The Ponto-Caspian Plain, the Great Depression of the Old World.
This second vast lowland is the direct continuation of the central European lowland, with a decided sinking toward the Black and the Caspian Seas, indicated by the course of the rivers of that region. It extends from the month of the Danube over the lower Dniester, Bog, Dnieper, Don, and Volga, as far eastward as the Sea of Aral. To the last named the Siberian plain gradually declines. The southern plain of Europe stands in unbroken connection, so far as its formation is concerned, with the West Siberian plain, (2,213,400 square miles in extent,) and is, therefore, one of the most extensive lowlands on the globe. The Baltic-Sarmatian plain is separated from the West Siberian merely by the long Ural chain, (from 50° to 67° N. lat.,) whose elevation is only from 4000 to 5000 feet, and whose breadth is unimportant. Take away the Ural, and a continuous line could be drawn from Breda, near the confluence of the Meuse, Rhine, and Scheldt, across Europe and Asia, following the line of 50° N. lat as far as the Chinese frontier, passing over a continuous series of low, insignificant hills, heathlands, and steppes, and traversing a space estimated by Humboldt to be three times the length of the Amazon!
Toward the south, the Cis-Ural, European side of the Ponto-Caspian lowland, is separated from the Black Sea by a ridge of granite knolls, which passes from Volhynia and Podolia eastward as far as the cataracts of the Dnieper, and thence southeast, with diminished breadth, reaching its limit at Taganrog, on the lower Don, and the Sea of Azof. This ridge separates the narrow steppes of the northern shore of the Black Sea from the lowland of South Russia, the fruitful district of Ukraine. The height of these hills in the west, where they appear to have the greatest elevation, has been estimated to be about 1000 feet above the sea. Toward the Dnieper they have not yet been carefully measured; but probably there they do not rise above 700 feet.
The small sand steppe south of these granite hills runs from the Crimea eastward as far as the North Caucasian steppe, between the Don, Volga, and the Caspian, and indeed may be traced to the northeast as far as the Bashkiric-Ural chain. Lakes of marked saltness are found there: Elton, for instance, which lies 24 feet above the sea; while farther eastward they are found, as for example on the Kamysh and at Samara at a depression of—138 feet, 60 feet below the level of the Caspian Sea. Yet this lacks confirmation.
From this lowland, only a few elevations arise, and these of insignificant absolute height; yet, on account of the extreme uniformity of the whole country, they are objects of amazement to the whole steppe world. The Little Bogdo, south of Lake Elton, and yet farther south, Great Bogdo, 504 feet above the sea, according to Humboldt, and Mount Arsargar, 331 feet in absolute height, according to Murchison, are the only important hills. The Great Bogdo is composed of calcareous limestone and of sandstone, with rich deposits of salt.
The Kirgheez steppe separates, by a plain of very moderate elevation, the north Siberian lowland from the Caspian-Ural depression. It was formerly supposed, and indeed represented on the map, that a mountain range passes through this district from the Ural chain to the Altai. The Kirgheez steppe appears to range from 780 to 960 feet in elevation; while the Siberian plain is but 280 feet above the sea at Omsk, 192 feet at Tora, and 108 feet at Tobolsk. It has been considered by some that the Kirgheez steppe, as well as the granite hills of southern Russia, belong to an undeveloped system of mountains, an early cooling having solidified them before reaching the elevation which they would have attained; and that they partake of the direction which analogy would teach us such a chain would have, from northeast to southwest, parallel with the Carpathian and the Caucasus ranges.
The great depression of the Old World begins with the deepening of the Volga basin below Simbeersk; and at the place (51¾° N. lat., near Orenboorg and Saratov) where it breaks through the last row of hills in the Obstshei-Syrtis, it commences a rapid descent toward the Caspian and the Aral Seas. This great concavity, on the confines of Europe and Asia, at the center of the greatest land-mass, and far removed from any ocean, is remarkable as having no parallel on the globe. Humboldt remarks that perhaps a similar phenomenon would be repeated at the interior of other continents, if the tertiary formation and the parts found by marine deposit did not exist. It would be profitable to follow out so weighty a thought, with the surface as it now is.
The Obstshei-Syrtis is the moderate range of hills which runs westward in two branches from the Bashkiric-Ural, at Orenboorg, the northern spur running by Uralsk and the Ural River; the southern by Samara, rising on the east shore of the Volga to a height of 600 feet, and ending at Sarepta.
Orenboorg, on the Obstshei-Syrtis, where it leaves the Ural chain, is 255 feet above the sea. Uralsk lies somewhat lower, being 234 feet above the sea. The surface of the Volga, where it breaks through the high banks of Saratov, is only 36 feet above the ocean level; while the western shore, above Saratov, is 562 feet in height. Farther down the river, Sarepta lies 30 feet below the sea level; and there is, therefore, between Saratov and Sarepta, a distance of about 180 miles, a fall of 66 feet. West of the Volga, and following the river, is the continuation of the Obstshei-Syrtis, ranging in elevation from 562 feet down to 168 feet. At Sarepta, the low hills which thus far have skirted the Volga turn to the southwest, to the Manitsh steppe, sinking to an elevation of but 75 feet, and extending as far as the Sea of Azof. At Sarepta, too, the Volga turns from its normal southerly course, and strikes southeasterly across the Astracan steppe, entering the Caspian at the City of Astracan, 72 feet below the level of the sea. The level of the sea is 4 feet below the shore on which Astracan is built.