted northward to the low sea-level abreast Peru,
from whence they are moved by the south-east trade winds as a drift
current to the equatorial latitudes, thus meeting and mingling with the
returning Japanese current abreast Central America, and so giving head
to the great equatorial stream which moves westward over the Pacific
Ocean, partly impelled by the trade winds, and, on gaining the western
side of the ocean, sends off from a moderate high sea-level a large
stream to the low sea-level caused by the westerly winds abreast Japan,
from whence it is drifted by the same winds over to the north-west
coast of America, thus forming the great Japanese current.
Meanwhile the temperature of the Humboldt current, being governed by
the temperature of the southern ocean from which it takes its rise, is
cooling at a slow rate through the enlargement of ice-sheets in the
antarctic regions, while the increase of glaciers on Patagonia will
in time greatly add to its coolness, and so lower the temperature of
the equatorial current from which the Japanese current branches, the
latter current being made cooler through the increase of coldness of
the former streams. Therefore, the temperature of Alaska, which is
governed by the Japanese current, will slowly acquire a colder climate;
and, consequently, its glaciers will increase in size sufficient to
launch icebergs into the Pacific to be currented southward, and so
still further lower the temperature of the Eastern Pacific waters,
and consequently the equatorial current from which the Japanese stream
branches, and so eventually, under the above conditions, cause heavy
ice-sheets to spread widely over the north-west coast of North America.
It will be seen from the above explanations how an increase of cold
in the southern hemisphere is necessary to cause a wider spread of
ice-sheets on lands in the northern hemisphere.
Especially is this the case to promote the gathering of glaciers on the
west coast of North America. The great equatorial current while on its
way to the Indian Ocean not only sends off the Japanese stream, but
also the East Australian current, which is like the Japanese current,
having its temperature lowered in proportion as the equatorial stream
is cooled. Therefore, the southern ocean is slowly being deprived of
equatorial heat from this source.
I have explained how the increasing coldness of the superior oceans of
the southern hemisphere affects more or less the temperature of the
Gulf Stream, which meanwhile is only able to enter a small portion
of its waters into the Arctic Ocean after undergoing a long cooling
process as a drift current; and, while thus mingling with the arctic
waters, it is not able to prevent the gathering of ice-sheets on
Greenland, where glaciers are launching bergs to float southward as far
as the latitude of 40° north. Consequently, the northern seas are now
being cooled as well as the seas of the southern hemisphere.
Yet this cooling process is so slow there is a lack of data to
show that the temperature of the high latitudes is lowering. Our
thermometrical observations are of such recent date they cannot be used
to determine climatic changes which requires centuries to bring about.
Still, it is generally known that the climate of Northern Europe has
been accused of growing colder. The vine no longer flourishes on the
shores of Bristol Channel or in Flanders or Brittany; and vineyards
are no longer planted on the elevated shores of France where they
flourished three hundred years ago. Arago did not refuse to believe
that the laws regulating the temperature of Western Europe had notably
altered. This is proved, he said, by the general retrogradation of the
vineyards southward.
The recent deadly freezing of the orange groves of Florida makes it
uncertain whether the cultivation of the orange can again be successful
in the counties where during this generation it has been very
profitable.
Travellers visiting Iceland say that the old accounts of its prosperity
seem strange to those who now visit its shores; and it is narrated in
the Sagas that in early times sheep could shift for themselves during
winter, and that there were large forests and that corn ripened.
Several years ago a correspondent of the _Spectator_, writing from
Northern Russia where the Volga is locked with ice for six months in
the year, stated that “the people were beginning to show increased
resentment at the climate, and that there was reason to believe that
the northern government of Russia would be abandoned to the desert. The
people silently glide south by the tens of thousands every year, so the
life of Russia was concentrating in the south.”
It is now the opinion of travellers in arctic lands that the
inhabitants of the Esquimaux regions are decreasing, as are also the
inhabitants of Northern Siberia.
