THE CLIMATIC RECORD AS A WHOLE
It is a remarkable fact that one of the oldest known sedimentary
rocks is glacial in origin, and indicated the presence of an
ice-sheet at a very early stage in the earth’s history. This is a
“tillite,” or boulder-clay, discovered by Prof. Coleman at the base
of the Lower Huronian (Early Proterozoic) of Canada. It extends in
an east and west direction for 1000 miles across northern Ontario,
and northward from the northern shore of Lake Huron for 750 miles. It
rests on a scratched or polished surface of various rocks, and the
included boulders are not always local, but some have been brought
from a considerable distance. All these characters point to a large
ice-sheet.
Traces of Proterozoic glaciations have been discovered in various
other parts of the world, and some of these may be of the same age
as the Canadian ice-sheet, but they cannot yet be dated exactly. An
interesting example is western Scotland, which J. Geikie considered
to have been glaciated by ice from the north-west which has since
sunk into the North Atlantic. Other glacial remains have no doubt
been destroyed or deeply buried, while some may still await
discovery, and at present we must be content to note the occurrence
of a glacial period at this time without attempting any description
of the distribution of climates over the globe.
Followed a long period of milder climate indicated in America by
thick deposits of limestone with the remains of reef-building
organisms and other marine life. This period may have been
interrupted at least once by the recurrence of glacial conditions,
but the evidence for this is doubtful. It must be remembered that the
duration of the Proterozoic was very great, at least as long as all
subsequent time, while the relics of it which are now known to us are
few and scattered, so that much which happened during that time is a
closed book. It is not until the very close of the Proterozoic that
we again find indisputable evidence of widespread glacial action.
This second great glaciation was placed originally in the earliest
Cambrian (see table of geological formations at the end of Chapter
I), but later evidence shows that it is slightly older than the
oldest deposit which can be referred to this series, and it may
be designated the Pre-Cambrian glaciation. Tillites of this age
have been found in the middle Yangtse region of China and in South
Australia (where they extend from 20 miles south of Adelaide to 440
miles north, with an east-west extension of 200 miles). Glacial
deposits which probably refer to this period have been found also
in India, both in the Deccan and near Simla, over a wide area in
South Africa, and in the extreme north of Norway. This distribution
suggests the presence of two centres of glaciation, one between
China, India and Australia, and the other north-west of Europe. The
south-eastern of these was the most extensive, and probably indicates
a ring of glaciated continents surrounding the pole, rather than a
single enormous ice-sheet.
During the Cambrian all evidence of glacial action ceases, and we
have, instead, evidence of a warm, fairly uniform climate in the
abundant marine life. This continued during the Ordovician and became
accentuated during the Silurian period, when reef corals lived in the
seas of all parts of the world. Terrestrial deposits are curiously
lacking in all this series, and this suggests that in the absence
of any great mountain-building and elevation shallow seas extended
over almost the whole of the surface, accompanied by mild oceanic
climates extending to high latitudes.
At the close of the Silurian there was a period of mountain-building
and the formation of continents. The extinction of numerous species
of marine organisms and the rapid evolution of others point to the
seas becoming cooler and the stress of life more acute. In the
succeeding Devonian period there is evidence of glacial conditions
in South Africa in the form of a thick series of quartzites
with striated pebbles up to fifteen inches long, but no typical
boulder-clay has been discovered. There are also some doubtful traces
from England. The most noteworthy development of the Devonian in the
British Isles is, however, a thick deposit of red sandstone (Old Red
Sandstone) of the type that is formed in shallow lagoons or inclosed
basins, and suggesting desert conditions, so that the rainfall of the
British Isles was probably slight.
These continental conditions passed away towards the close of the
Devonian period, and once again extensive warm oceans appear to
have spread over a large part of the globe, associated with the
development of reef-building corals. Climate continued warm and
equable throughout the greater part of the Carboniferous. The
important feature of this period is the great system of coal-beds
which extends through North America and Europe to China, with
northern and southern limits in 80° N. (north-east Greenland and
Spitzbergen) and 15° S. (Zambesi River). Wegener, summing up the
evidence, and considering especially the absence of annual rings
in the trees, concludes that the coal-beds are the remains of the
tropical rain-forest when the equator lay across Europe some 30
degrees north of its present position.
