the post-glacial climatic optimum extended to the Antarctic coast,
and this is additional argument for extending the much greater
interglacial oscillation southward beyond its known limits in Tierra
del Fuego, Australia and New Zealand, but here the matter must be
left.
BIBLIOGRAPHY
Hedley, C. “The palæographical relations of Antarctica.” _London,
Proc. Linnæan Soc._, 124, 1911-2, p. 80.
Lydekker, R. “A geographical history of mammals.” Cambridge
University Press, 1896, pp. 125 ff.
David, T. W. E. “Antarctica and some of its problems.” London,
_Geogr. J._ 43, 1914, pp. 605-30.
Nordenskjold, O. “Antarktis.” _Handbuch Regional Geologie_, Heft
15, 1913.
CHAPTER XIII
THE CLOSE OF THE ICE AGE—THE CONTINENTAL PHASE
In Chapter V we left the climatic history of northern Europe at the
point where the ice in its final readvance had once more reached
the German coast. But Scandinavia was now sinking, and the margin
of the ice soon began to retreat again. At the same time the Alpine
glaciers diminished in size, while the Irish and Scottish glaciers
disappeared. This is the critical period in the change from glacial
to temperate conditions, and, thanks to the researches of the
Swedish geologists, and especially G. de Geer, H. Munthe and Gunnar
Andersson, we are very well acquainted with it. The change was
not uniform; at first the recession was very slow, and there were
periods when for scores of years the ice-edge remained stationary
or even readvanced, but on the whole the time was one of persistent
amelioration. The following description is based chiefly on W. B.
Wright’s summary of de Geer’s work.
After leaving the coast of Germany the ice-edge appears to have
remained in the western Baltic, retreating slowly for some 8000
years. About 10,000 B.C. it lay along the southern coast of Sweden,
and during the next 2000 years it withdrew to about 59° N. This was
the Gotiglacial stage. Here came a pause, when, for 200 years, about
8000 B.C., owing presumably to a change for the worse in the climate,
the ice-edge remained in one position, forming a great moraine. Then
came another period of very rapid retreat, the Finiglacial occupying
nearly 3000 years, followed by a further halt of some duration near
Ragunda, about 5000 B.C. After this the ice-sheet split into two
portions, and the Glacial period is regarded as over.
In the Alps there were similar periods of regression and of halting
or readvance. The first, known as the Bühlstadium, corresponded to
the Baltic readvance (Chapter V). The second, the Gschnitz-stadium,
with a snow-line 2000 feet below the present (i.e. mean temperature
about 6° F. lower than now), has not been dated, but probably
occurred about 8000 B.C. This was followed by a warmer period,
probably as warm as and drier than the present, after which the
glaciers readvanced about 5000 B.C. in the third or Daun-stadium,
when the snow-line was depressed 1000 feet (temperature 3° F. lower
than now).
In the lower Nile valley the deposition of gravel ceased, and that of
mud began about 8000 B.C., indicating that at this time the climate
of north-east Africa reached its present state of dryness.
It is at present difficult to give more than a tentative explanation
of these oscillations of climate during the Retreat Phase. Northern
Europe was at the time passing through a complicated series of
geographical changes. As the ice left the Baltic basin the latter
became the site of a cold ice-lake, with narrow outlets to the
Atlantic by way of the Sound and the Belts. At this time the
recession was slow. Then the retreat of the ice opened a connexion
with the White Sea, and elevation closed the outlet to the west.
This probably made the waters still colder, and the Fennoscandian
pause occurred. Elevation now closed the connexion with the White
Sea, and an entirely closed-in ice-lake resulted. During this stage
the retreat was slow, until between 7000 and 6000 B.C., when the
ice-sheet vacated Scania, and direct communication between the Baltic
and the Atlantic was opened across Lakes Wener and Wetter, and the
climate, though still arctic at first, became appreciably warmer by
6000 B.C.
For more than 10,000 years of the retreat, or until 6000 B.C., the
ice-sheet was still sufficiently large and powerful to maintain a
border of Arctic anticyclonic conditions on its southern edge. During
the retreat the mean annual temperature of southern Sweden increased
from 17° F. to 35° F., equivalent to a change from North-east
Greenland to South Greenland. The July temperature rose to about 43°
F. On the North German Plain still lived the reindeer and the fauna
and flora of the sub-Arctic tundras; the mean annual temperature rose
to 45° F. by the close of the period. The land flora in Sweden was
entirely xerophilous, indicating a slight rainfall. There is also
geological evidence of a small annual rainfall on the south-west
coast of Norway. This period covers the transition from Palæolithic
to Neolithic culture.
It seems probable that the continental character of the climate of
the final stages of the retreat phase was slightly increased by
astronomical causes, the obliquity of the ecliptic being probably
nearly one degree greater about 7500 B.C. than it is now. In Germany
and Sweden this would have the effect of lowering the winter
temperature and raising the summer temperature by rather more than 1°
F.
While the land was still falling rapidly in the north of Scandinavia
and the Gulf of Bothnia, the coasts of Germany and Denmark began to
rise, and about 6000 B.C. again closed[5] the outlet of the Baltic,
converting it into a large fresh-water lake, the _Ancylus_ lake. A
similar lake was formed farther east in central Finland. At this time
the south-west Baltic lands stood more than 100 feet higher than at
present. The land was probably still largely under the influence of
dry easterly winds, and the shutting out of the Atlantic accentuated
the continental conditions, and this stage in the climatic history
of Europe is known as the “Continental Phase.” The winter climate
was severe; at first the summers were not especially warm (July
temperature about 54° F. in southern Sweden). This is probably the
period of formation of the Ragunda moraines, and of a readvance of
the glaciers on the Norwegian side of the divide, when the snow-line
lay 200-300 metres lower than at present; it was also the time of
the Daun readvance in the Alps. But as the land sank in the north
and rose in the south, the waters of the _Ancylus_ lake retreated
farther and farther north, and the summers became hot and dusty, with
a mean July temperature of about 60° F. Everywhere in the Baltic
regions the older _Ancylus_ beds show a monotonous pine-wood, but
in the upper _Ancylus_ these are followed closely by a number of
plants and shrubs of southern type—black alder, curled birch, linden,
etc. The temperature continued to rise, and oak, Norway maple, ash,
and finally, in the southernmost parts of Sweden, the common maple
appeared. The last-named plant has been found below the present level
of the sea in Ystad Harbour.
Under the influence of these conditions the remnant of the
Scandinavian ice-sheet again decreased in size, until it split into
two portions, the break occurring at Ragunda, and this is considered
by Scandinavian geologists to mark the end of the Ice Age in Europe.
Gunnar Andersson compares the climate of southern Sweden at this time
to the Baraba Steppes in western Siberia, with an annual rainfall
of 12 to 16 inches, but this seems an extreme estimate. The “Karst”
flora of the limestone areas of south-east Europe immigrated into
eastern Sweden during this period, and south-east Europe probably
gives a better idea of the climate of Sweden during the continental
phase. Farther east, in Finland, Kupffer describes the climate as
resembling that of central Russia. In central Germany the climate was
dry, with a mean temperature in the four summer months of 63° F.;
it resembled that of south-west Russia. This period of warm summers
began earlier in Germany than in Sweden, and throughout this phase
Scandinavia was occupied by a rich forest flora. The hazel extended
several degrees north of its present position, and to higher levels,
indicating a July temperature about 7° F. higher than the present. In
southern Norway the pine extended to much greater heights. But the
ivy and yew, whose limits depend on the winter rather than on the
summer temperature, showed no such extension, indicating that the
winters remained severe. In Denmark there was a dry climate, fairly
warm at the close, with fir forests, though western Denmark is now
too wet for this tree. On the coast of Norway the seas were still
cold, so that there is a contrast between the animal life of the sea
and the plant life of the land. The Alps also became warm and dry,
and were occupied by a xerophilous flora.
As the glacial anticyclone decreased in intensity, depressions from
the Atlantic began to take a more northerly course, but were held up
near the British Isles and materially increased the rainfall. This is
the first peat-bog period of these islands, when the birch and pine
forests which had covered the non-glaciated lands during the cold
dry period gave way to extensive growths of peat-bogs. Southern and
eastern England, however, largely escaped this damp period, sharing
in the dry climate of the Continent.
The absence of storms off the north-west coast of Norway is shown by
the forests which at this period covered all the outermost islands
of Norway as far as Ingo Island, off North Cape. These islands are
now barren, and their afforestation indicates a drier and especially
a less stormy climate than the present, with a decreased frequency
of winds from the sea. These conditions were well developed about
5000 B.C. This is the Early Neolithic period. Owing to the great
development of forests, this period is sometimes called the _Early
Forest period_.
The late-glacial history of North America was equally complicated.
Consider first the region of the St. Lawrence Estuary and the Great
Lakes. As the Wisconsin ice-sheet retreated across the present
site of the Lakes, the latter underwent a remarkable series of
fluctuations of area and outflow, which have been made the subject
of brilliant studies by several American geologists. The opening
stage began when the ice abandoned the high ground south of the
lakes, leaving depressions bounded on the south by the hills and
on the north by the ice. The earliest of these in the basins of
Lakes Erie and Huron are known as the first and second Lake Maumee.
These gradually grew in size and coalesced, forming several series
of connected lakes, to which various names have been given; thus
Lake Warren extended well outside the present limits of Lake Erie
and southern Huron, and was held up by ice over Lake Ontario and
northern Huron. At a later stage an enormous Lake Algonquin extended
beyond the combined limits of Lakes Superior, Michigan and Huron, and
communicated by broad channels with an enlarged Lake Ontario known
as Lake Iroquois, and with Lake Erie. But even before this time the
northern shores of the lakes, relieved of the major portion of their
ice-load, had begun to rise rapidly, and ultimately reduced the lakes
to their present size.
These great areas of ice-cold water, bathing the southern edges
of the ice-sheet, must have had an unfavourable influence on the
climate, keeping it cold and damp, and preventing dry continental
conditions from becoming established. They probably retarded the
ice-retreat in these regions quite considerably, so that a lobe of
ice was left here long after the edge had retreated northwards on
either side. At the same time the climate further south was dry, with
æolian deposits; but as the anticyclonic winds blew off the Atlantic
the evidence of drought is not so marked as in Europe.
After this slow retreat had been in progress for a considerable time
a submergence, known as the “_Champlain stage_” set in, reaching a
depth of at least 600 feet and opening the St. Lawrence regions
wide to the Atlantic, which penetrated into Lake Ontario. The ice
now retreated rapidly under the influence of a maritime climate
little colder than the present. In phase this period corresponds
to the second _Yoldia Sea_ stage of Scandinavia; in point of time
it was probably somewhat earlier. This was followed by elevation,
the first result of which was to cut off the warm water and cause a
sharp fall of temperature exactly analogous to that of the Ragunda
moraines, but a few thousand years earlier and probably more marked.
