THE MEDITERRANEAN REGIONS DURING THE GLACIAL PERIOD
Our knowledge of the history of the Mediterranean basin during the
Glacial period is not nearly so complete as is that of the more
northern regions, chiefly for the reason that during most of the
period the land lay above its present level, and except for local
glaciers in the mountain regions there was no ice to leave us a
record of the changing climates. Most of what we do know relates to
the relatively brief periods of submergence.
At the beginning of the Glacial period the sea lay some 500 feet
above its present level, and we can trace the first appearance of a
northern marine fauna. This stage is known as the Calabrian; it is
divided into two horizons—a lower, in which northern forms are still
rare, and an upper, in which they are becoming abundant. The most
typical species are two mollusca whose present habitat is the coast
of Iceland—_Chlamys (Pecten) islandicus_ and _Cyprina islandica_.
The Calabrian beach is not found on the coast of Spain or at
Gibraltar, and in Algeria it probably occurs at a lower level. This
suggests that the subsidence at this period was local, and the
western lands stood up as a barrier against the Atlantic. There
must have been a channel of some sort, however, on the site of the
present Straits of Gibraltar, to provide an inlet for the immigrating
northern mollusca. In the Maritime Alps, and again in the eastern
Mediterranean, the Calabrian beaches are at a much greater height
owing to local elevation.
After the formation of the Calabrian beach the whole Mediterranean
region was elevated above its present level. This elevation must be
contemporaneous with the period of maximum elevation in north-west
Europe associated with the great Mindelian glaciation. It is
suggested that the “sill” of the outlet channel at Gibraltar was
raised above the level of the Atlantic, and the Mediterranean became,
first a closed salt lake, and then a pair of lakes, the eastern fresh
draining into the western, which was salt, the two being separated
by a ridge of land between Italy and Tunis. This period of elevation
was long enough for a great deal of denudation to take place. Even in
the Mediterranean this was a time of severe climate. On the eastern
side of Gibraltar there are breccias, known as the “Older Limestone
Agglomerate,” which reach a thickness of 100 feet in places, and are
now much weathered. Similar agglomerates are found in Malta. These
resemble the “head” of the south of England, and appear to be due
to frost action in a severe climate. In Corsica there are traces of
four periods of mountain glaciation, and the two oldest of these are
provisionally correlated with the Gunzian and Mindelian of the Alps.
In the Balkan highlands there are traces of two distinct glaciations:
the older, which was the more general and reached the greater
intensity, probably corresponding to the Mindelian. In the Atlas
Mountains there are great boulder moraines which seem to belong to
three distinct glaciations, the oldest extending to about 2000 feet
above sea-level, and the second terminating at about 4000 feet, while
the third glaciation consisted of small valley glaciers only.
Towards the close of the Mindelian glacial period the land sank or
the ocean rose again, and the waters of the Atlantic poured in,
bringing with them a great number of high northern and Arctic
mollusca. The theory has been put forward that this influx was in the
nature of a debacle and carved out a deep gorge through the present
Straits of Gibraltar. The beaches deposited by this sea lie at a
height of 250 to 330 feet above the present sea-level. The fauna
resembles that inhabiting the northernmost parts of Europe at the
present day, and the waters must have been several degrees colder
than at present. This stage is termed the Sicilian.
As the climate improved the land gradually rose again, and the
next general raised beach lies at a height of only about 100 feet
in southern Italy (except where it has been elevated by local
earth-movements). Further west it lies still nearer the present
sea-level—twenty feet in the Balearic Islands and only seven feet on
the coast of Spain. On the coast of Algeria and Tunis this beach is
found at a height of about forty-five feet.
The beach contains no trace of the northern fauna found in the
Sicilian stage; instead it is marked by an assemblage of mollusca
of a sub-tropical aspect, including _Strombus bubonius_, _Mytilus
senegalensis_ and _Cardita senegalensis_. The bones of large mammals
are also found, including the hippopotamus and southern forms of
elephant (_E. antiquus_) and rhinoceros (_Rh. merckii_). This warm
stage corresponds to the Chellean interglacial fauna of northern
Europe, though so far as I am aware no Chellean implements have been
found associated with it.