Towards the close of the Carboniferous period great mountain-building
set in, resulting in the formation of the famous Gondwanaland,
including south and central Africa, southern Asia, part of Australia
and possibly Brazil. From a consideration of the glacial evidence,
however, it appears, as will be seen later, that this was probably
a ring of neighbouring and partly adjoining land areas rather than
a single enormous continent. At the same time the climate became
cooler, and a hardier vegetation, known as the _Glossopteris_ flora,
developed in the southern hemisphere, including woody trees with
annual rings indicating seasons. The large insects of the coal
forests which did not undergo a metamorphosis were replaced by
smaller types which did pass through such a stage; this change of
habit is considered to be due to the winters having become severe,
so that the insects learnt to hibernate through them. In the early
Permian, Gondwanaland was occupied by great ice-sheets, remains of
which in the form of tillites of great thickness, ice-worn surfaces
and striated boulders have been found in South Africa, Belgian Congo,
and Togoland, Tasmania and widely separated parts of Australia,
peninsular and north-western India, and probably also Afghanistan.
In India the glacial striæ show that the ice-sheet was moving north,
while in South Africa it was moving south, i.e. away from the present
equator in both cases. Widespread glacial remains have been found
also in Brazil, northern Argentine and the Falkland Islands, and
there are probable traces near Boston in North America, in Armenia,
the Urals and the Alps, and possibly also in England.
Wegener’s reconstruction of the geography of this period places the
south pole a little to the south-east of South Africa, surrounded by
a great continent composed of the junction of Africa, South America,
Antarctica, Australia, and an extended Deccan added to by smoothing
out the folds of the Himalayas. This great circumpolar continent he
considers to have been the site of an immense ice-cap. The North Pole
lay in the Pacific Ocean, so that almost all the remaining land areas
enjoyed temperate or tropical climates.
It is admitted that this peculiar distribution of glacial
remains apparently necessitates a position of the pole somewhere
near that described by Wegener, but the theory of a single polar
ice-cap extending beyond 50° latitude on nearly all sides presents
difficulties. From the mechanism of the supply of snow to an
ice-sheet described in the preceding chapter it follows that, except
close to the edges, all the moisture precipitated must be brought in
by upper currents. But even if we take into account the increase in
the strength of the atmospheric circulation due to the introduction
of an ice-cap, there is a limit to the supply of moisture by this
process. All such moisture has to cross the periphery, and with
increasing radius; the number of square miles of area to each mile
of periphery becomes greater, slowly at first, then more and more
rapidly. We shall see in Chapter VIII that even the North American
Quaternary ice-sheet became unwieldy from this cause, and suffered
several changes of centre.
Hence it seems that the _rapprochement_ of the continents in
Permo-Carboniferous times need not have been so complete as Wegener
supposes, the glacial phenomena being more readily explicable by a
ring of continents surrounding a polar sea, as in the case of the
Quaternary glaciation of the northern hemisphere. The local Permian
glaciations of Europe and North America, some of which fell close to
Wegener’s equator, are easily explicable as due to mountain glaciers
similar to those of Ruwenzori and other tropical mountains during the
Quaternary. There were interglacial periods in South Africa, Brazil
and New South Wales, which increase the resemblance between the
Permian and Quaternary Ice Ages.
In Upper Permian times there was a widespread development of arid
climates, especially in the present temperate parts of North America
and Europe. Wegener attributes this to the northerly position of the
equator bringing the sub-tropical desert belt (Sahara, Arizona) to
these regions. In the Trias these conditions gradually gave place
to another period of widespread warm shallow seas, with abundant
marine life and corals extending over a large part of the world, even
to Arctic Alaska. In the United States there are the remains of the
forests of this period, in which the tree-trunks show very little
evidence of annual rings, indicating that the seasonal changes were
slight, so that the climate had again become mild and oceanic.
