MONGOLIA, the name given to a vast tableland in Central Asia, the traditional home-country of the Mongol peoples and formerly a definite "dependency" of the Chinese Empire, but now divided into two very distinct cultural and political entities, Inner Mongolia, which is becoming absorbed into China, and Outer Mongolia, a republic in close relations with the Russian Soviet Federation. The later stages of the political evolution and the significance of the present situation can most easily be appreciated in the light of the physical geography of the country, its natural divisions and their respective orientation.
Mongolia in the physical sense is essentially a plateau varying from 3,00o to 5,00o feet in altitude and composed of granites, gneisses and crystalline schists of Archean and Primary age. It is bordered on the north-west by the extensive mountain-complex of the Altai, Tannu-ola and Sayan groups, and on the north-east by the Trans-Baikalian Highlands. On the south-east it is sepa rated from the Plain of north China by a belt of pronounced scarps whose continuation northwards in the high faulted scarp of the Great Khingan delimits the plateau on the side of Man churia. On the south-west it abuts against the lofty Nan-shan, the outermost of the great chains which enclose the Tibetan plateau. Only on the west does the Mongolian tableland sink to lower levels in the Jungarian depression and the Tarim basin on either side of the Tien-shan. Physiographically Mongolia falls into three clearly marked divisions : I. The central or Gobi region (see also under GoBi) on the whole the lowest and most uniform portion of the plateau. It forms an elongated shallow depression within the plateau and has as its rim on the south-east and east the plateau-edge formed by the In-shan series of scarps in Inner Mongolia and the faulted scarp-edge of the Great Khingan ; while on the north-west it is delimited by the sloping edge of the broad Altai-Khangai-Kentei mountain-complex. It thus runs with successive decrease of alti tude from the foothills of the western Nan-shan in the south-west to the Khingan foothills in the north-east. Isolated and weathered hill-ranges of low altitude characterize the surface of Gobi, which is generally covered with gravel, sand or even marked by bare rock. Rivers are absent save on the northern margin where the
Kerulen drains an area really foreign to Gobi—an area of moun tains and valleys constituting the first outliers of the Yablonoi mountain-system of Trans-Baikalia.
2. The north-western mountain-complex which has been com pared in its character with the Bohemian block in Europe. It is in the nature of an uplifted massif enclosed by mountain ranges (Altai, Khangai, Sayan) in places marked by faulting. It may even be the counterpart, on a smaller scale, of the Tibetan plateau. The general level of the valleys in this region is about 3,000-4,500 feet above sea-level. The whole is well drained by rivers, some of which, e.g., the Kobdo and Tess, enter brackish or salt lakes with no outlet. The largest lake, the Kosso-gol (5,320 feet), close to the Siberian frontier, occupies the highest part of this mountainous region. At least two high basin-areas can be distinguished : a. That of the Yenisei headstreams (or Urianghai basin) ; entirely mountain-girt by the Sayan on the north, the Tannu-ola on the south. The floor is dissected by rivers but is nowhere less than 1,700 ft. in altitude. b.. A series of lake-basins at different levels contained between the Tannu-ola and Altai further south. These are remnants of larger lakes which have shrunk as the result of progressive desiccation. The largest is Ubsa Nor, occupying an extensive plain, while further south is Kirghiz Nor, and a third group round Kobdo (Kaira Ussa and Durga Nor). This sub-region is one of lofty gravel plains pene trating north-westward between the Khangai and Altai. The latter mountain range (or series of ranges) rises abruptly out of the Jungarian depression, in which its southern base lies at an altitude of only 1,000-3,000 feet, to heights of 0–I i,000 feet (decreasing eastward). The northern base of the Altai rests on the high plateau at a considerable height (4,260 ft. at Kobdo). Thus the long steep southern scarp-face and the short northern slope to the high plateau behind give this system the character of a series of tilted fault-blocks. The Kentei and other mountain ranges to the west of Urga seem partly to enclose the basin formed by the headstreams of the Selenga-Orkhon.