A writer in the North China _Herald_, of Shanghai, says that “the
climate of Asia is becoming colder than it formerly was, and its
tropical animals and plants are retreating southward at a slow rate.
In the time of Confucius elephants were in use on the Yangtse River.
A hundred and fifty years after this Mencius speaks of the tiger, the
leopard, the rhinoceros, and the elephant as being in many parts of
China.
“It is also said that the ferocious alligator, that formerly infested
the rivers of South China, has retreated southward.
“The flora of the country is also affected by the increasing coldness
of the climate. The bamboo is not found in the forests of North China,
where it grew naturally two thousand years ago, but is still grown in
Pekin, with the aid of good shelter, as a sort of garden plant only.”
A letter from Hong Kong, published in the London _Standard_, reports
that on the 15th of January, 1893, the temperature of Hong Kong, a
tropical seaport of China, was below freezing for three days, and was
colder than ever before known. The rocks and also vegetation were
covered with a coating of ice. The thermometer at times stood at 23°
and 26° Fahrenheit.
I have previously explained how the slow increasing coldness of the
northern temperate zone is also being carried out in the southern
hemisphere. The meteorological records for the lofty table lands of
Ecuador, although very incomplete, furnish strong evidence to show that
the mean temperature of that region is gradually lowering.
Observations made by Boussingault at Quito in 1831, compared with those
from 1878 to 1881, showed a decrease from 15.2° Centigrade to 13.27°
Centigrade.
Records made by Hall from 1825 to 1827 give averages of 16.1°
Centigrade, 15.52° Centigrade, and 15.6° Centigrade. This decrease
holds good for all points in the inter-Andean region where records have
been kept.
Yet we know that the falling temperature in the northern temperate
latitudes is not brought about by a yearly increase of cold, because,
when the arctic channels are somewhat obstructed with icebergs, the
movement of arctic waters through them is lessened; and, therefore,
during such times the Gulf Stream, meeting with less opposition from
arctic currents while flowing northward, is able to move a larger
volume of its waters into the arctic seas, thus warming their waters
sufficiently in a few seasons to clear the obstructed channels, and
also somewhat soften for several successive years the temperature of
such lands as border on the seas of that region.
And in this way we account for the mild seasons which at times follow
those of lower temperature in high northern latitudes.
But, when the detained icebergs are set adrift, and currented into the
temperate North Atlantic, the heat consumed while melting such numerous
bodies of ice is able to more than overcome the warmth gained during
the temporary detention of ice in the northern seas. Thus, under such
considerations, it appears that the conditions are favorable for the
growth of glaciers in the high northern latitudes.
I have pointed out the manner in which the superior oceans in the
southern hemisphere are obtaining a lower temperature, and how
they impart their coldness to the tropical currents, and in this
way slowly cool the waters of all oceans. Thus it appears that the
northern temperate zone, with all other parts of the earth, is slowly
approaching a cold epoch.
Several writers on climatic changes have expressed their views as to
the number of glacial and mild periods that have been perfected since
the conditions have been favorable for their appearance on the globe.
According to my views, while considering the reasons for the occurrence
of the great glacial periods which have left such extensive traces on
the land, it seems certain that two very cold epochs have possessed
the earth, separated by a warm period; and, possibly, other preceding
cold epochs of less intensity have possessed the high latitudes, with
intervening periods of mildness. But the earlier cold periods, if they
ever existed, were comparatively short, because the Cape Horn channel
during such times possessed less capacity than in the later periods,
and, therefore, was more easily and quickly obstructed by the natural
methods previously explained.
Consequently, the independent circulation of the southern ocean was
sooner arrested than during the later epoch, when the channel had
become enlarged by erosion from heavy glaciers and icebergs; and
meanwhile the same conditions may have governed the arctic channels
which give an independent circulation to the arctic waters which
surround Greenland, and thus, in connection with cold epochs in the
southern hemisphere, have caused periods of cold of small intensity to
occur in the high northern latitudes, and it may happen in the future
that more ice periods will be perfected than the one now progressing.