The continuance of elevation brought on a long continental period
of extreme aridity, when trees grew on the peat-bogs of the eastern
States, while the lakes of the Great Basin further west were almost
or wholly dried up. At the maximum of the continental conditions the
summers at least were warmer than at present, as indicated by the
northward extension of various species of plants and fresh-water
mollusca. The winters were probably more severe. Possibly the great
aridity of this period was partly due to a sub-glacial continental
anticyclone obstructing the path of depressions across America from
west to east. The drainage area of the Great Basin received hardly
any rainfall and was a hopeless desert, but the Atlantic States were
able to grow trees on the old peat-bogs, probably with rainfall
derived from the Atlantic. By reference to the cutting of Niagara
gorge, we can infer that the warm dry period began about 6000 B.C.,
so that it corresponds exactly with the continental phase (_Ancylus_
stage) of Europe. This period of aridity was finally ended by a fresh
submergence, the “_Micmac_,” which carried the land about twenty feet
below its present level.
In Yukon and Alaska, where the glaciation was not nearly so severe as
further to the south-east, the depression of the land by the ice-load
and consequently the subsequent rise on its removal were not great.
There were no complicated geographical changes, and correspondingly
there appear to have been no fluctuations of climate, but only a
gradual passage to present conditions.
Even in Iceland there are indications of a dry period following the
last glacial maximum, for tree-trunks, buried in the peat-bogs, show
that the birch formerly had a much greater extension. It is also
quite possible that there was an accentuation of desert conditions in
Asia during the retreat of the glaciers in Europe and North America,
which may have played a part in the wave of Neolithic migration that
appears to have overwhelmed the artistic Palæolithic races of western
Europe; but of this we have as yet no direct evidence. The Neolithic
invasion of Europe took place along two main routes, the Nordics
passing from the centre of Asia north of the Caspian, across Russia
to the Baltic shores, where they became the Kitchen-midden people;
and the Alpine race passing from Transbaikalia, south of the Caspian
and Black Sea, into southern Europe. The Nordics drove before them
an older race, characterized by the transitional Maglemose culture,
which passed from east of Russia to the shores of the Baltic and
ultimately to England, where harpoons of Maglemose type have been
found beneath the peat of Holderness.
In the southern hemisphere the continental phase does not appear
to have been so well developed. The uppermost part of the Pampean
loess is possibly post-glacial; more certainly so are the sand-dunes
on the coast near Buenos Aires, in which human remains have been
found in association with the bones of some extinct animals. In New
South Wales, after the retreat of the glaciers, there was a period
with land a little above its present level, so that the stools of
Eucalyptus trees are now found ten feet below sea-level; but there is
no evidence as to the climate of this stage. In New Zealand we have
no definite post-glacial beds of continental type. The occurrence of
xerophilous plants, such as _Aciphylla_, still living in a climate
which is now decidedly moist, may be a remnant of a continental phase
in New Zealand, or may date back to the steppe conditions of the
loess. As to Antarctica, we have, of course, no evidence.
BIBLIOGRAPHY
Brooks, C. E. P. “The evolution of climate in north-west Europe.”
London, _Q. J. R. Meteor. Soc._, 47, 1921, p. 173.
Wright, W. B. “The Quaternary Ice Age.” London, 1914. (Ch. 17,
Late-glacial changes of level in North America.)
_Bericht Internat. Geologenkongress_, Stockholm, 1910. “Die
Veränderungen des Klimas seit der Maximum des letzten Eiszeits.”
Numerous papers, dealing with Europe and North America.
Munthe, H. “Studies in the Late-Quaternary history of southern
Sweden.” Stockholm, _Geol. Foren. i Forh._, 22, 1910, pp. 1197-1292.
Antevs, E. On the late-glacial and post-glacial history of the
Baltic. _Geogr. Rev._, New York, 12, 1922, p. 521.
CHAPTER XIV
THE POST-GLACIAL OPTIMUM OF CLIMATE
In most of the polar and temperate regions of the world the Glacial
period seems to have been separated from the present by a short
interval of slightly more maritime climate. The existence of this
phase was the chief point brought out in the great collection of
papers communicated to the Stockholm meeting of the International
Geological Congress, which has frequently been referred to in
this volume. The pioneer work on the subject has been done by the
Scandinavian geologists, and we may commence with a discussion of
this period in the countries bordering on the Baltic.
About 4000 B.C., at the conclusion of the continental phase referred
to in the preceding chapter, a rapid movement of submergence set
in over the whole of the southern Baltic, and shortly afterwards
the land-bar which had formerly separated the fresh waters of the
_Ancylus_ lake from the Atlantic gave place to a wide strait, through
which the waters of the ocean flowed into the Baltic across southern
Sweden. Ultimately this channel became wider than the present outlet
between Sweden and Denmark, and maritime influences penetrated to
all parts of the Baltic. In recognition of this influence the period
was termed by Blytt the “Atlantic stage.” The much greater freedom
with which the waters of the Atlantic were able to enter is shown by
a comparison of the “isohalines” of this period with those of the
present day. Isohalines indicate the degree of saltness of the water;
those of to-day can, of course, be measured directly, and show that
in the Gulf of Bothnia the water becomes continually less salt as we
go northward, for which reason many species of marine mollusca are
unable to live. By studying the distribution of the mollusca in the
_Littorina_ Sea the isohalines of that period have been reconstructed
also, and show that the salt content was much greater than at the
present day, indicating a greater influx of oceanic waters.
If we take a map showing a reconstruction of the geography of
_Littorina_ time, and apply to it the formulæ given in the Appendix,
comparing our results with the inferences of Scandinavian and north
German geologists as to the temperature, we find that there is a
remarkably good agreement. Many of the palæo-botanists comment on
the prolongation of the autumn into the present winter, which is
especially characteristic of a more insular climate. The amounts of
change in each case are also in good agreement, except perhaps in
the Christiania region and in north Denmark, where the geologists
require a greater change than that calculated from the land and sea
distribution; this is probably accounted for by a higher temperature
in the waters of the Atlantic. The maximum change as calculated
is shown in south-west Finland (winter 6° F. warmer, summer 2° F.
cooler). Finland is described as having at that time the climate of
western Europe, which we may take as meaning winter 8° warmer, summer
3-4° cooler. There was thus a great change from the extreme climate
of the continental phase with its hot summers and severe winters and
little rain, to an extremely temperate climate with cool summers,
mild winters and a heavy rainfall. The warmth-loving plants which had
begun to immigrate during the later part of the continental phase
continued to spread, and probably the highest average temperatures
were reached at the time of maximum submergence, but now they were
accompanied by plants for which a large rainfall is necessary, and it
seems that the average rainfall of southern Sweden must have been
about 40 inches a year. The oak began to dominate the forests in
place of the hazel, and the peat-bogs, which during the preceding dry
period had hardened into a firm surface on which birch and pine were
able to take root, again became moist, so that the trees were choked
by growths of bog-plants. On the shores lived men of the Transition
and Early Neolithic. As the land rose again and the _Littorina_ Sea
decreased in area the climate again became drier and more rigorous.
In Denmark the forests of the _Ancylus_ period gave place to oak as
the land sank, and there are also remains of two water plants, the
water-nut (_Trapa natans_), which is no longer found in Denmark,
and _Najas marina_, still living in one isolated locality. Northern
Denmark was broken up into islands, among which marine deposits were
formed, containing the remains of southern mollusca, many of which
are found in the kitchen-middings. Most of the wood used by Neolithic
man was oak; there is little fir and no beech.
In Norway the work of C. Brögger has made us familiar with the
_Tapes_ beds, which correspond in point of time to the _Littorina_
stage of the Baltic. _Tapes decussatus_ is itself a southern species
of mollusc, and it is associated with a very rich warmth-loving
fauna. In southern Norway the geographical conditions were different
from those in Sweden, for the land reached its lowest level
relatively to the sea about the close of the Glacial period, and has
been rising throughout the post-glacial. The seas show a progressive
rise of temperature from 8° F. below present at the close of the
Glacial period to 4° F. above the present in the older _Tapes_ beds.
The littoral climate at this stage resembled that prevailing at
present on the coast of northern England. After this, as the land
approached its present level, the temperature fell again, and in the
upper _Tapes_ stage was only 2° F. above the present.
The warm period represented by the _Tapes_ beds is found at intervals
along the west coast of Norway, and we again find evidence of a
submergence of the land contemporary with the maximum temperature.
These conditions extend even as far north as Tromsö, within the
Arctic circle. In Spitzbergen there are raised beaches 30 to 80
feet above the sea, containing remains of molluscs and a species of
_Fucus_, none of which are now living so far north. On the land there
are old peat-bogs of great thickness, though peat-mosses cannot now
grow, since the ground never thaws below a depth of 6-10 inches. It
has been pointed out that a great number of the plants now living in
Spitzbergen are unable to ripen their seeds under present climatic
conditions, though they must have done so in the past. Ripe seeds
of some species, in fact, have been found in the peat-bogs, which
are contemporaneous with the raised beach. There is thus evidence
of a very well-marked warm period associated with submergence in
Spitsbergen.
In Franz Josef Land, Nansen found raised beaches with mussels 10 to
20 feet above the present level; this shell does not now live so far
north. In the White Sea and on the Murman coast there are also raised
beaches with a southern fauna. The warm period shown in the beds of
the New Siberian Islands has already been referred to (p. 79).
Returning to the British Isles we find that in the south the land
was above its present level throughout the whole of the post-glacial
period. On the other hand, a 25-foot beach is found in north-west
England (Formby and Leasowe marine beds), but without any evidence
as to climate; the same applies to the 25-foot beach of Scotland.
It is only when we come to the north-east of Ireland that we find
evidence of conditions appreciably warmer than the present, in the
section of the Alexandra Dock, Belfast, where marine clays overlie
beds of grey sand and peat. The lower estuarine clay is essentially a
littoral clay, known as the _Scrobicularia_ zone. It is brownish-blue
and sandy, and contains in abundance the roots and leaves of the
grass-wrack (_Zostera marina_), and a vast number of shells of a few
species which live between tide-marks, indicating that the land stood
10 feet or so above its present level at first, while the climate
cannot have differed greatly from that prevailing at the present day.
It must have been formed during a period of gradual depression, for
throughout its six feet or more of thickness it preserves identical
littoral characters. After a time this depression became more rapid,
and the upper estuarine clay began to form—a light blue clay, very
pure and unctuous, with a very rich and well-preserved fauna, known
as the _Thracia_ zone. The fauna has a decidedly southern aspect,
and indicates that the coasts of north-east Ireland had the present
temperature of Bantry Bay—an increase of at least 3° F. in the mean
annual temperature. The _Thracia_ zone is followed by a bed of yellow
shore sand, indicating re-elevation to about seven feet above the
present level.
Corresponding to the upper estuarine clay are raised beaches at
a height of 25 feet in north-east Ireland, falling to 15 feet at
Dublin, and to only 6 or 7 feet in western Donegal and Sligo. The
mollusca indicate a somewhat higher temperature than the present. In
the beaches have been found flint scrapers and arrowheads of early
Neolithic type.
Looking further westwards, we find that Iceland, which had undergone
a slight elevation during the continental phase, so that peat was
formed below present sea-level, again subsided, falling to 10 or 12
feet below its present level. During this subsidence the temperature
rose, the greatest warmth coinciding with the lowest level of the
land. Species from the south-west shores, where the temperature of
the water is directly influenced by the Gulf Drift, extended to the
cold northern coast. In some places the marine clays of this period
have been ploughed up by a subsequent readvance of the glaciers.
