North-east of this anticlinal zone is the immense "Moscow basin" floored mainly by Palaeozoic rocks mainly still horizontal, with some Triassic deposits, but there are patches of Jurassic and Cretaceous strata as for example around Moscow and in the great bend of the Volga, and also farther north from Kostroma to Ust-Sysolsk on a tributary of the Vychegda. There was an in trusion of the sea in Jurassic times which became much more ex tensive in the Cretaceous period; no upper Cretaceous is known, however, in northern Russia. Around the Moscow basin in the south-west and west the lower Carboniferous system (with poor coal at Tula, etc.), outcrops in a great curved belt from the re gion of Tula almost to the White Sea. Beyond this belt north-west ward is the area of Devonian rocks towards Latvia and Estonia and across to the White Sea. They rest unconformably on Silurian rocks which outcrop along the southern shore of the Gulf of Finland and of lake Ladoga. The Devonian rocks are partly lacustrine (old red sandstone) and partly marine in origin, and the two types are often interstratified ; a basal red sandstone is covered by a dolomitic limestone which in turn has a sandstone over it.
The Timan hills (p. 684) on the north-east may be said to be an upfold bordering the Moscow basin. The main axis of the basin is parallel to the course of the Volga above Nizhniy-Nov gorod, continued north-westward by the Mologa, i.e., rivers run ning broadly parallel to the sections of the Dnieper, Donetz and Don already noted. Permian rocks outcrop over large areas on the north. The dips of the strata are small and anticlines and synclines are here really slight undulations which might be fol lowed right across Russia from the Dniester to the Timan hills always with lines roughly north-west to south-east (or west north-west to east-south-east) well marked in geology and drain age, though not much indicated in relief save that there is a marked low line of the Dnieper valley above Dnepropetrovsk, the Pripet marshes or Rokitno swamps and the upper Bug, where it continues the line of the Dnieper and Pripet in Poland. This belt of lowlands is of special importance because of the great marshes just named ; they form the effective barrier between Russia and peninsular Europe. (See EUROPE : Geography.) Turning now to the east one notes the virgations of the Tian Shan, orographically very subdued in the Aralo-Caspian lowland. One stretches north-westward parallel to and north of the Syr Darya river and apparently re-emerges from the lowland in one of the southern branches of the Urals which are at first a plateau and only rise to any considerable height north of lat. 52° N.
Another virgation stretches parallel to the first, this time just north-north-east of the Amu-Darya or Oxus river, and is con tinued into the Mangishlak peninsula that projects into the north east of the Caspian Sea. Farther south still are the hills of the Persian border reaching the Caspian Sea between the Gulf of Kara Boghaz and the town of Krasnovodsk, and continued on the west of that sea by the mighty Caucasus. In the Aralo-Cas pian lowland the rocks of these virgations are masked by a qua ternary covering with Tertiary deposits south of the lower Oxus and on the Ust Urt plateau between Aral and Caspian. Between the Aralo-Caspian-Balkhash area and the Arctic drainage area of the Oh and its feeders, is the higher land of Semipalatinsk related structurally to the Altai and floored by Palaeozoic and igneous rocks. West of this the land continues to be somewhat above the level of the lowlands on either side and is floored by Tertiary deposits, but the higher land narrows down in the Turgai region which according to general opinion until Oligocene times had a sea-communication right away to the Arctic on the north. The two great rivers of the region are the Amu-Darya (Oxus), 1,500 m. long, and the Syr Darya, 1,500 m. long, both reaching the Aral Sea.
Structural geologists have spread familiarity with the idea of a great sea called Tethys reaching in Mesozoic and Eocene times from the present Mediterranean area eastwards through the region occupied later by the young folded ranges of Asia Minor, Armenia, the Hindu-Kush and Himalaya, etc., out to the Malaya. Without venturing into details of advances and retreats of the sea it may be said that the uprise of the fold mountains of Asia Minor, Armenia, the Elburz Range and the Hindu-Kush left the part of that sea to the north of this more or less isolated as the Pontic brackish lake, losing its former connection via the Turgai low-line with the Arctic Ocean probably in Oligocene times and becoming much modified by the sinking of the southern parts of the Black and Caspian Seas (q.v.) which are both deep. Apart from these two depressions the land shelves easily from land to sea around the northern parts of the Black and Caspian Seas while the shallow Aral Sea is a part of the Pontic lake, now cut off by lowland from the north-east of the Caspian Sea. Balkhash is a freshwater lake on higher land farther east now, at any rate, without outlet, whatever may have been the condition of things in the past.