We thus have a series of enclosed seas and lakes (q.v.) of which Balkhash, the Aral and the Caspian have no outlet, and the two latter are salt, while the Sea of Azov communicates with the Black Sea through a break in the mountain line between the Caucasus and the Yalta mountains of the Crimea, and the Black Sea has communicated with the Egean and the Mediterranean since the sinking of the former. The rivers Dnieper (strictly 1,064 m. or with minor windings 1,328 m.), and Don (strictly 98o m. or with minor windings 1,123 m.), both flow south-eastwards for a considerable distance and then turn sharply south-westward to the Black Sea, probably as a consequence - of the recent dominance of flow towards the Egean following the sinking of the latter. Prior to that sinking it is thought that the Danube for a time made its way eastward across the lowland to the Cas pian Area which it may have reached via the curious Manych depression. The Volga (1,977 m. or with minor windings 2,107 m.), is the longest river of Europe flowing broadly east-south east parallel to the axis of the Moscow basin and meeting the Kama as it turns south below Kazan. On its course southward it has a sharp eastward loop with Samara at the head opposite the southward bend of the Don, and quite near it, the Volga bends south-east, thus continuing the previous line of the Don, and so it reaches its delta on the Caspian Sea. The great rivers of the Aral basin, the Amu-Darya or Oxus and the Syr Darya have already been mentioned and the relation of their courses to structural lines has been indicated.
Apart from the borders of the Pacific Ocean, Mongolia, Sin kiang, Afghanistan, Persia and the Caucasus, the Russian Area is really great lowland broken only by the Ural mountains and the residual heights on the Angara block. It is a region of great lakes and inland seas, of which the north-west European group is discussed in the article Europe, while the Caspian, Aral, Balkhash and Baikal are subjects of separate articles. As there are immense tracts covered by glacial clays, there are areas, for example, in the south-east of the Ob basin, with myriads of small lakes in the hollows.
The Caucasus (q.v.) and Transcaucasia form very distinct re gions structurally and physically, the Armenian mountain-knot being separated from the Caucasus by a relatively low narrow line occupied by the river Rion on the west and the river Kur on the east, but a long stretch of this "low line" is well over 1,500 ft. above sea. This line has to the east the deep basin of the southern Caspian and westward the deep basin of the southern Black Sea.
The Pleistocene Ice Age affected Russia profoundly and its effects have been studied more particularly in European Russia. In north Russia river-drainage has re-established itself post glacially to a greater extent than in Finland, which lay near the Scandinavian ice centre of the time. A great morainic belt may be traced between the Petchora-Vychegda-Northern Dwina basin and the Kama-Vyatka basin, and this belt continues south-west ward past Kostroma, Moscow and the northern watershed of the Dnieper and Pripet system into Poland. North-west of this great belt, in the west, are many great remnants of moraines enclosing lake or swamp areas. To the south of it are swamp-areas such as the Pripet basin, the Riazan district and the tongue between Volga and Oka before they join. Boulder clay stretches some way south of the great morainic belt above mentioned, and this extension is enormously increased by the occurrence of great tongues of boulder clay in the basins of the Don and Dnieper on the left banks of their long north-west-south-east sections. In
each case these boulder-clay extensions are much dissected by streams as would be expected on an area with impervious floor. The main morainic belt and the more southerly morainic limit above described are sometimes held to represent two separate glacial advances, the former being the more recent.
There seems little doubt that the Caucasus and the great high lands of Central and north-east Asia (including the Verkhoyansk arc and the Stanovoi range, as well as part of the Angara plateau) were centres of Pleistocene glaciation, but towards the north east the low precipitation probably limited the extent of the glaciers. Beyond the boulder-clay zone follows the loess, an aeolian deposit due to the outblowing winds from an ice sheet or other cold centre. They pick up fine-grained material from terminal moraines across which they blow out and deposit this over a wide zone as a porous deposit in which, frequently, grasses have grown and decayed, leaving vertical tubes up which water may be brought, and depositing organic matter which has pro moted the fertility of loess when it is exploited by man. Loess occupies practically the whole of South Russia and interlocks with the two tongues of boulder clay, sending north a broad tongue between the Don and the left bank feeders of the Dnieper to Orel, forming a region much poorer in streams than the boulder clay tongues on its two flanks. East of the boulder-clay tongue of the Don basin, the loess reaches as far north as the Vyatka river and eastward into Turkestan. In Asiatic Russia the loess seems to occur mainly along the foot of the western highlands and across Turkestan towards the Caspian.
The essential feature of the climate of the U.S.S.R. is its continental character ; the regions of somewhat more temperate type became independent states after 1917. From the western border to the mountains fringing the Pacific and from the Arctic to the mountain zone of Central Asia, this great stretch of land lies open to the winds, the Urals being too low to form an effective climatic barrier, though they have some influence on iso therms and especially on isohyets. Oceanic influences play some part in the west, but the Arctic Ocean to the north has prac tically no moderating effect. Even if eastern Russia were not cut off from the ocean by a mountain fringe, moderating influences would hardly be felt, since the winds blow off shore most of the year and moreover the Sea of Okhotsk is under the influence of a cold Polar drift and is a region of floating ice and fog. During the long winter the land mass of Asiatic Russia becomes ex tremely cold especially towards the north-east, and the cold, dry, heavy air which settles above it so intensifies the high pressure belt of Central Asia that it is attracted northwards. Lake Baikal and the region to the east of it have a January pressure of 30.5". The north-east coast of the Arctic lies north of the 30.1" isobar and a belt of high pressure extends from Asiatic Russia in a south-west direction. The most intense tongue of January high pressure stretches west along lat. 50° N. and forms a vortex of variable winds. To the north of it the prevailing winds are south and west, while to the south of it in the steppes and steppe deserts of European South Russia and of Turkestan the winds are north, north-east and east, so that severe winter cold is carried far to the south. The region west of the Ob river is under the influence of the Icelandic low-pressure system of western Europe.