From Greenland comes abundant evidence of a post-glacial warm period
coincident with a subsidence of about 80 feet. Raised beaches all
along the west coast contain mollusca, some species of which are not
now living north of the St. Lawrence estuary. On the other hand,
some northern species which lived off the west coast during the
glacial maximum retreated northwards during this period, and have
not re-established themselves, though the climate is now suitable.
Further, K. Steenstrup describes the occurrence of “dead ice” at
several places in North Greenland—masses of ice which have become
separated from their parent glaciers owing to rapid recession,
and are now buried in morainic matter. Subsequently the ice again
advanced, and in some cases a new glacier has advanced over these
masses of “dead ice.”
Passing to the mainland of North America, we find in eastern Canada
colonies of southern mollusca, especially oysters and quohogs,
separated from their main area of distribution south of Cape Cod
by a wide area of cold seas—the Gulf of Maine and Bay of Fundy. At
the beginning of the warm phase the land lay slightly below its
present level, but subsequently rose above it. The climate became
still warmer, until its temperature resembled that of the middle New
England States. At the same time the rainfall diminished and the
peat-bogs were replaced by forests of hardwood trees. In the basin of
the Great Lakes the warm period is represented by gravel beds in the
Niagara gorge, which from their position must, according to the most
recent determinations, have been formed about 4000 to 3000 B.C. These
gravels contain shells of fresh-water mollusca, especially species of
_Unio_, which are not now living in the St. Lawrence system, but are
found in tributaries of the Mississippi further south. Further south
on the eastern coast of the United States there are marine deposits
indicating a slight submergence, with a climate somewhat warmer than
the present.
Passing to South America, we find in southern Patagonia and Tierra
del Fuego exactly similar evidence of a post-glacial subsidence with
a warmer climate than the present. Raised beaches at a height of 50
feet contain mollusca, some of which are now rare or extinct in that
locality, and in sheltered situations plants are found still living
whose nearest neighbours are some way to the north.
In the same way, in southern and eastern Australia there are beaches
a few feet above present level, containing warmth-loving species of
mollusca and indicating a post-glacial warm period. There is some
evidence in the distribution of plants and marine mollusca that this
warm period extended to New Zealand. Raised beaches at a height of
50 to 180 feet are also known from many places in Antarctica, and
these contain mollusca, some of which are not now living south of
the sub-antarctic islands. An interesting confirmation of this has
been given by E. Philippi, from the results of an examination of the
sea-floor at four points in about 63° S., 75-95° E., all within the
present limit of pack-ice. The deposit at present forming is poor
in pelagic foraminifera, and consequently contains little lime, but
this deposit is very thin, and beneath it is a much more calcareous
clay especially rich in _Globigerina_. The latter deposit is still
forming north of the limit of pack-ice, and Philippi concludes
that at no very distant date the limits of ice were further south,
indicating warmer conditions. It is interesting to note that a
similar sequence has been found in the Norwegian North Sea, the brown
foraminiferous deposit (in this case containing _Biloculina_) being
known to be underlain as well as overlain by an unfossiliferous
grey clay attributed to the Glacial period. Finally, with regard to
Cape Colony, A. W. Rodgers says: “It is possible that the presence
of marine mollusca belonging to species that are only known in the
living state from the coast north of Pondoland, in the raised beaches
of Mossel and Algoa Bays, indicates that the sea on the south coast
was formerly warmer than now.”
Thus we have evidence of a period of submergence and climates warmer
than the present from a large number of places, including the Arctic
Ocean and Greenland, the temperate coasts of North America and
Europe, the Southern Ocean and Antarctica. The stage appears to be
missing on the temperate coasts of the Pacific, on both the Asiatic
and North American sides, and from the whole of the Tropics. It is
fairly certain that the warm period occurred at the same time in
eastern North America and western Europe; in the case of the southern
hemisphere we have no direct proof of this, but in all cases the
deposits are comparatively recent, and since they obviously refer to
a similar state of affairs we may assume that they are of the same
date.
In the Baltic area we know that the great change of level was due
largely to a subsidence of the land and only to a small extent to
a rise of the sea. But in other parts of the world the amount of
submergence was remarkably uniform at places in the same latitude,
and decreased steadily from the polar regions to about latitude
40-50°, where it became zero. Now such a general change suggests that
it was the sea which rose rather than the land which sank, and points
to some general cause which piled up the waters of the oceans in the
higher latitudes. A possible cause of this nature has been adduced
by O. Pettersson, which he terms the “tide-generating force,” which
reached one of its maxima in an 1800-year cycle about 3500 B.C. This
possibility will be dealt with more fully in Chapter XVII.
BIBLIOGRAPHY
_Bericht Internat. Geologenkongress_, Stockholm, 1910. “Die
Veränderungen des Klimas seit der Maximum des letzten Eiszeits.”
Numerous papers ranging from the Arctic to the Antarctic.
Brooks, C. E. P. “The evolution of climate in north-west Europe.”
London, _Q. J. R. Meteor. Soc._, 47, 1921, p. 173.
Praeger, R. Ll. “Report on the estuarine clays of the north-east of
Ireland.” _Proc. R. Irish Acad._, ser. 3, Vol. 2, 1892, pp. 212-89.
Goldthwait, J. W. “The twenty-foot terrace and sea-cliff of the
lower St. Lawrence.” _Amer. J. Science_, ser. 4, Vol. 32, 1911, pp.
291-317.
Cowles, H. C. “A remarkable colony of northern plants along the
Apalachicola River, Florida, and its significance.” _Rep. 8
Internal. Geogr. Congress_, 1904, p. 599.
Shimer, H. W. “Post-glacial history of Boston.” _Amer. J. Science_,
ser. 4, Vol. 40, 1915, pp. 437-42.
Halle, T. G. “On Quaternary deposits and changes of level in
Patagonia and Tierra del Fuego.” _Bull. Geol. Inst._, Upsala, 9,
1908-9, pp. 93-117.
Süssmilch, C. A. “An introduction to the geology of New South
Wales,” Sydney, 1914.
Marshall, P. “New Zealand.” _Handbuch regional Geologie_, Heft 5,
1911.
CHAPTER XV
THE FOREST PERIOD OF WESTERN EUROPE
Hitherto we have been dealing with climatic changes which can be
recognized with more or less certainty over most of the polar and
temperate regions of the world, but we have now to describe a stage
which appears to have been peculiar to Europe and possibly Asia—the
Forest period. By 3000 B.C., or towards the close of the Neolithic
period, considerable elevation had again taken place over the central
latitudes of western Europe (the northern parts of Norway and Sweden
were still several hundred feet below their present level). The
southern part of the British Isles, which had remained slightly
elevated since the last Glacial period, had now emerged to a height
of nearly ninety feet above its present level; the area of Ireland
had increased appreciably and part of the North Sea was land. The
geographical changes were not great, but they were sufficient to turn
the scale in the direction of a continental climate in the British
Isles. The more or less complete closing of the Straits of Dover, and
the consequent bar to the free circulation of the Gulf Drift, must
have had an appreciable effect on the climate in the direction of
continentality. At the same time the low level of northern Norway,
and possibly the persistence of warm conditions in the Arctic basin,
more and more attracted depressions to the northernmost track, so
that the British Isles especially, and to a lesser extent Holland,
Germany, southern Scandinavia and Russia, came more persistently
under the influence of anticyclonic conditions. The rainfall of
these countries diminished, and the surface of the bogs dried
sufficiently to enable forests to grow in the western countries; in
Germany heath-plants took the place of bog-plants, while in Russia
steppe conditions supervened. The normal meteorological conditions at
this time in fact resembled those of the memorable drought of 1921,
which was characterized by low pressure and stormy conditions in
the Arctic Ocean and a belt of high pressure and persistently fine
weather across central Europe.
During this phase the winters may have been severe, but the summers
were warmer than the present, for in the peat-bogs of Ireland and
Scotland are the remains of trees larger than any now found in the
neighbourhood. The Irish bogs dried so completely that they were
extensively inhabited; corded oak roads have been found at this
horizon, while in 1883 a two-story log house, surrounded by an
enclosure, was found in Drumkelin Bog, Co. Donegal; it was twelve
feet square and nine feet in height, and a roadway led to it across
the bog. Both house and roadway were entirely constructed of oak.
With the hut were found a stone chisel and a flint arrowhead. Beneath
the floor were fourteen feet of bog, and above the floor twenty-six
feet. This time was also one of relatively little wind movement,
for stools occur even in exposed positions on the mountain slopes
of western Ireland, where trees will only grow now in sheltered
positions near sea-level.
Further evidence of the very dry climate of this phase is the
frequent occurrence of trees apparently _in situ_ beneath the surface
of fresh-water lakes, both in Ireland and Scandinavia. I was able to
examine one very good example near Lough Toome in north-west Ireland.
An unusually dry spring had lowered the surface of the water and
a large number of tree-stools were exposed; when these trees were
growing the water-surface must have been at least two feet below
the level of the present outlet. Most of the lakes in which these
stools are found are shallow upland basins with a small drainage
area, and if the present climate became drier they would more or less
completely disappear.
Mr. Fairgrieve has noted the action of blown sand on the westward
side of broken-off tree-stumps in a submerged forest on the shore in
south Wales, which, though not conclusive, suggests dry conditions.
Mr. Fairgrieve also noted the direction of fall of twenty-one trees,
and found that in the great majority of cases they were blown down by
westerly winds.
The forest phase was short; according to the late C. Reid the land
again began to subside shortly after 3000 B.C., and by 1600 B.C.,
in Britain at least, had reached its present level; this carries
us to the beginning of the Bronze Age. In connexion with Ellsworth
Huntington’s theory that the dampness of Ireland lowers the energy
of its inhabitants, it is interesting to note that this dry period
apparently corresponds to the legendary Heroic Age, when the vigour
of the Irish reached a level never since attained. Civilization in
Scandinavia also seems to have benefited by the drier conditions,
for Scandinavian technique advanced rapidly to a high level about
1800 B.C. But though there is evidence of a considerable sea-borne
commerce with Britain and Ireland, there appears to have been
comparatively little land traffic between different parts of
Scandinavia at this time. In fact, to primitive man dense forest with
thick undergrowth was almost impenetrable. But at the close of the
forest phase and the beginning of the peat-bog phase the trees were
weakening under conditions becoming unfavourable. Such dying forests
are marked by the absence of undergrowth and young trees, and afford
safe and easy land communication. Accordingly we find that by 1500
B.C. a considerable traffic had developed across Scandinavia by land.
Although we have no direct evidence, the meteorological conditions
suggest very strongly that the dry belt extended across Russia into
Siberia as a marked period of desiccation, possibly worse than any
droughts of the historic period. At present Siberia receives its
rainfall mainly from depressions which cross Russia from the Baltic
or Black Seas, and follow a well-marked track north of the central
Asiatic mountains. But during the forest period these tracks were
abandoned, and the majority of the depressions passed north-eastward
off the coast of Norway into the Arctic Ocean. The result must
have been a great diminution of rainfall over the continent. We
shall see later (Chapter XIX) that this period of drought was of
extraordinary importance in human history. For during the moist
maritime phase central and eastern Europe, and probably also Asia,
had become extensively peopled by neolithic nomads of Aryan and
Semitic races, while the great river valleys of the south were in
the possession of dense agricultural populations in a more advanced
state of civilization. As the climate became progressively drier and
the pasture diminished, the land was unable to support such a large
nomadic population, and there was a great outburst of raiding and
conquering expeditions directed southwards and westwards, resulting
in a succession of empires in the rich Mesopotamian regions and
neighbouring countries, which form the beginnings of our history. The
beginnings of history in China also, about 2500 B.C., show that at
this time the settled peoples of that country were in trouble with
the nomads of the interior.