Still, it is well to bear in mind that the Cape Horn channel, which
is the real cause of glacial periods having occurred in both the
northern and southern hemispheres, in the manner previously explained,
is being made wider and deeper during each succeeding ice age. For
this reason the latest cold epoch will require a longer continuance
of cold to obstruct the channel than the cold period preceding.
Therefore, it appears that the time will come when there will be such
great accumulations of ice stored on the land and in the sea before
the enlarged Cape Horn channel can be closed that, when it is closed,
there will not be sufficient warmth remaining in the tropical seas to
unite with the sun’s rays to subdue the intense cold stored in the
immense gatherings of ice. And thus the earth, which began its career
with a warm temperature, and so continued for long ages, will finally
terminate in an endless glacial age.
The statements made by General Cowell in _Science_ of Nov. 25, 1892, in
reference to the alleged discovery of the second rotation of the earth
by Major-general Drayson, represents the discovery as affording a new
solution for the cause or causes of an ice age.
The second rotation as defined consists in the pole of the heavens
describing a circle around a point which is ascertained to be
situated six degrees distant from the pole of the ecliptic. And it
is asserted that by a knowledge of the second rotation it is proved
that a variation of twelve degrees in the extent of the arctic circle
and the tropics occurred not later than 13,500 B.C., “the tropics
varying in distance from the equator from the minimum of 23° 25′
47″ to the maximum of 35° 25′ 47″, thus extending the torrid zone
during its widest expansion from Cape Hatteras to the river Plate....
It is calculated that at this date we are about 403 years distant
from the time when the pole of the heavens in its revolution, the
pole of the ecliptic and that of the second rotation, will be in
the same colure,--that is, in the year 2,295 A.D.; and then the
least differences in temperature between summer and winter will be
experienced. From that time forward this difference will increase, and
about 6,000 years later, or about the year 8,300 A.D., the earth will
enter the next glacial period, and attain its greatest severity about
the year 18,136 of our era.” General Cowell does not state how the
widening of the tropical zone, as above set forth, would bring about
a glacial period. The winters of the temperate zones would evidently
be colder than now; but, on the other hand, the summers would be
proportionally warmer, while the westerly winds above the latitudes of
40° would prevail the same as now.
Therefore, their general effect on the surface waters of the ocean
in the high latitudes would not be changed with such an extension of
the tropical zone, neither would the trade winds change their general
direction with a wider torrid zone; yet the boundaries of the trade
winds and also the westerly winds would be more shifting according to
the declination of the sun, such winds being governed as now by the
position of the sun during the summer and winter solstice. Yet the
natural process for moving tropical water into the high latitudes, or
excluding it therefrom, would not be greatly changed.
Consequently, the expansion of the torrid zone to the latitudes named
by General Drayson would not affect the climate of the hemispheres
sufficiently to cause a frigid epoch. On the contrary, the summer
monsoons, which now blow from the north-east, along the shores of
Eastern Africa, and also along the coast of Southern Brazil, would be
much stronger with a vertical sun in midsummer as far south as river
Plate, thus forcing the surface waters of the tropical oceans into the
higher latitudes with greater facility than at this age.
Moreover, according to the statements of General Cowell, the present
period of mildness should be on the increase, and obtain perfection
in the year 2,295, or about 400 years hence; while, on the contrary,
according to the explanations we have given in the preceding pages,
there is much to show that an ice age is advancing, and has made
considerable progress in the high latitudes of both hemispheres.
Furthermore, if the second rotation, as claimed by General Cowell,
is able to perfect a glacial period at regular intervals of 31,600
years, it seems that traces of frigid epochs should not be confined to
late geological records, as there appear to be little or no traces of
glacial work prior to the Quaternary or Post-tertiary periods.
It appears that explanations so far given, which depend on the
astronomical theory to account for the ice age, are not in harmony with
well-known geographical facts. The explainers neglect the attention due
to the great prevailing winds which since the earlier geological ages
have, in connection with continents, moved the surface waters of the
ocean from torrid latitudes to colder zones, and from the colder zones
to the warmer latitudes.
