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.