BIBLIOGRAPHY
Reid, C. “Submerged forests.” Cambridge University Press, 1913.
Lewis, F. J. “The history of the Scottish peat-mosses and their
relation to the Glacial period.” Edinburgh, _Scot. Geogr. Mag._,
22, 1906, p. 241.
CHAPTER XVI
THE “CLASSICAL” RAINFALL MAXIMUM, 1800 B.C. TO A.D. 500
About 1800 B.C., or the beginning of the Bronze Age in Britain, the
subsiding land finally attained approximately its present level. At
the same time the climate of western Europe deteriorated, becoming
much more humid and rainy, and there set in a period of intense
peat-formation in Ireland, Scotland and northern England, Scandinavia
and North Germany, known as the Peat-Bog Period or Upper Turbarian.
The peat-beds choked and killed the forests which had developed on
the older peat-bogs, and grew up above the stools and fallen trunks,
so that we have two layers of peat separated by an old forest. The
forest level contains neolithic articles, the peat contains gold
collars, bronze swords and pins, and other objects of the Bronze
Age. This growth also went on even over high ground, which had not
previously been covered by peat, for Professor Henry informs us that
on Copped Mountain, near Enniskillen, and at other places in Ireland,
Bronze Age cairns and tumuli are found resting on rock and covered
by several feet of bog. Peat-beds on the Frisian dunes between two
layers of blown sand are dated about 100 B.C., and some bogs in
northern France were formed during the Roman period. There is also
some much-disputed contemporary Latin evidence that at the time of
the Roman occupation the climate of Britain was damp and boggy, while
Gibbon (“Decline and Fall of the Roman Empire”), referring to the
climate of central Europe at the beginning of the Christian era,
points to some evidence that the climate was colder. This is, that
the Rhine and the Danube were frequently frozen over, so that the
natives crossed them with cavalry and wagons without difficulty,
although at the present time this never happens. It is possible
that this severe climate is referred to in the Germanic legend of
the “Twilight of the Gods,” when frost and snow ruled the world
for generations. The Norse sagas point to a similar cold period in
Scandinavia. This lapse of climate occurred in the Early Iron Age,
about 650 to 400 B.C., when there was a rapid deterioration from the
high Scandinavian civilization of the Bronze Age. This deterioration
of culture was probably the direct result of the increased severity
of the climate.
This Pluvial period has been made the subject of special studies
by Ellsworth Huntington in several important books and papers; he
finds evidence of a distinctly Pluvial period in three regions—the
Mediterranean, central and south-western Asia, and an area including
the southern United States and northern Mexico. In the first of
these, the Mediterranean, Huntington considers that the Græco-Roman
civilizations grew up in a period of increased rainfall which lasted
from about 500 B.C. to A.D. 200. These states were able to develop
in comparative peace because during this time there were no great
invasions of nomadic peoples from eastern Europe or central Asia,
a fact which points to good rainfall in these comparatively dry
regions, so that their inhabitants had no need to emigrate in quest
of a living. In the Mediterranean itself the heavier rainfall allowed
a solid agricultural basis which produced a sturdy race of peasants
who made good soldiers. Owing to the greater cyclonic control of
climate and consequent changeable weather, these inhabitants were
more vigorous in mind and body, for Huntington’s researches have
demonstrated that long spells of monotonous weather, either fine
or rainy, are unfavourable for human energy. Finally the heavier
rainfall maintained a perennial flow in the rivers, giving plentiful
supplies of good drinking water. These conditions broke down earlier
in Greece than in Italy, as the latter naturally has a heavier
rainfall. Huntington considers that the decline of Greece was largely
due to malarial poisoning, the decreasing rainfall causing the
river-flow to break down in summer, leaving isolated pools forming a
breeding ground for mosquitoes.
After A.D. 200 the climate of Italy also deteriorated. The decrease
of rainfall, combined with gradual exhaustion of the soil, made
wheat-growing more and more difficult for the small agriculturalist,
and the farms came into the hands of large landowners, who worked
them by slave labour, and in place of wheat either grew vines or
olives or raised flocks and herds. The agricultural population
gravitated to Rome and a few other large cities, and had to be fed by
imported wheat. The decline was probably aided by the introduction of
malaria, as in Greece.
In north Africa and Palestine the question is more debatable. C.
Negro, who has investigated the supposed desiccation of Cyrenaica,
concludes that there has been no change of climate since Roman times,
but a careful study of his evidence suggests that his conclusions are
open to criticism. All that he has proved is that there has been no
marked _progressive_ decrease of rainfall since about A.D. 200; he
has ignored the possibility of great fluctuations before and since
that date. In north Africa it seems difficult to believe that the
great cities of antiquity could have existed under present climatic
conditions, but when we turn to Palmyra in the Syrian desert we have
practically incontrovertible proof in the great aqueducts, built to
carry from the hill-springs to the city large volumes of water which
these springs no longer deliver, so that even where they are intact
the aqueducts now carry only the merest trickle.
In Persia we find numerous ruins, which point to a much greater
population two thousand or more years ago. This population lived
by agriculture, and the remains of their irrigation works are now
found in regions where running water never comes. Even the scanty
population of to-day can hardly live on the present rainfall of the
country, and it is unbelievable that the much greater population
indicated by these ruined cities could have existed without a very
much greater supply of water. The same condition is indicated by the
ruined cities of the great deserts of central Asia. These cities were
inhabited by agriculturalists, and the remains of tilled fields,
terraces and irrigation works abound in places where the supply of
brackish water would now be barely sufficient for drinking purposes
for such a large population. Huntington has also made a careful study
of the water-level of the Caspian Sea in classical times, and finds
that there was a great period of high water extending from unknown
antiquity to about A.D. 400.
There is only one region in central Asia where the population appears
to have been less in classical times than now, and that is the high
basin of Kashmir. Huntington points out that this basin is at present
near the upward limit of agriculture, and any fall of temperature
and increase of snowfall would drive out the inhabitants. But local
legends point to such a period in the remote past, corresponding to
the period of increased habitability of the central Asian deserts; at
its close there were extensive migrations from Turkestan into Kashmir.
Passing to America, we come to interesting evidence of a very
different class—I refer to the “big trees” (_Sequoia_) of California.
Since these trees live in a semi-arid climate, the amount of rainfall
is the chief factor in their growth, which finds an expression in
the breadth of the annual rings measured on the stump of the tree
when it is cut down. The method of utilizing the data was due to
A. E. Douglass. A careful comparison was first made between the
measurements of rings and the rainfall measured at neighbouring
stations, and a formula was developed by which the rainfall of each
year could be reconstructed from the tree-growth with a high degree
of accuracy. In extrapolating to find the rainfall for earlier years
before rainfall measurements began, various corrections had to be
applied, for instance trees grow more rapidly when young than when
they are old, while trees which are likely to live to a great age
grow more slowly at first than trees which die younger. These methods
were applied to nearly two thousand “big trees,” some of which were
found to be four thousand years old, but it is pointed out that the
corrections eliminate any progressive variation of climate which may
have occurred, so that the results show only “cycles” of greater or
lesser length. Summing up, Huntington says: “Judging from what we
have seen of the rainfall of to-day and its relation to the growth
of the Sequoias, high portions of their curve (of growth) seem to
indicate periods when the winters were longer than now, when storms
began earlier in the fall and lasted later into the spring, and
when mid-winter was characterized by the great development of a
cold continental high-pressure area, which pushed the storms of the
prevailing zone of westerly winds far down into sub-tropical regions
and thus caused sub-tropical conditions to invade what is now the
zone of equatorial rains.” Neglecting later favourable periods,
which are relatively short and unimportant, it is found that these
conditions prevailed very markedly between 1200 B.C. and A.D. 200,
with maxima about 1150 B.C., 700 B.C., and from 450 B.C. to 250 B.C.
Thus over the greater part of the temperate regions of the northern
hemisphere we have evidence of an important rainy period between the
extreme limits of 1800 B.C. and A.D. 400 or 500. This period was best
developed from 1200 B.C. to A.D. 200, and reached its maximum about
400 B.C. It constitutes a remarkable wave of climatic variation,
which is hitherto without adequate explanation. A somewhat similar,
though less intense, wave which occurred about A.D. 1200-1300, and
which is described in the following chapter, was associated by Wolf
to a great outburst of sunspots which took place about A.D. 1200. It
is well known that sunspots are an index of solar activity, the sun’s
radiation being greater at times of spot maximum than at times of
spot minimum. Greater solar radiation increases the evaporation over
the oceans, so that the air becomes more humid. This moist air is
carried by atmospheric currents over the land, where the moisture is
condensed into clouds and greatly increases the rainfall. At the same
time the cloud canopy shuts off some of the direct heat of the sun,
and we have the curious paradox that at times of sunspot maximum, or
greatest solar radiation, the temperature of the earth’s surface is
lowest.
The connexion outlined above is, however, extremely problematical
for temperate regions. Since the absolute sunspot maximum at A.D.
1200 is also very doubtful, it will be realized that the evidence for
the sunspot hypothesis of the mediæval rainfall maximum is extremely
slender. Furthermore, since we know nothing whatever about the solar
activity during the classical rainfall maximum, we are still less in
a position to extend the sunspot hypothesis to that period also.
The interesting theory recently put forward by O. Pettersson, already
alluded to, provides a plausible alternative explanation of the
severe stormy climate of the Peat-bog period, which reached a maximum
near 400 B.C. Without going into details this theory is that the
strength of the tides depends on the relative positions of the sun
and moon, and the tides are greatest when these act in conjunction,
and also when they are nearest to the earth. This fluctuation of
strength passes through various cyclic variations with periods of
nine years, about ninety years and about 1800 years, though the
lengths of the periods are not constant. The latter cycle is most
important to our purposes; according to Pettersson’s calculations
the fluctuations of the “tide-generating force” were as follow:
Maxima 3500 B.C. 2100 B.C. 350 B.C. A.D. 1434
Minima 2800 B.C. 1200 B.C. A.D. 530.
Increased range of the tides means increased circulation in the
waters of the oceans, especially an increased interchange between the
warm North Atlantic and the cold Arctic waters. It also means than an
unusual amount of ice is brought down from high into low latitudes.
Wide local variations of temperature of the surface waters of the
oceans cause increased cyclonic activity, and hence we may expect a
generally increased storminess at times of maximum “tide-generating
force,” and the reverse at times of minimum.
For the last maximum (A.D. 1434) Pettersson is able to adduce a good
deal of historical evidence of increased storminess in north-west
Europe and bad ice-conditions near Iceland and Greenland, while
Huntington has found an increase of rainfall shown by the big trees
of California. The next preceding maximum, that of 360 B.C., marks
the culminating point of the Peat-bog phase. The Norse sagas and
the Germanic myths point to a severe climate about 650 B.C., which
destroyed an early civilization. This was the “Twilight of the Gods,”
when frost and snow ruled the world for generations. The period
was the Early Iron Age, when civilization deteriorated greatly in
north-west Europe.