This exchange of ocean waters between the zones is as old as the
continents which shape their courses. The important change wrought in
the ocean currents sufficient to have caused the glacial age which
ended the early warm epochs was brought about through the action of the
prevailing winds, which, in connection with the form of continents,
became able to move the ocean waters from the northern hemisphere into
the southern sufficient to submerge the low lands of the southern
hemisphere, causing a great diversion of the tropical currents from
the high southern latitudes, such as I have pointed out in preceding
chapters.
Those writers who believe that ocean currents have been the cause of
great climatic changes have suggested that the existence of an ancient
channel through the isthmus of Panama would have caused a frigid period
on lands bordering on the northern shores of the Atlantic by turning
the head-waters of the Gulf Stream into the Pacific Ocean.
Professor Agassiz thinks that such a channel existed during some remote
geological age, judging from the semblance of the fauna pertaining to
the Caribbean Sea and Pacific Ocean.
Yet it may be said that an open channel through Central America would
have connected two high sea-levels.
For this reason there would be little or no exchange of water between
the Caribbean Sea and the Pacific Ocean.
The high sea-level on the Pacific side is caused by the prevailing
north-west winds which blow down the North American coast past
California as far south as Central America; while, on the other hand,
the south-east trade winds impel the surface waters of the South
Pacific along the coast of Peru down to the equator, and so onward 5°
to 8° north latitude. Thus the space between the ending of the two
ocean winds obtains a high sea-level, corresponding to the high level
of the Caribbean Sea. This has been proved from levellings for the
Nicaragua ship canal.
Consequently, the Atlantic waters would not run into the Pacific Ocean,
even if a channel opened through Central America.
Therefore, the Gulf Stream has never been turned away from the North
Atlantic.
Writers, while seeking a cause for the mild climate of ages preceding
the glacial epochs, have thought that during such times channels
opening through Asia from the Indian Ocean by the way of the Persian
Gulf into the arctic seas would be the means of furnishing the Arctic
Ocean with warm water. But it is evident that such a movement of water
could not be brought about, because the winds would not be favorable
for it. For, when we reflect that the prevailing winds would blow in
the same direction as now, and that the seas of Eastern Europe and
Western Asia were enlarged during the warm epochs, it seems that they
would obtain high levels superior to the high level seas of the Indian
Ocean.
Besides, we should consider that there is a continuous range of high
land separating the Persian Gulf from the northern seas, which probably
existed anterior to the ice age. Still, during later periods, while
the ice-sheets were being melted from the northern hemisphere and also
on the ending of the last ice age, the Isthmus of Suez was submerged,
as were all other low lands in that latitude; but it is probable that
the waters of the high sea-level of the Indian Ocean abreast tropical
Africa did not flow largely into the Mediterranean Sea for the reason
that the enlarged European seas, being within the westerly wind-belt,
maintained a high sea-level, while at the same time the high level
tropical Indian Ocean waters were strongly attracted into the southern
oceans through the Mozambique and Agulhas currents in the manner I
have previously explained. Yet the waters of the high sea-level of the
southern European seas must have been strongly attracted to the low
sea-level abreast the Canary Islands.
While considering the causes which brought about the glacial periods,
it is well to reflect that the natural mode of action which could have
produced a frigid age was as extensive as the surface of the globe;
and, therefore, any geographical change that would affect only a
comparatively small portion of the earth cannot serve to account for
ages of warmth which extended over the globe, or for glacial epochs
which were separated by warm periods of time, which seem to have
affected all lands and seas.
And it appears from the geographical explanations given in preceding
pages of the general movements of the winds and currents of the sea how
impossible it is for heat to be conveyed to the antarctic latitudes
sufficient to prevent the growth of glaciers on their lands while the
Cape Horn channel is in possession of its present capacity.
