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.