Of the maximum of 2100 B.C. there is no trace. It is possible that
the great Atlantic submergence of the Maritime phase is connected
with the tidal maximum of 3500 B.C., but the phenomena were on a
scale so much greater than those of the more recent maxima that this
can hardly have been the sole cause.
The minima should have been characterized by periods of relatively
quiet stable climate with little ice near Iceland and Greenland.
That the last minimum, in A.D. 530, was such a period there is
considerable evidence in the high level reached by civilization at
that period in Scandinavia and by the revival in Ireland. Again,
about 1200 B.C., in the early part of the Peat-bog phase, there
is evidence of considerable traffic by sea between Scandinavia
and Ireland. The Irish Museum has lately discovered a hoard of
gold objects dated about 1000 B.C., in which the designs show a
Scandinavian origin. The minimum of 2800 B.C., which occurred in the
Forest phase, may have contributed to the dry climate of that period,
but otherwise has left no trace.
Although at first sight the effect which Pettersson sets out to
explain seems out of all proportion to the smallness of his cause,
the coincidences after 2000 B.C. are extremely interesting, and
suggest that after the land and sea distribution reached its present
form the astronomical cause adduced by Pettersson was possibly
effective, but before that date the astronomical cause, if it
existed, was masked by the much greater climatic variations due to
changes in the land and sea distribution.
The opinion has frequently been expressed that the “Classical” and
“Mediæval” rainfall maxima were phenomena similar to the Glacial
period, but less intensive. This view is often carried to its logical
conclusion, that the thirty-five-year cycle, the eleven-year,
and still smaller cycles of climate, are also part of the same
series, and that the Glacial period and, let us say, the three-year
periodicity of rainfall are therefore due to variations of the same
agent, in this case the sun. This logical extension of the theory
is, however, completely untenable. The eleven-year periodicity is
admittedly connected with variations in the solar activity, but there
are other cycles which are completely independent of such variations,
such as, for instance, the annual variation undergone by all
meteorological elements, which depends entirely on the inclination
of the earth’s axis. There is a well-marked 4.8 year period in the
amount of ice off Iceland, the half-cycle of which is exactly equal
to the distance travelled by the water taking part in the North
Atlantic circulation, divided by the velocity with which it travels.
There is, therefore, no _a priori_ reason for assuming that the cause
of the Glacial period was identical with the cause of the Classical
and Mediæval rainfall maxima. Further, in the latter case, the chief
phenomenon was the increase of rainfall; the decrease of temperature
was merely incidental, but in the Glacial period the outstanding
feature was a great lowering of temperature in the polar and
temperate regions, and in this case it was the increase of rainfall
which was incidental.
BIBLIOGRAPHY
Lewis, F. J. “The history of the Scottish peat-mosses and their
relation to the Glacial period.” Edinburgh, _Scot. Geogr. Mag._,
22, 1916, p. 241.
Brooks, C. E. P. “The correlation of the Quaternary deposits of
Great Britain with those of the Continent of Europe.” _Ann. Rep.
Smithsonian Inst._, 1917, p. 277.
Huntington, Ellsworth. “The pulse of Asia.” Boston and New York,
1907.
——. “The climatic factor as illustrated in arid America.”
Washington, _Carnegie Institution_, 1914.
——. “World power and evolution.” New Haven, 1919, pp. 186-207.
Pettersson, O. “Climatic variations in historic and prehistoric
time.” _Svenska Hydrogr.-Biol. Komm. Skrifter_, Heft 5.
CHAPTER XVII
THE CLIMATIC FLUCTUATIONS SINCE A.D. 500
The question of climatic changes during the historic period has been
the subject of much discussion, and several great meteorologists
and geographers have endeavoured to prove that at least since about
500 B.C. there has been no appreciable variation. It is admitted
that there have been shiftings of the centres of population and
civilization, first from Egypt and Mesopotamia to the Mediterranean
regions, and later to northern and western Europe, but these have
been attributed chiefly to political causes, and especially to the
rise of Islam and the rule of the “accursed Turk.” Recently, however,
there has arisen a class of evidence which cannot be explained away
on political grounds, and which appears to have decided the battle
in favour of the supporters of change; I refer to the evidence of
the trees, explained in the preceding chapter. The conclusions
derived from the big trees of California have fallen admirably into
line with archæological work in central America, in central Asia
and other regions, and have shown that the larger variations even
of comparatively recent times have been very extensive, if not
world-wide, in their development.
Let us consider first the evidence of the trees. These indicate
that after the moist period ending about A.D. 400, described in the
preceding chapter, the rainfall was generally light until about A.D.
1000, when it showed a sharp rise, probably to the level attained in
A.D. 1. (The correction for age renders an exact comparison between
periods a thousand years apart difficult.) This period of abundant
rainfall lasted some fifty years, followed by a gradual decline to a
brief minimum, shortly before A.D. 1200. About 1300 occurred another
rapid rise, reaching a maximum before 1350; the period of heavy rain
continued a short while after 1400, when a decline set in, reaching
a minimum at 1500, after which the rainfall recovered somewhat, and
subsequently maintained approximately its present level, with a
slight maximum about 1600 to 1645.
In the desert of Arizona, in regions at present too dry for
agriculture, there are abundant ruins, which are attributed by
Huntington to three periods:
(_a_) Pueblo ruins, dating back to just before the coming of the
Spaniards (i.e. about A.D. 1600), and indicating merely an increase
of population at the present centres.
(_b_) Ruins of an older civilization, termed by Huntington the
Pajaritan, during which numerous inhabitants lived in places where
at present no crops can be raised. “These people, as appears from
their pottery, their skulls and their methods of agriculture, belong
to a different civilization from that of the modern Pueblos who
inhabited Gran Quivera at the time of the coming of the Spaniards.
They had evidently disappeared long before that date, as is evident
from the present ruins of their villages, and from the absence of any
hint of their existence in the early annals of the country” (_Geogr.
Journal_, 40, 1912, p. 396).
The largest ruins of this type invariably lie near the main lines of
drainage. They consist of villages with houses of several storeys.
But digging down beneath these ruins we find (_c_) traces of an
older occupation, and ruins of a primitive type are also found on
the plateaus remote from any except small valleys. “They are usually
small, and are greatly ruined, and seem to belong to a time long
anterior to the main large ruins.” Huntington terms this type the
Hohokam; unfortunately this and the Pajaritan occupations cannot
be accurately dated, but it is reasonable to connect them with the
rainfall maxima shown by the trees, about the time of Christ, and in
A.D. 1000 to A.D. 1300.
A similar succession has been found in the neighbourhood of Mexico
City. The earliest trace of occupation is a crude “mountain pottery,”
in ordinary river sand and gravel. These deposits are succeeded by
finer sand with better pottery known as the “San Juan” type, above
which comes a culture layer with the remains of houses. This is
covered by a bed of “tepetate,” a white calcareous deposit frequently
found in dry regions where much water evaporates. The gravels suggest
the occasional heavy rains of arid countries. The San Juan pottery
extends throughout the “tepetate,” which probably corresponds to the
dry period of A.D. 400-1000 in California.
Historical records in Mexico date back to the coming of the Aztecs in
A.D. 1325. They show that in 1325 and again in 1446 the level of the
lake of Mexico was high, but towards the end of the fifteenth century
the water was much lower. In 1520 it was high again; in 1600 it was
low, but high from 1629 to 1634. From 1675 to 1755 was a long dry
period. On the whole the climate from 300 to 600 years ago seems to
have been moister than that of to-day.
Still further south in the Peninsula of Yucatan recent explorations
have yielded results of extreme interest. Yucatan lies within the
tropical rain-belt, and is covered by almost impenetrable forests.
The climate is enervating and unhealthy, and the present inhabitants
are greatly lacking in vigour. In the forest, however, have been
found the ruins of ninety-two towns, some of them of great size,
and all remarkable for the beauty as well as the solidity of their
architecture.
These ruins belong to the great Mayan civilization. Mayan history
has been briefly summarized by Huntington as follows: “First we have
a long period of active development, during which the calendar was
evolved and the arts of architecture and sculpture were gradually
developed.... This time of marked growth must have preceded the
Christian era. Then comes ... the building of the great cities of
Copan, Quirigua, Tikal and others. These first great cities were in
the southern part of the Maya area, on the borders of Honduras or in
eastern Guatemala. They lasted perhaps three or four centuries; then
quickly declined. So far as we have any evidence, civilization never
revived in this southern area, for the structures of the great period
have not been rebuilt by later inhabitants. Towards the end of the
period of greatness the centre of Mayan culture moved northward....
The great period, according to Bowditch, lasted from 100 B.C. to
A.D. 350 ... then came a time of very low civilization, lasting for
centuries.... A revival ensued about A.D. 900 or A.D. 1000, and
architecture once more reached a high pitch, but ... only in northern
Yucatan; all the rest of the country seems to have remained in
darkness. Moreover, this mediæval revival was relatively shortlived.
Since that time the condition of the Mayas has fluctuated more or
less, but on the whole there has been a decline.”
Now at the present day the densest and most progressive population in
Yucatan is found in the driest part of the country, where the forest
gives place to jungle. If the line of separation between jungle and
forest were moved southward 300 miles, the former would include
all the districts where ruins are now found. We see from the above
summary that the prosperous periods of Mayan history were just those
periods which in California were moist; in Yucatan they must have
been dry. Huntington’s explanation is the theory of the “shifting of
climatic belts”; during the rainy period in California the temperate
storm-tracks were shifted further southward. At the same time the
sub-tropical high-pressure belt, which at present lies over the
West Indies, was also shifted southwards, and this brought a dry
cool winter to Yucatan, with an increased contrast of seasons, and
consequently a more invigorating climate.
In Asia, Huntington and other explorers have found similar traces of
past variations of climate, a fascinating account of which is given
in “The Pulse of Asia.” Space will not permit of a summary in detail,
but the following general conclusions may be quoted:[6]
“If we omit the Volga and the European portions of the Caspian
drainage area, the limits (of the six basins considered) lie over
sixteen hundred miles apart from north to south and over three
thousand from east to west. All this great area seems to have been
subject to the same great waves of climatic change.
“In the ancient days when the Oxus River entered the Scythian Gulf
of the expanded Caspian Sea, and Lake Gyoljuk discharged permanently
to the Tigris, the lake of Seyistan had not been converted into dry
land by the giants. Kashmir was so cold and snowy that agriculture
was impossible.... In the Lop basin the rivers were full of water;
Lop-Nor was the “Great Salt Lake”; the desert was comparatively small
and the zone of vegetation extensive; and on all sides there was a
density of population and a degree of prosperity far beyond those of
to-day. And in the Turfan basin the same was probably true.
“A great change took place throughout the six basins during the early
centuries of the Christian era. The lakes of Gyoljuk, Seyistan, the
Caspian, Lop-Nor and presumably Turfan were greatly reduced in size.
In the case of the first three, parts of the old lake-beds were used
as sites for villages. Except in Kashmir, the change of climate
appears to have brought disaster....