For, as has been shown, this channel furnishes opportunity for the
westerly winds to impel the surface waters of the great southern ocean
constantly around the globe, and so largely turns away the tropical
currents from the high southern latitudes.
Consequently, there seems to be no method yet devised through nature’s
mode of action that can carry sufficient heat into the antarctic
latitudes to melt the ice-sheets from the southern continent, or even
arrest their growth, while the Cape Horn channel maintains its present
width and depth.
Therefore, the increase of glaciers and icebergs will slowly continue
until a glacial epoch is perfected.
And it seems that this arrangement for bringing about a frigid age
made slower progress in its early stage than at this date, owing to
there having been a lack of glacial ice in the polar regions to produce
icebergs for cooling the ocean waters. But the independent circulation
of the great southern ocean, after turning away the tropical currents
from the high southern latitudes for thousands of years, did at length
cause glaciers to form on the antarctic lands, which have been slowly,
but constantly increasing; and, consequently, the cooling of the ocean
has been accelerated proportionate to the increase of ice-sheets.
Therefore, with the cooling process so well advanced as it now appears
to be, it seems that more than half of the time required to bring a
frigid age to perfection has been expended since ice-sheets began to
gather on the antarctic shores. For, when we realise how the facilities
for making ice have advanced through the increase of glaciers in both
hemispheres, and how large a portion of the ocean waters have been
cooled below a temperate or tropical temperature even in the torrid
latitudes where the warm upper waters of the ocean have been reduced
to a comparatively thin stratum when compared to the vast bulk of the
cooled under-waters, it appears that the cold will increase at a faster
rate for the next thousand years than was the case during the last ten
centuries. Therefore, the climate will be less favorable for plants and
animals existing on lands in the high latitudes for the next thousand
years than during the ten centuries preceding; and, when we take into
consideration the accelerative growth of a frigid epoch, it seems that
the increasing cold will in a few thousand years drive the greater
portion of both plants and animals from the now temperate latitudes to
maintain an existence in the tropical zone, where a large part of the
existing species of such life must have taken refuge during the last
ice period.
And, from what can be learned from the relics of man’s prehistoric
life, it seems to point to the lands of the tropical latitudes as
having been his home during the frigid ages; and, because of his long
undisturbed residence in favored portions of the tropics, he there
attained his earliest civilization. For it appears that the tropical
zone was not only less burdened with ice in glacial times than the
higher latitudes of the globe, but was also more exempt from the
great flooding of lands which obtained in the more northern latitudes
through the shifting of the ocean waters, from causes set forth in the
preceding pages. Yet it may be said that the low lands of the tropical
zone south of the equator during cold epochs were much more extensive
than at this age, on account of the shrinkage of the sea, because of
the great amount of water evaporated from its surface, and stored in
ice-sheets on the great continents and islands. Hence the reefs and
shallows which surround such tropical islands as include the Seychelles
Archipelago, and also the extensive banks covered with shoal water
in that portion of the Indian Ocean, were during the glacial period
elevated above the surface of the sea, possessing a climate favorable
for vegetable and animal life. But, owing to the great rain-fall of
that region, it is probable that the highest lands were glaciated,
as it is reported that granite bowlders still rest on the mountain
slopes of the highest island. The numerous islands and shoals of the
south-western tropical Pacific must also have afforded wide land areas,
with a temperate climate, owing to their having been situated on one of
the warmest regions of the earth during the ice age.
Moreover, it is probable that these tropical lands afforded space for
numerous lagoons which had little connection with the surrounding
oceans, and consequently were able to maintain, in their secluded
shallow basins, a warmer temperature than obtained in the open seas;
and at the same time, owing to the great rainfall in such tropical
portions of the Indian and Pacific regions, the waters of the lagoons
were rendered less salt than the briny depths of the shrunken oceans
of a cold period. Hence because of such conditions the fauna of the
tropical seas were preserved from the destructive rigor which beset the
earth during the frigid epochs.
Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.
Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.
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