In the Lias (Lower Jurassic), there was crustal movement and
volcanic action accompanied by land-formation and a gradual lowering
of temperature. There was a great reduction in the abundance and
geographical range of corals, and most of the species of insects are
of dwarf types. There is, however, no evidence of glacial action.
The Upper Jurassic period appears to have been warmer than the
Lias. Insects of a large size and corals again attained a very wide
distribution, but there is enough difference in the marine faunas of
different regions to indicate a greater development of climatic zones
than in the extremely oceanic periods such as the late Triassic.
Schuchert points out that the plants of Louis Philippe Land in 63° S.
are the same, even to species, as those of Yorkshire.
In the Cretaceous period the climate was at first similar to that of
the Jurassic, and trees grew in Alaska, Greenland and Spitzbergen.
These trees, however, show marked annual rings, indicating a
considerable differentiation of seasons, while trees of this age
found in Egypt are devoid of rings. Towards the close of the
Cretaceous there were many crustal movements and great volcanic
outbursts, accompanied by a considerable reduction of temperature,
which led to the extinction of many forms of life and the rapid
evolution of others. There is no evidence of glacial action during
the Cretaceous, however, though at the beginning of the Eocene
there was a local glaciation of the San Juan Mountains of Colorado.
According to W. W. Atwood this glaciation was double, the first
stage being of the Alpine mountain glacier type, separated by an
interglacial from the second stage, which was of the Piedmont type
(mountain glaciers spreading out on the plain at the foot of the
mountain). This Eocene glaciation has been found nowhere else,
however, and the climate of the Tertiary, which is discussed more
fully in the next chapter, was in general warm and oceanic, becoming
rigorous towards its close.
Summing up, we find that in the geological history of the earth two
main types of climate seem to have alternated. Following on periods
of great crustal movement, and the formation of large land areas,
the general climate was cool, with a marked zonal distribution
of temperature, culminating during at least four periods in the
development of great sheets of inland ice. It is in such a period,
though, fortunately, not at its worst, that we are living at present.
During quiescent periods, on the other hand, when these continents
largely disappeared beneath the sea, climate became mild and equable,
and approached uniformity over a great part of the world. At these
times, as soon as the surface water of the sea in high latitudes
began to cool, it sank to the bottom, and its place was taken by
warmer water from lower latitudes. The oceanic circulation was very
complete, but there were practically no cold surface currents.
Instead, there was probably a general drift of the surface waters
from low to high latitudes (with an easterly trend owing to rotation
of the earth), and a return drift of cooled water along the floor of
the ocean. The formation of sea-ice near the poles became impossible,
while the widespread distribution of marine life was facilitated.
The alternation of periods of crustal deformation with periods of
quiescence has frequently been noticed, and has been termed the
“geological rhythm.” It may be attributed to the gradual accumulation
of small strains during a quiescent period until the breaking point
is reached, when earth-movements take place until equilibrium is
restored, when the process is repeated.
The gradual erosion of the land by river and wave-action and the
consequent shifting of the load provides a certain amount of stress;
but this is local, and calls for local readjustments only. A more
generally effective agency may be the gradual slowing down of the
earth’s rotation under the influence of tidal friction. The mechanism
of this process was described by A. E. H. Love (“Nature,” 94, 1914,
p. 254): “The surface of the ocean, apart from waves and tides, is at
any time a figure of equilibrium answering to the speed of rotation
at the time, more oblate when the speed is greater, less oblate when
it is slower. Let us imagine that the lithosphere also is at some
time a figure of equilibrium answering to the speed of rotation at
that time. If the speed remained constant, the lithosphere would
retain this figure, and the matter within it would remain always
in the same configuration without having to support any internal
tangential stress. Now suppose that the speed of rotation gradually
diminishes. The surface of the ocean will gradually become less and
less oblate. The lithosphere also will gradually become less oblate,
but not to such an extent as to make it a figure of equilibrium
answering to the diminished speed of rotation, while the matter
within it will get into a state of gradually increasing internal
tangential stress. The effect on the distribution of land and water
will be that the depth of the ocean will gradually diminish in lower
latitudes and increase in higher latitudes, the latitudes of no
change being 35° 16′ N. and S.