“Again there came a change (about A.D. 700). The process of
desiccation gave place to a slight but important tendency toward
increased rainfall and lower temperature. Kashmir became colder and
more snowy, and hence more isolated; the rivers of Lop and Turfan
gained greater volume; and the lakes of Lop, the Caspian and Turfan
expanded once more. The habitability of the arid regions began to
increase; migrations came to an end; and central Asia was prosperous
for a time. Finally (about 1350) a latest and slightest change took
place in the other direction, and we seem to-day to be in the midst
of an epoch of comparative equilibrium, with no marked tendency
towards climatic change in either direction.” There was, however, a
period of comparatively high water in the Caspian in the early part
of the seventeenth century.
In Europe the evidence for climatic changes during historical times
is more difficult to follow, since variations of rainfall leave fewer
traces in a moist than in an arid or semi-arid country. A certain
amount of material is given by Brückner in his “Klimaschwankungen.”
He finds that there was a great advance of the Alpine glaciers from
1595 to 1610, while two Italian lakes without outlet, the Lago di
Fucina and Lake Trasimeno, attained a high level about the same
time. Other evidence for western Europe is derived from the date
of the wine-harvest and from the records of severe winters. Like
the growth-curves of the big trees, they need a secular correction
to alter the general slope of the curve, especially in the case of
severe winters, but the larger irregularities probably correspond to
real variations of climate. I have added in column 4 the numbers of
winters with sea-ice on some part of the Danish coast, as tabulated
from the records compiled by Captain C. I. H. Speerschneider. The
results are in general agreement with column 3, particularly as
showing that the period 1401-50 was relatively mild; but the first
half of the seventeenth century is less instead of greater than its
two neighbours in this column.
The figures for the wine-harvest refer to the average for the period
1816-80; - indicates that the harvest was so many days earlier than
normal, corresponding to a high summer temperature (May to August).
The table shows that cold winters were especially numerous in the
first half of the twelfth century and again in the thirteenth. The
end of the fifteenth century was marked by hot summers and mild
winters, or a warmer climate; the beginning of the seventeenth
century by cold (presumably snowy) winters and cool summers. Thus the
periods of increased rainfall in the arid regions of Asia and America
were marked by a colder climate in the rainy regions of western
Europe.
--------+---------+---------+-------------+-------------
1. | 2. | 3. | 4. | 5.
| | | |
Period,| Date of | No. |Winters with |
50 years| Wine |of Severe|Ice on Danish| Remarks.
about. | Harvest.| Winters.| coast. |
--------+---------+---------+-------------+-------------
825 | | 4 | |
875 | | 7 | |
925 | | 5 | |
975 | | 6 | |
1025 | | 6 | |
1075 | | 10 | |
1125 | | 15 | | Cold winters
1175 | | 10 | |
1225 | | 13 | } |
1275 | | 13 | } | Cold winters
1325 | | 13 | 7 } |
1375 | | 11 | 4 |
1425 | +5 | 13 | 7 |
1475 | +1 | 7 | 2 | Warm
1525 | +2.9 | 10 | 5 |
1575 | +2.2 | 14 | 14 } |
1625 | +4.1 | 17 | 11 } | Cold
1675 | +2.7 | 15 | 14 } |
1725 | +0.1 | 10 | 5 |
1775 | -0.2 | -- | 22 |
1825 | -0.9 | -- | 21 |
--------+---------+---------+-------------+-------------
The date of the break up of the River Dwina at Mitau was recorded
intermittently from 1530 to 1709, and regularly since that date, and
the figures have been discussed by Rykatchef. Recasting them in our
unit of fifty years we find the mean dates to be:
----------+----------+-----------+----------+-----------+----------
1551-1600 | 1601-50 | 1651-1700 | 1701-50 | 1751-1800 | 1801-50
March 29 | March 30 | March 5 | March 26 | March 26 | March 28
----------+----------+-----------+----------+-----------+----------
This again points to a cold period about the beginning of the
seventeenth century.
The climate of Iceland and Greenland in the Middle Ages has been
the subject of much controversy, the view that there were extensive
changes during that period being warmly upheld by one party and as
warmly combated by the other party. The case for climatic change has
been well set out by O. Pettersson[7]. The Roman authors (Pliny,
Solinus, etc.) wrote that there was a frozen sea about Thule
(Iceland), but a party of monks who visited the island about A.D.
795 during the months of February to August, in which the ice is
normally most abundant in Icelandic waters, found the coast free,
though they met with a frozen sea a day’s journey to the northward.
In the ninth century the Norsemen visited Iceland regularly, and at
times sailed round it, apparently without interference from ice.
The early settlers practised agriculture with some success. In the
thirteenth century, however, the reports of ice off Iceland became
frequent—apparently the conditions were worse than those of the
present day, and much more so than in the eighth and ninth centuries.
According to Rabot, it appears from ancient records that considerable
areas cultivated in the tenth century are now covered with ice. The
first spread of the glaciers took place in the first half of the
fourteenth century. In the fifteenth and sixteenth centuries the
climate of Iceland ameliorated somewhat, but in the seventeenth there
was a readvance, which destroyed several farms about 1640 or 1650.
Since then there has been a slight retreat.
The ice-conditions of Greenland are closely related to those of
Iceland, and the records of the Norse colonization of Greenland bear
out the conclusions drawn from the latter island. Up to the close
of the twelfth century ice is hardly ever mentioned in the accounts
of voyages, though it is now a great hindrance. Eric, the pioneer
explorer of West Greenland, spent three successive winters on the
islands in Juliaanehaab Bay (latitude 60° 45′ N.), and explored the
country during the summer; “this cannot be explained otherwise than
by assuming that the Polar ice did not reach Cape Farewell and the
west coast of Greenland in those days.” In the thirteenth century ice
is first specifically mentioned as a danger to navigation, and at
the end of the fourteenth century the old Norse sailing route was on
account of ice definitely abandoned in favour of one further south.
Shortly afterwards the Norse colonies were wiped out by a southward
migration of the Eskimos. Even in Norway itself the fourteenth
century was a time of dearth, short harvests and political troubles,
when corn had to be imported from Germany instead of being exported
to Iceland as in former years.
It should be noted that Pettersson’s conclusions are considered
invalid by H. H. Hildebrandsson[8] on the ground of the
incompleteness of the records.
For the southern hemisphere our records are naturally much rarer and
of less antiquity than for the northern hemisphere, and until the
tree-rings are investigated we cannot carry our study back beyond
the sixteenth century. From some researches into the municipal
archives of Santiago de Chile, latitude 33½° S., published by B. V.
Mackenna in 1877, we can infer, however, that the general course of
variation since 1520 was similar to that of corresponding regions in
North America. Santiago lies in a semi-arid region where a temporary
shortage of water is severely felt, the average annual rainfall
being only 364 mm. (14.3 inches). The early travellers, however,
make no specific mention of drought, and in 1540 Pedro de Valdivia
crossed the desert of Atacama with a column of troops and cattle
without inconvenience—a feat which would be difficult nowadays. In
1544 there were heavy rains and great floods in June. The next record
is for the year 1609, recording another heavy flood on the Mapocho,
which was repeated nine years later in 1618. The first recorded
drought occurred in the years 1637 to 1640; there was another flood
in 1647, after which came a series of severe droughts interrupted by
occasional floods, which lasted until the close of the eighteenth
century. The first half of the nineteenth century was again
comparatively rainy. The records thus indicate a wet period centred
about 1600, followed by a dry period during the eighteenth century,
exactly parallel to the records from the United States and Europe.
BIBLIOGRAPHY
Huntington, E. “The climatic factor as illustrated in arid
America.” Washington, Carnegie Institution, 1914.
——. “The fluctuating climate of North America.” _Geogr. J._ 40,
1912, pp. 264, 392.
——. “The pulse of Asia.” Boston and New York, 1907.
——. “World power and evolution.” New Haven, 1919.
Pettersson, O. “Climatic fluctuations in historic and prehistoric
time.” _Svenska Hydrogr.-Biol. Komm. Skrifter_, H. 5.
Rabot, Ch. “Essai de chronologie des variations glaciaires.” _Bull.
géogr. historique et descriptive_, No. 2, 1902.
Brückner, E. “Klimaschwankungen seit 1700” ... Vienna, 1890.
Hildebrandsson, H. H. “Sur le prétendu changement du climat
européen en temps historique.” Upsala, _Nova Actae Regiae Soc.
Sci._ (4), Vol. 4, No. 5, 1915.
Brooks, C. E. P. “An historical notice of the variations of climate
in Chile.” Washington, _Dept. Agric., U.S. Weather Bureau, Monthly
Weather Rev._ 47, 1919, p. 637.
CHAPTER XVIII
CLIMATIC FLUCTUATIONS AND THE EVOLUTION OF MAN
The origin of man from an ape-like ancestor[9] is generally admitted,
but owing to the incompleteness of the palæontological record we
are still in ignorance as to the circumstances, while the place is
generally put vaguely as somewhere in Asia, and the time as the late
Tertiary (Prof. Elliot Smith places it near the Siwalik hills in the
Miocene). For this early period we are reduced to speculation, in
which we may reasonably utilize the facts which we have gained about
climatic variation.
The chief problem to be explained is why man’s arboreal ancestor
left the safe shelter and easy food supply of his primæval forest
and ventured forth into the plains. An article by Professor J.
Barrell,[10] of Yale University, gives a plausible account of the
change, putting it down to necessity, and not to choice. His theory
is that the human ancestor lived in the forests spread over Asia,
then a vast well-watered plain, during the middle-Tertiary period.
Then the gradual uplifting of the Himalayas and other mountain
ranges caused a decrease in the rainfall of central Asia, so that
ultimately the forests were unable to thrive, and gradually gave
place to steppe conditions. The change was slow enough to give the
less specialized inhabitants of the forest time to change their
habits and evolve into forms suitable to a terrestrial life, and the
chief of the animals which took advantage of this period of grace
was the pre-human. Forced to live on the ground, with a diminishing
food supply, only the most progressive individuals were able to
survive, and evolution was rapid. The changing type was saved from
being submerged in the great mass of the original type in the forests
which continued to exist further south by the impassable wall of
mountains. Major Cherry[11] considers that there is sufficient
evidence to prove that a portion of this evolution took place on the
seashore, an environment which would have been much more favourable
to a small ape-like animal than the open steppe would have been. It
is quite likely that the earliest migrations, such as that which
carried _Pithecanthropus_ to Java, took place along the shore. But
after a time, when increasing brainpower and the use of primitive
stone implements enabled man to take the offensive against the
larger animals, the centre of activity changed to the steppes. A
familiar view of the early development of man was advocated by W.
D. Mathew,[12] who writes: “In view of the data obtainable from
historical record, from tradition, from the present geographical
distribution of higher and lower races of men, from the physical
and physiological adaptation of all and especially of the higher
races, it seems fair to conclude that the centre of dispersal of
mankind in prehistoric times was central Asia, north of the great
Himalayan ranges, and that when by progressive aridity that region
became desert it was transferred to the regions bordering it to the
east, south and west. We may further assume that the environment in
which man primarily evolved was not a moist tropical climate, but
a temperate and more or less arid one, progressively cold and dry
during the course of his evolution. In this region and under these
conditions, the race first attained a dominance which enabled it to
spread out in successive waves of migration to the most remote parts
of the earth.”
