If we examine the hydrographic basins of the three divisions of Asia thus indicated we find that the northern division, including the drainage falling into the Arctic Sea,the Aralo- Hydro- Caspian depression, or the Mediterranean, embraces an graphs area of about 6,394,500 sq.
Fills the deepest part of a vast depression, sometimes known as the Aralo-Caspian depression, once an inland sea, the Eurasian Mediterranean or Sarmatian Ocean.
And it is indeed the fact that large portions of the vast region comprised between the lower Volga, the AralIrtysh water-divide, the Dzungarian Ala-tau, and the outliers of the Tian-shan and Hindu-kush systems are actually covered with Aralo-Caspian deposits, nearly always a yellowish-grey clay, though occasionally they assume the character of a more or less compact sandstone of the same colour.
The exact limits of the ancient Aralo-Caspian sea are not yet settled, except in the north-west, where the Ergeni Hills of Astrakhan constitute an unmistakable barrier.
During the early Tertiary age it belonged to the Sarmatian Ocean, which reached from the middle Danube eastwards through Rumania, South Russia, and along both flanks of the Caucasus to the Aralo-Caspian region, and westwards had open communication with the great ocean, as indeed the ancient geographers Eratosthenes, Strabo and Pliny believed it still had in their day.
This communication began to fail, or close up presumably in the Miocene period; and before the dawn of Pliocene times the Sarmatian Ocean was broken up or divided into sections, one of which was the Aralo-Caspian sea already discussed.
During the subsequent Ice Age the Caspian flowed over the steppes that stretch away to the north, and was probably still connected with the Black Sea (itself as yet unconnected with the Mediterranean), while northwards it sent a narrow gulf or inlet far up the Volga valley, for Aralo-Caspian deposits have been observed along the lower Kama in 56° N.