According to Existingcountries, Outer Mongolia, or NW, is a very extensive (almost 800,000 sq km) and very complex mountainous region, included between high ranges such as the Saians, the Altai and the Khangai, and raised so as to have the largest valley bottoms between 800 and 1600 m. of altitude. An area rich in forests and pastures, crossed by large rivers which partly feed vast inland lakes, draws its humidity from the winds that reach it through the Siberian plains. There are three main basins: that of the high Jenissei between the Saiani and the transversal chain of the Tannu-ola, with wide and flat valleys and very high mountains (3000-3500 m.); that of Selenga, between the eastern Saians and the jagged groups, but with flat peaks and not too high, of the Khangai and western Kentai; finally the so-called Valle dei Laghi, between the Tannu-ola and the Mongolian Altai.
Central Mongolia, or Gobi, is the lowest part of the plateau, and is very uniform in its immense extension, which measures up to 1200 km. in the meridian direction (towards 104 ° long. E.) and over 2000 km. in the transverse direction (towards 44 ° lat. N.). The term “Gobi” with which it is usually designated, is a Mongolian word that indicates depressions with a rocky bottom and sandy filling, mostly divided into small more or less hollow basins, from which the great Gobi desert is essentially formed. The general configuration of central Mongolia is that of an immense flat basin, or rather an immense raised plateau, with a wavy bottom and more or less salient edges; its altitude decreases from the margins (high 1600-2200 m.) to the center of Sair-Usu (850 m.), and, with secondary undulations, from SW. (foothill edge of Nanshan) to the NE. (towards the raised edge of the Great Khingan, where there are depressed areas up to 750 m.). The very long continuation of the Mongolian Altai to the east divides the Gobi into two sections; the western Gobi, with a more moved relief, between Altai and Nan-shan, and which extends to the West without delimitation lines in the deserts of Zungaria and Eastern Turkestān; and the eastern Gobi, flatter, between Altai and Khingan, and which extends for almost 500 km. to E. of central Khingan. Between the two sections is the Golyb Gobi corridor, between the last offshoots of the Mongolian Altai and the Khara-Narin mountains. The faint reliefs of the surface are isolated and eroded series of low hills, arranged mainly in the direction SO.-NE., among which s’ they interpose flat depressions filled with gravels in the peripheral parts and thin debris in the rest, up to salty clays in the center. There is no shortage of large areas of quicksand with semilunar dunes (barcane). But they occupy a very large surface only in the western part, which can be called a real desert; the remainder is rather a desert steppe, with even parts that can be cultivated. Region essentially with closed basins, in which salt lakes are also scarce, it has perennial rivers only at the extreme N., where with the great basin of Kerulen it enters the source region of Amur, and at the extreme S., where the Hwang -ho descends like a large river from the Nan-shan, and therefore crosses the marginal strip of central Mongolia and Inner Mongolia for a long stretch.
Inner Mongolia, or SE, or Chinese Mongolia, comprises two sets of chains. The first, more northerly, at N. del Hwangho, includes, from E. to O., the Shuma-Chada, Ta-Tsingshan, Khara-Narin chains, etc., which, although structurally different from the Great Khingan, seem continue it as the southeastern edge of the Mongolian plateau. The second, more southern, includes various ranges on which the Great Wall runs in part (eg, near Kalgan) as a border between Mongolia and the northern China plain. The area located between the two series of chains is the best part of Inner Mongolia, except for the steppe tabular plateau of the Ordos: the latter, embraced by the great loop of the Hwang-ho, is tectonically linked to western China, but similar to the Gobi for the characters of the soil.
The backbone of the Mongolian plateau is made up of archaic schist-crystalline soils (Tai-shan system) and Algonchian (Wu-tai and Khangai systems) densely corrugated, and huge granite masses intruded between them in the ancient Paleozoic. Apart from devonic sediments in the far north, there follow, discordant and involved in new folds, marine sediments of the recent Paleozoic (Permic), and, again in discordance, Jurassic sediments, corrugated together with the previous ones by a new and intense orogenic diastrophism. From the end of the Jurassic onwards, the region had always emerged, and the work of sub-aerial demolition of the reliefs (revived, at times, by partial lifting and sinking) led to covering large areas the corrugated base with horizontal eontinental deposits (fluvial, lacustrine and wind turbines), accumulated with great thicknesses during the Cretaceous, Tertiary and Quaternary times against and above the dismantled wrecks of the ancient chains. In these deposits, the recent American scientific expedition discovered in central Mongolia very important deposits of large Cretaceous reptiles (including dinosaur eggs) and of Tertiary and Pleistocene mammals. The tertiary and quaternary orogenic movements had the consequence of accentuating the elevation of the Mongolian plateau, varied in its various elements, and also of accentuating the depression of the sunken clods; the unequal lifting of the fractured parts led to the formation of tabular reliefs, while in the marginal chains, brought to greater height, a new cycle of erosion sank the valleys.