We do not know anything of the migrations of the Eolithic and
earlier Palæolithic races, except that they spread rapidly
over a considerable portion of the earth. Both migration and
evolution, especially mental evolution, must have been accelerated
by the great changes of climate which were taking place. In
the Mindel-Riss interglacial period we know of two types, the
Piltdown man (_Eoanthropus dawsoni_) and the Heidelberg man (_Homo
heidelbergensis_), the latter a true man, though probably not on
the direct line of evolution of _Homo sapiens_. The stress of the
succeeding second Glacial period was too great for _Eoanthropus_,
which appears to have died out, but _Homo_, probably an Asiatic or
African type similar to _H. Heidelbergensis_, survived. The next
form, associated with Mousterian implements, is Neanderthal man (_H.
neanderthalensis_), who closely resembled modern man, and all the
remains of races which lived subsequently to the last glaciation
are those of modern man (_H. sapiens_), including the magnificent
Cro-Magnards and the negroid Grimaldi race. Thus each glaciation
has been marked by a step upwards in the scale of humanity; does
this mean that the coming of the super-man is contingent on another
glacial epoch?
BIBLIOGRAPHY
Barrell, J. “Probable relations of climatic change to the origin of
the Tertiary Ape-man.” _Scientific Monthly_, New York, 4, 1917, p.
16.
Mathew, W. D. “Climate and evolution.” _Annals New York Acad.
Sci._, 24, 1915, p. 212.
London, British Museum. “A guide to the fossil remains of man....”
London, 1918.
CHAPTER XIX
CLIMATE AND HISTORY
It is a remarkable fact in human history that civilization began in
regions which are at present inhabited chiefly by backward races,
and the centres of progress have shifted from one country to another
with the passage of time. Many accidental factors—position on
trade-routes, possession of special mineral advantages, and so on,
have undoubtedly played a part in this, but it will not be difficult
to show that climatic fluctuations have also had their share.
A brilliant study of Ellsworth Huntington[13] has shown that there
are certain optimum conditions of climate which are most suitable
for efficient work. These conditions, which were determined by an
analysis of the output of work in American factories, were then
found to be just those which prevail in the most progressive regions
of the globe, which are located in the temperate storm-belts, and
it is shown in certain instances that fluctuations in the position
of this storm-belt coincided with fluctuations in the centres of
civilization. A few additional examples of this may be given.
The beginnings of civilization may reasonably be placed with the
transition from the Palæolithic to the Neolithic type, a transition
which involved much more than just the polishing of stone weapons.
It involved also the beginnings of agriculture, crude pottery, and
later, the domestication of animals. One of the earliest Neolithic
cities known is probably that of Anau, near Askabad in Transcaspia,
excavated by Pumpelly in 1904. From the thickness of the accumulated
debris the date of first settlement is placed at or before 8000
B.C., i.e. 10,000 years ago, or during the period which in Europe is
assigned to the concluding stages of the Wurm glaciation. Pumpelly’s
time-estimates are based on careful comparison with accumulations
in Merv and other cities. At present the mean annual rainfall in
that part of Turkestan is below ten inches a year, and the country
is practically desert, and is entirely unfitted for agriculture.
But with the remains of the ice-sheet still over Scandinavia and
depressions following a more southerly course along the Mediterranean
basin and into southern Asia, the rainfall was considerably heavier,
and the climate in general was more suited to a progressive race.
At the outset we find this Neolithic race living in rectangular
houses built of uniform sun-dried bricks; they were skilful potters,
cultivating cereals, but at first without domestic animals.
The beginning of Neolithic civilization in Crete is placed by Evans
at about 12000 B.C., while on the basis of excavations by de Morgan
at Susa in Persia, Montelius places the origin of Neolithic culture
in this part of Asia as early as about 18000 B.C. At Susa the
deposits are 130 feet thick, and of these the upper 40 feet cover a
period of 6000 years.
Thus we see that what may be considered as the great step from
savagery to civilization took place while the present centres of
progress in Europe and America were still in the Ice Age. At this
time the climate of southern Asia must have resembled the present
climate of north-west Europe in heavier rainfall and the day-to-day
fluctuations of weather—in fact, the districts where civilization
began probably had at that time the most stimulating climate in the
northern hemisphere.
With the vanishing of the ice-sheets and the setting in of the mild
climate of the Maritime phase the Neolithic culture spread rapidly to
Europe, and by 2000 B.C. even the Baltic regions were well inhabited,
and it is probable that the Aryan race was developing in the Russian
steppes. About this time Anau was abandoned owing to increasing
aridity.
With the coming of the Bronze Age in western Europe, about 1800
B.C., however, the climate again became colder and rainier,
corresponding to the Peat-bog phase or “Classical” rainfall maximum,
the deterioration culminating in the Early Iron Age. This period
was marked by a great southward spread of the Aryan peoples, and
ushered in the Heroic Age of Greece. The races of the Mediterranean,
as we have seen, continued to thrive throughout this rainy period,
and their power did not diminish until its close, about A.D. 400.
This downfall was accelerated if not caused by the pressure of nomad
peoples driven out of Asia by the increasing drought. These Asiatic
migrations included the great marches of the Tartar hordes and, aided
by religious enthusiasm, the conquests of the Moslems.
The early Middle Ages, after the downfall of Rome, appear to have
been characterized by a dry warm climate. This was the age of the
Vikings, when the Norse races rose to dominance in western Europe,
finally invading and occupying large areas of France and Britain,
and even extending their power to Sicily. With the increasing cold
and wet of the “Mediæval” rainfall maximum came a final burst
of Norse migration, which left the homeland poor and scantily
populated, and the centre of activity and progress lay once again
with the Mediterranean peoples, and especially with Italy and Spain.
The Tartar invasions ceased, and against the increasing power of
Europe the Moslem wave broke and receded. At the close of this
rainy period political dominance again moved north. From that time
the fluctuations of climate have been of minor importance, and
correspondingly there have been no great shiftings of political
power from latitude to latitude.
BIBLIOGRAPHY.
Tyler, J. M. “The New Stone Age in Northern Europe.” London, 1921.
Huntington, Ellsworth, “World Power and Evolution.” New Haven, 1919.
Haddon, A. C., “The wanderings of peoples.” Cambridge University
Press, 1919.
APPENDIX
THE FACTORS OF TEMPERATURE
To calculate the probable temperature of January or July at any
point, the following procedure should be adopted:
Draw a circle round the point of angular radius ten degrees (i.e. set
the compass to cover ten degrees of latitude) and divide this into
two halves by a line passing from north to south through the centre.
By means of squared tracing paper, or otherwise, measure: (_a_) the
amount of ice in the whole circle; (_b_) the amount of land in the
western half; (_c_) the amount of land in the eastern half. (_a_) is
expressed as a percentage of the area of the whole circle; (_b_) and
(_c_) as percentages of the area of a semicircle.
The term “ice” includes ice-sheets such as that of Greenland or
Antarctica, and also frozen sea or sea closely covered by pack-ice;
the latter figure may vary in different months.
The temperature in January or July is then calculated from the
following formula:
Temperature = basal temperature + ice coeff. x per cent. of ice +
land west coeff. x per cent. of land to west + land east coeff. x per
cent. of land to east.
The basal temperatures and the appropriate coefficients are given in
the following table.
In calculating the effect of a given slight change of land and sea
distribution, it is not necessary to employ the basal temperature.
Instead the equation can be treated as a differential, and the change
of temperature due to the change of land and ice calculated from the
figures in columns 3 to 5. The figures are given in degrees absolute,
273°0 = 32° F. To convert differences to Fahrenheit, multiply by 1°8.
---------+-------------+----------+-----------+-----------
Latitude.| Basal Temp. |Ice Coeff.| Land, | Land,
|(Water Zone).| |West Coeff.|East Coeff.
---------+-------------+----------+-----------+-----------
Jan. | a. | | |
70 N. | 298.8 | -0.49 | -0.43 | -0.20
60 | 277.4 | -0.07 | -0.31 | -0.01
50 | 276.8 | -0.09 | -0.29 | 0.09
40 | 282.5 | -- | -0.17 | 0.04
30 | 289.6 | -- | -0.08 | 0.03
20 | 294.2 | -- | -0.01 | -0.01
10 | 298.6 | -- | -0.01 | 0.03
0 | 299.3 | -- | 0.01 | 0.00
10 S. | 298.2 | -- | 0.04 | -0.01
20 | 296.2 | -- | 0.07 | 0.00
30 | 293.5 | -- | 0.06 | 0.03
40 | 289.3 | -- | 0.09 | -0.03
| | | |
July. | | | |
70 N. | 279.3 | -0.16 | 0.02 | 0.02
60 | 280.7 | -- | -0.01 | 0.11
50 | 285.8 | -- | 0.04 | 0.06
40 | 291.1 | -- | 0.05 | 0.07
30 | 296.8 | -- | 0.08 | -0.01
20 | 297.6 | -- | 0.07 | 0.02
10 | 298.8 | -- | 0.03 | -0.01
0 | 298.6 | -- | 0.02 | -0.01
10 S. | 296.9 | -- | 0.04 | -0.03
20 | 293.1 | -- | 0.02 | -0.02
30 | 288.2 | -- | -0.01 | -0.01
40 | 284.0 | -- | 0.00 | -0.03
---------+-------------+----------+-----------+-----------
In the case of the calculation of the effect of comparatively slight
and irregular changes in land and sea distribution in a limited area,
such as those of the _Littorina_ Sea referred to on p. 128, it may
be found that a ten-degree circle is too wide an area to employ, the
changes from land to sea at one point being nullified by changes
from sea to land at another more distant point. In such a case a
smaller unit such as a circle of five degrees radius can be employed.
As a rough approximation it may be said that the effect of the
conversion of a square mile of land into sea, or _vice versa_, on
the temperature of a neighbouring point is inversely proportional to
its distance. Since the area of a five-degree circle is one-quarter
that of a ten-degree circle, while the average distance of the
land composing it is one-half, we have to divide our regression
coefficients by two in order to fit the new data.
This method was applied to obtain the probable temperature
distribution on the shores of the _Littorina_ Sea at its maximum
extension, and gave results which agreed remarkably well with those
calculated by geologists from the animal and plant life of the time.
See London _Q. F. R. Meteor. Soc._, 43, 1917, pp. 169-171.
INDEX
A.
Acheulian, 52
_Aciphylla_, 125
Africa, 103, 133, 142
Aftonian, 87
AHLMANN, 51, 61
Alaska, 43, 124
Algonquin, Lake, 123
Alps, dry period, 122
glaciation, 29, 52, 56
retreat stadia, 119
Altai Mountains, 77
Anau, ruins, 163
_Ancylus_, 120, 127
ANDERSSON, 118, 121
Andes, 98
Antarctica, 114, 133
Anticyclonic circulation, 55
Antipodes Is., 112
Aral Sea, 83
Argentine, 100
Arizona, 94, 150
ARRHENIUS, 19
Artesian water (Australia), 110
Aryans, 164
Asia, 76, 125, 139, 143, 153
Astronomical theory, 17
Atlantic Stage, 126
Atlas Mountains, 69
Australia, 109, 125, 155
B.
