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.
The old statements that the level of the Caspian is 300 feet below the ocean, rested solely on conjectures made by the naturalist Pallas. The influence of this great depression on the warmer climate of that region, the peculiar vegetation of the salt steppes, and the salt morasses which exist where the land is perfectly level, as well as the great beds of oyster-shells and other crustaceous remains, led him to the hypothesis that the whole neighboring district is the dry and deserted bed of a former sea, now shrunk to the comparatively insignificant dimensions of the Caspian. The broken line of bold bluffs which bounds the Obstshei-Syrtis on the south seemed to him to be the northern boundary of this inland sea, into which the Volga entered below the pass of Kamyschin and Saratov. Parrot and Engelhardt supposed that their barometrical elevations in 1811 confirmed Pallas’ theory, that the Caspian lies 300 feet below the ocean. Many hypotheses were based upon their observations; but the whole were at length brought into discredit by Humboldt, who distrusted the accuracy of instruments made at that time. Nothing but a trigonometrical survey from Taganrog to Astracan could give conclusive results, and this was accomplished in 1837, under the auspices of the Russian government. The result proved that, so far from being 300 or 350 feet below the ocean, the Caspian is not 100 feet. Its depression, as already stated, is about 76 feet.
The level of the Aral Sea, which is evidently closely linked to the Caspian, has not yet been determined with absolute certainty. Barometrical observations were instituted for this end by the expedition under General Berg, which explored that region in the winter of 1826, but the cold was severe, and the results are questionable. The result of their investigations was, however, that the surface of the Aral lies 110 feet higher than that of the Caspian. This would make the Aral to be 34 feet above the sea level. More careful inquiries may, however, determine the level of the two seas to be the same; but at present we have to be content with the results of the expedition referred to, and accept its elevation as 34 feet above the level of the ocean.
Without, however, going into details respecting the Aral, the region around the Caspian and directly connected with it, which is below the ocean level, embraces an area of not less than 131,400 square miles. This survey extends from the Volga to the Ural River, thence to the Emba and the northernmost point of the Sea of Aral, and thence to the salt lakes of Aksakal-Barbi, lying to the northeast of this sea. The tracing of this line from the higher to the lower stages of depression gives clear indications, in the nature of the soil, of the existence of a great sea once occupying that whole tract.
Thus much for the configuration of the Caspian lowland. If to these 131,400 square miles be added the 153,000, or, according to Humboldt, 164,000 square miles of the Caspian itself, the entire depression embraces almost 318,000 square miles, and is greater than France, greater than Germany, and only to be compared with the whole Austrian empire! If to this great region be added the district around the Aral, which sea alone covers nearly 25,000 square miles, and then to this the yet unmeasured surface covered by seas yet to the eastward, the entire region of depression is immensely increased. And then if to this be added the great Siberian plain, whose level is not greatly above the sea, the combined district would be at least once and a half as great as all Europe.