About this time the Older Limestone Agglomerate of Gibraltar had
been worn into caves, in which are found the bones of ibex, wild
boar, leopard, spotted hyena, _Rhinocerus leptorhinus_, _Elephas
meridionalis_, lion, southern lynx, bear, wolf, stag, horse, etc.,
so that the rock must have been covered by a rich vegetation, and
must have had a greater extent than now, and a connexion with
the continent of Africa. This is said to have been followed by a
submergence of about 700 feet with numerous oscillations. This
submergence, if it is really attributable to the interglacial, must
have been extremely local, and possibly it is much older.
After the warm Chellean period the Mediterranean region rose again,
probably contemporaneously with the rise which caused the Rissian
glaciation of northern Europe. But the climate was nothing like
so severe as in the Sicilian. We have no old beaches containing a
molluscan fauna of this period, but at the Grotte au Prince near
Mentone, investigated by M. Boule, the _Strombus_ beach is overlain
by a bed of cemented pebbles and “hearths” containing Mousterian
implements and bones of a temperate fauna. The Newer Limestone
Agglomerate on the east of Gibraltar may have been formed during this
period. The Mediterranean lands remained above their present level
until the close of the Glacial period.
Each glaciation of northern Europe must have been a time of greater
rainfall as well as of lower temperature in the Mediterranean. The
glacial anticyclone in the north displaced the storms from the
Atlantic, which now mostly either skirt the north-west coast of
Norway or pass across Denmark into the Baltic. These storms had
to take a more southerly course, and entered the Mediterranean
basin either across the south of France or in the neighbourhood of
Gibraltar: tracks which are still occasionally followed in winter.
These storms brought a rainfall much heavier than the present, and
of a different character. The Mediterranean is now a “winter rain
region,” and the north of Africa is entirely rainless for several
months in the summer. But during the “Pluvial periods” it is probable
that rain fell throughout the year, though the winter still had more
than the summer. The winter rains were in the form of steady falls
of long duration, such as we experience now in England, while the
summer rains fell in short, heavy showers, perhaps accompanied by
thunder. The Older Pluvial period, which corresponds to the Mindelian
glaciation, had these conditions in their greatest development.
Depressions cannot live long without a supply of moisture, either
from the sea or from transpiring vegetation, and at present such
winter storms as enter the Mediterranean are almost confined to its
surface, and on the African side rarely penetrate more than one
hundred miles inland. But at the period of greatest elevation the
shrunken Mediterranean offered no such great attraction, and with a
comparatively well-watered Sahara the storms were able to pass much
further south. Consequently, northern Africa possessed a number of
large and permanent rivers which reached the sea. It was along these
rivers and their banks that the fauna still inhabiting the Saharan
oases made its way, to be isolated there by the decrease of the
rainfall, so that crocodiles and many species of fish now live in
isolated pools and in rivers which lose themselves in the sand.
In Egypt and Syria the first Pluvial period is double, corresponding
to the Gunz and Mindel glaciations, with an intervening phase of
feeble desert conditions, during which, however, the rainfall
remained greater than the present. The second stage, corresponding to
the Mindelian, indicates very great activity; at this time the Jordan
Sea (Dead Sea) reached its greatest area, extending to the northern
end of the Sea of Tiberias.
Conditions in Egypt at this time are very interesting. South of Cairo
the alluvial Nile muds are at most thirty to thirty-five feet thick,
and ten feet of this thickness has been deposited since the time of
Ramesis II. If the rate of deposition has been uniform, this gives a
period of only 14,000 years for the deposition of the whole thickness
of the muds. The theory put forward by Hume and Craig (British
Association Report, 1911, p. 382) is briefly as follows: The mud
deposits of the Nile valley are carried down with the flood waters
of the Blue Nile, Atbara, etc. These rivers rise in the highlands
of Abyssinia, where they are fed by the rains of the south-west
monsoon. The incidence of the monsoon is determined by a number of
factors, prominent among which is the temperature of southern Asia.