Balearic Is., 70
Balkans, 69
Baltic Interstadial, 64
Banded clays, 49, 93
Baraba steppes, 121
Barkans, 65
BARRELL, 159
BEDDARD, 115
Belfast, 130
_Biloculina_, 133
BLYTT, 127
Bonneville, Lake, 93
Brazil, 101
British Isles, 57, 62, 64, 136
BRÖGGER, 129
Bronze Age, 138
BRÜCKNER, 49, 57, 154
Buenos Aires, sand-dunes, 125
Bühlstadium, 119
C.
Calabrian, 68
Cambrian, 33
Campbell Is., 102
Canada, post-glacial, 132
Cape Colony, raised beaches, 133
Carbon dioxide, 19
Carboniferous, 34
Caspian, 83, 143, 153
CHAMBERLIN, 19
Champlain Stage, 123
Chellean, 51
CHERRY, 160
Chile, rainfall fluctuations, 157
China, 81, 139
Chronology, 48, 92
CHUDEAU, 106
Classical Rainfall Maximum, 140
Climatic Record, 132
COLEMAN, 92
Colorado, 94
Continentality, 25
Continental Phase, 120
Continents, movement of, 21
Cordilleran glaciation, 87
Corsica, 69
CRAIG, 72
Cretaceous, 37
Crete, Neolithic, 163
CROLL, 18
Cro-Magnards, 161
Cyrenaica, desiccation, 142
D.
Daun-stadium, 119
DAVID, 110
Dead ice, 132
Denmark, continental phase, 122
Depressions, path of, 47, 60, 71, 122, 139
Devonian, 34
Diluvium, 48
Don Valley, 91
DOUGLASS, 143
Drakensberg Mountains, 103
Drought in Forest Period, 139
Drumkelin Bog, 137
Dunes, fossil, 65
Frisian, 140
E.
Early Iron Age, 141
Earth’s Orbit, eccentricity of, 18
Earthworms, 115
East Anglia, 47, 57
Eccentricity of Earth’s Orbit, 18
Ecuador, 99
Egypt, 72
_Eoanthropus_, 161
Eocene glaciation, 37
Etosha Pan, 107
Europe, 49, 55, 118, 127, 136, 154
EVANS, 163
Evolution of Man, 155
F.
FAIRGRIEVE, 138
Falkland Is., 97
Fennoscandian Pause, 119
Finiglacial, 118
Finland, post-glacial, 120, 128
Florida, 95
Forest bed, 47, 51
period, 122, 136
Forests, submerged, 137
Formby and Leasowe Beds, 130
Fossil ice, 59, 78
Franz Josef Land, 130
FRECH, 20
FREYDENBERG, 106
Frisian dunes, 140
Fucino, Lago di, 154
_Fucus_ in Spitzbergen, 130
G.
Gable Island, 98
_Galaxiidæ_, 115
GEER, G. DE, 49, 93, 118
GEIKIE, J., 51, 81
Geographical theory, 22
Geological formations, 31
rhythms, 38
GIBBON, 140
Gibraltar, 69, 70
Gila conglomerate, 95
Glacial anticyclone, 55
stages, 48
_Globigerina_, 133
_Glossopteris_, 35
Gondwanaland, 34, 35
Gotiglacial, 118
Graham Land, glaciation, 114
Great Basin, America, 89, 93, 124
Great Lakes, history, 123
Great Salt Lake, 93
Greece, Heroic Age, 164
Greenland, 131, 156
GREGORY, 104
Grimaldi Race, 161
Gschnitz Stadium, 119
Gunz Glaciation, 56
Gunz-Mindel Interglacial, 50, 51, 56
H.
_Haplochitonidæ_, 115
Hazel, post-glacial extension, 122
HEDLEY, 116
Heidelberg Man, 161
Height and temperature, 26
HILDEBRANDSSON, 157
Himalayas, 81
HOBLEY, 105, 107
Hohokam, 150
HUME, 72
HUMPHREYS, 20
HUNTINGTON, 141, 144, 150, 153, 162
I.
Ice on Danish coasts, 155
Iceland, 125, 156
Illinoian glaciation, 90
Ingo Is., forests, 122
Iowan Glaciation, 90
Ireland, glaciation, 57, 62, 64
Heroic Age, 138
Iroquois, Lake, 123
Isohalines, 127
J.
Japan, 81
Jurassic, 37
K.
Kalahari, 107
Kamchatka, 80
Kansan, 88
Karst flora, 121
Kashmir, 143, 153
Keewatin, 88, 91
KEIDEL, 99
Kenya, 103
Kilimanjaro, 103
Kioga, Lake, 104
Kitchen-midden, 125
Kosciusko, 109
KREICHGAUER, 20
KUPFFER, 121
L.
Labradorean Glaciation, 87, 89, 90
Lahontan, Lake, 93
Lena Valley, 78
LEVERETT, 91, 92
Limestone Agglomerate, 70
_Littorina_, 128
Loess, 52, 83, 91, 112
Lofoten Islands, 61
Lop-Nor, 83, 153
M.
MACKENNA, 157
Maglemose culture, 125
Malta, 69
Mammoths, frozen, 79
Marsupials, 115
MATHEW, 160
Maumee, Lake, 123
Maya ruins, 151
Mediæval Rainfall Maximum, 164
Medicine Bow Range, 94
Mediterranean, 68, 142
Mesopotamia, Empires, 139
Mexico, culture, 151
MEYER, 99
Micmac Stage, 124
Mindelian Glaciation, 49, 69
Mindel-Riss Interglacial, 50
Miocene, 44
Mombasa, 105
Mono Basin, 94
MONTELIUS, 163
Mousterian Man, 63
MUNTHE, 118
MURGOCI, 66
Murman coast, 130
N.
_Najas_, 129
Neanderthal Man, 161
NEGRO, 142
Neolithic, 122, 131, 136, 163
migration, 125, 163
Neudeckian, 51
NEUHAUSS, 111
Newfoundland, 87, 90
New Guinea, 111
New Siberian Islands, 78
New South Wales, 125
New Zealand, 111, 125, 133
Ngami, Lake, 107
Niagara, 93, 132
Nile, 72, 119
NORDENSKJOLD, 117
Nordic Race, 125
Norfolkian, 51
North America, 86, 122, 132, 141, 149
North Sea, 56, 61
Norway, 51, 55, 129
O.
Obliquity of Ecliptic, 16, 120
Old Red Sandstone, 34
Optimum of Climate, 127
Ordovician, 33
P.
Pajaritan, 150
Palmyra, 142
Pamirs, 77
Pampean, 100, 125
Patagonia, post-glacial, 133
Patom Highlands, 78
Peat-bog Period, 140
PENCK, 49, 51
Pendulation Theory, 20
Peorian, 91
Permian, 35
Persia, 84, 142
Peru, 99
PETTERSSON, 134, 145
Piedmont ice-sheets, 57, 109
Piltdown Man, 161
_Pithecanthropus_, 160
Pliocene, 47
Pluvial periods, 71, 140
Poles, motion of, 20, 40
Pre-Cambrian Glaciation, 33
Proterozoic Glaciation, 32
Pueblo ruins, 150
Pulse of Asia, 153
PUMPELLY, 84, 163
Pyrenees, 57
Q.
Quaternary Ice Age, 47
R.
Ragunda, Lake, 49
moraines, 121
REID, 138
Retreat of the Ice, 49
Riss Glaciation, 49, 61
Riss-Wurm Interglacial, 50, 53
Rixdorf, 62
RODGERS, 133
Romania, 66
Ruwenzori, 103
S.
Sagas, 141, 146
Sahara, 74, 105
Sangamon, 90
Scania, 49
SCHMIDT, 53
Scotland, 57, 61, 64
_Scrobicularia_ Zone, 130
Selsey, 58
_Sequoia_, 143
Shell-banks, 47, 56
Siberia, 78
Sicilian, 70
Sierra Nevada, 93, 94
SIEVERS, 99
Silurian, 33
Skærumhede, 63
Slugs, 115
SMITH, ELLIOTT, 159
Solar radiation, 15
South America, 97, 125, 132, 157
South Georgia, 97
South Orkneys, 114
SPITALER, 18
Spitzbergen, 80, 130
Stanovoi Mountains, 79
Steppe climate, 53
Stone rivers, 98
Submerged forests, 137
Suess, Lake, 104
Sunspots, 145
Susa, Neolithic, 163
SVEN HEDIN, 84
Sweden, 49, 56, 118
Syria, 72
T.
_Tapes_, 129
Tasmania, 109
Tchad, 106
Tertiary, 42, 116
_Thracia_ Zone, 131
Tian-Shan Mountains, 77
Tibet, 82
Tidal friction, 39
Tide-generating force, 134, 145
Tierra del Fuego, 97, 133
Tillite, 32
Titicaca, Lake, 101
Toronto Stage, 91
_Trapa_, 129
Trasimeno, Lake, 154
Tree-rings and rainfall, 143
Turbarian, 140
Triassic, 37
TYNDALL, 19
U.
Uinta Mountains, 94
_Unio_ in Niagara, 132
Ural Mountains, 57
V.
Venezuela, 100
Verkhoiansk Mountains, 79
Victoria Nyanza, 104
Vikings, 164
Volcanic dust, 16, 20
VOLLOSSOVITSCH, 79
W.
Wales, 57, 64
Warren, Lake, 123
Wasatch Mountains, 93, 94
WAYLAND, 104
WEGENER, 20, 34
WERNERT, 53
White Sea, 130
Wine harvest, 155
Winters, severe, 155
Wisconsin Glaciation, 91, 92
WOLF, 145
Wurm Glaciation, 48
Y.
Yarmouth Stage, 88
_Yoldia_ Sea, 50, 124
Yucatan, 151
Yukon, 124
_Printed in Great Britain by Jarrold & Sons, Ltd., Norwich._
FOOTNOTES:
[1] By this term we shall in future understand only that part of it
which is responsible for thermal effects.
[2] If the figure of the earth is adjusted to its speed of rotation
before the development of ice-sheets, the latter renders it too
prolate, and there will be a tendency for readjustment by the
transference of mass towards the equator.
[3] This has been the subject of much discussion recently. For a
summary see _Science Progress_, 17, 1922, October, p. 233.
[4] Leverett, F. (see Bibliography).
[5] See reference to Antevs in this connexion.
[6] “The pulse of Asia,” p. 356. See also a new work by E.
Huntington, entitled: “Climatic changes.”
[7] “Climatic variations in historic and prehistoric time.”
[8] “Sur le prétendu changement du climat européen en temps
historique.”
[9] Or lemur-like ancestor. There is evidence to show that man’s
ancestor was a nocturnal animal, whose food supply was governed by
the phases of the moon.
[10] “Scientific monthly,” New York, 4, 1917, pp. 16-26.
[11] “Science progress,” 15, 1920, p. 74.
[12] “Climate and evolution.”
[13] “Civilization and climate.”
TRANSCRIBER’S NOTE
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
Some hyphens in words have been silently removed, some added,
when a predominant preference was found in the original book.
Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained.
Pg 64: ‘powerful conviction’ replaced by ‘powerful convection.
Pg 97: ‘and Tierra del Fuega’ replaced by ‘and Tierra del Fuego’.
Pg 103: ‘Drakenberge Mountains’ replaced by ‘Drakensberg Mountains’.
Pg 150: ‘modern Pueblas who’ replaced by ‘modern Pueblos who’.
Pg 166: ‘coffiecients are’ replaced by ‘coefficients are’.