The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Brooks, C. E. P. “The correlation of the Quaternary deposits of the British Isles with those of the continent of Europe.” Ann. Rep. Smithsonian Inst., 1917, pp. 277-375. [Full list of references.]

Penck, A., and Brückner, E. “Die Alpen in Eiszeitalter.” 3 Vols. Leipzig, 1901-9.

Gagel, C. “Die Beweise für eine mehrfache Vereisung Norddeutschlands in diluvialer Zeit.” Geol. Rundschau, 4, 1913, p. 39.

Wahnschaffe, F. “Die Oberflächengestaltung des norddeutschen Flachlandes.” Stuttgart, 1910.

Svastos, R. “Le postglaciare dans l’Europe centrale du nord et orientale.” Ann. Sci. Univ. Jassy, 4, 1908, p. 48.

CHAPTER VI
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.