During the winter, at present, the low temperature of the Himalayan
and Tibetan region results in a great outflow of cold air, which
strikes the coast of Africa as the cool dry north-east monsoon.
During this time there is very little rain in Abyssinia. It is only
when the Asiatic land-mass warms up in summer that the south-west
monsoon is established.
But during the Glacial period, as we shall see, there was a great
development of snow and ice on the Himalayas. The result was that
winter conditions, i.e. the north-east monsoon, prevailed more
or less throughout the year, and the rivers which feed the Nile
contained only a small volume of water. Hence they lost themselves in
the desert before reaching Cairo, and the Nile in its present form
did not exist. On the other hand, the westerly winds which at present
bring a moderate winter rainfall to the coast of Syria were greatly
increased in intensity and extended further south, replacing the dry
north and south winds now occupying the Nile valley. The northerly
winds prevailing in the Nile valley in summer are associated with
the low pressure area over the neighbourhood of the Persian Gulf,
which in turn is due to the extremely high temperature experienced
there. Even at the present day the highest hills of Sinai penetrate
above the north winds into a westerly current, and a moderate fall
of temperature over the Persian Gulf would inhibit the north winds
in the Nile valley altogether and allow the westerly winds to reach
the surface. These strong westerly winds brought a heavy rainfall
to the hills, now almost rainless, between the Nile and the Red
Sea. Powerful streams descended the western slopes of these hills,
bringing great quantities of debris, which formed delta-terraces
forty of fifty feet thick where the streams debouched on to the
Egyptian plain. These are especially well developed at Oina, the
meeting place of several dry valleys from the hills, and it is
remarkable that they actually cross the present site of the Nile
valley and reach the desert on its western side, additional evidence
that the Nile was not then in existence.
These gravel terraces contain numerous stone implements of early
(pre-Chellean) types, showing that at this time Egypt had sufficient
rainfall of its own to support human life.
The moist westerly winds carried the climate of the Mediterranean
coast far into the desert. For instance, in the oasis of Khargeh, in
latitude 25°, grew the evergreen oak and other plants not now found
south of Corsica and southern France.
The Mindelian Pluvial period was followed by a long dry period
corresponding to the Chellean, when desert conditions supervened. The
Nile as we know it first appeared during this period. Terraces were
formed on the sides of the valley, probably during the submergence
which produced the _Strombus_ beaches of the western Mediterranean;
these contain Chellean implements. During the succeeding elevation
the Nile cut its bed below the present level.
The Rissian glaciation of northern Europe is represented in Egypt by
a second rainy period, the Lesser Pluvial period. Rain again fell
on the Red Sea hills, forming a newer set of gravel terraces, but
these are much smaller than the great Mindelian terraces. No terraces
are known representing the Wurmian period, and the country does not
seem to have been inhabited at this time. Probably the climate was
semi-desert, with not enough rainfall of its own to support human
life, and yet without the fertilizing Nile floods to enable human
life to exist without rainfall. As has been said, the present regime
did not begin until the last glaciation was nearly over, about 12,000
B.C.
BIBLIOGRAPHY
Gignoux, M. “Les formations marines pliocènes et quaternaires de
l’Italie du Sud et de la Sicilie.” _Ann. l’Univ. Lyon_, n.a., Vol.
1, fasc. 36, Paris, 1911, pp. 693.
Depéret, C. “Les anciennes lignes de rivage de la côte française de
la Mediterranée.” _Bull. Soc. Geol. de France_, ser. 4, Vol. 6, pp.
207-30.
Douvillé, R. “Espagne,” _Handbuch regional Geol., H. 7_, 1911.
(Includes Gibraltar and Balearic Is.)
Hume, W. F., and Craig, J. I. “The Glacial period and climatic
change in North-east Africa.” _Rep. Brit. Assoc._, 1911, p. 382.