“The internal tangential stress in the matter within the lithosphere
may increase so much that it can no longer be supported. If this
happens a series of local fractures will take place, continuing until
the lithosphere is again adjusted much more nearly to a figure of
equilibrium, which will be less oblate than the original figure. The
effect on the distribution of land and water will be that the depth
of the ocean will increase rather rapidly and spasmodically in lower
latitudes and diminish in higher latitudes.
“Accordingly, the kind of geological change which the theory of tidal
friction would lead us to expect is a sort of rhythmic sequence,
involving long periods of comparative quiescence, marked by what
Suess calls ‘positive movements of the strand,’ in the higher
latitudes, and ‘negative movements’ in the lower, alternating with
comparatively short periods of greater activity, marked by rise of
the land around the poles and subsidences in the equatorial regions.”
The main periods of adjustment under this scheme fall at the
beginning and end of the Proterozoic, in the Permo-Carboniferous
and in the Quaternary. The two latter at least were periods of
great earth-movement, while the two former were also continental
periods, since the land-masses were large and high enough to develop
ice-sheets.
The difficult question raised by the low latitudes in which the
Pre-Cambrian and Permo-Carboniferous glaciations were chiefly
developed cannot yet be regarded as solved, but the geological facts
speak strongly in favour of ice-sheets rather than mountain glaciers,
and practically speaking it is meteorologically impossible for large
ice-sheets to extend to sea-level in the Tropics while the rest of
the world enjoys a temperate climate. The only escape seems to be to
assume a position of the South Pole somewhere between Africa, India
and Australia throughout the whole of the Proterozoic and Palæozoic
periods. On the other hand, from the Jurassic onwards, there is no
real support to the hypothesis that the positions of the poles were
other than they are now. Wegener’s explanation of the Quaternary Ice
Age we have seen to be untenable. The period of transition appears
to lie in the later Permian and Triassic. The Proterozoic and
Permo-Carboniferous glacial periods were much less definite in the
north than in the south-east; but such as they were they appear to
have been most severe in the east of North America, where the ice was
coming from the north; there are also some glacial traces in Europe.
This indicates that the position of the North Pole cannot have been
in the North Pacific Ocean, which is antipodal to the South Indian
Ocean. Hence it seems that what we have to consider is not so much
the wanderings of the poles at large among the continents as the
break-off at the close of the Palæozoic period of portions of the
Antarctic continent and their drift northwards towards the equator.
Without going into the mathematics of the question, it seems just
possible that the periodic overloading of circumpolar continents by
large ice-masses could have this effect in the course of time,[2] but
the suggestion is put forward tentatively for consideration rather
than as a definite hypothesis. We must be thankful that in the next
chapter we are on safer ground.
BIBLIOGRAPHY
Coleman, A. P. “Climates and physical conditions of the early
Pre-Cambrian.” _Geol. Mag._ (6), Vol. 1, 1914, p. 466.
Eckardt, W. R. “Paläoklimatologie.” Sammlung Goschen, Leipzig, 1910.
Geikie, J. “The Evolution of Climate.” Edinburgh, _Scot. Geogr.
Mag._ (6), 1890, p. 57.
Grabau, A. W. “Principles of Stratigraphy.” New York, 1913, pp. 74,
_et seq._
Neumayr, M. “Ueber klimatischen Zonen während der Jura- und
Kreidezeit.” Wien, _Denkschr. Ak. Sci._, 47, 1883, p. 211.
Ramsay, W. “Orogenesis und Klima.” _Ofvers af Finska Vetenskaps.
Soc. Forb._, 52, 1909-10, A, No. 11. Helsingfors, 1910.
Schuchert, C. “Climates of Geologic Time, In: The climatic
factor as illustrated in arid America,” by Ellsworth Huntington.
Washington, 1914, Pt. 2.