Western Russia makes up the largest part of one of the great lowland areas of the world, the Russian Plain (also called the East European Plain), which extends into Russia from the western border eastward for 1,000 miles (1,600 km) to the Ural Mountains and from the Arctic Ocean more than 1,500 miles (2,400 km) to the Caucasus Mountains and the Caspian Sea. About half of this vast area lies at elevations of less than 650 feet (200 metres) above sea level, and the highest point (in the Valdai Hills, northwest of Moscow) reaches only 1,125 feet (343 metres). Nevertheless, the detailed topography is quite varied. North of the latitude on which Moscow lies, features characteristic of lowland glacial deposition predominate, and morainic ridges, of which the most pronounced are the Valdai Hills and the Smolensk Upland, which rises to 1,050 feet (320 metres), stand out above low, poorly drained hollows interspersed with lakes and marshes. South of Moscow there is a west-east alternation of rolling plateaus and extensive plains. In the west the Central Russian Upland, with a maximum elevation of 950 feet (290 metres), separates the lowlands of the upper Dnieper River valley from those of the Oka and Don rivers, beyond which the Volga Hills rise gently to 1,230 feet (375 metres) before descending abruptly to the Volga River. Small river valleys are sharply incised into these uplands, whereas the major rivers cross the lowlands in broad, shallow floodplains. East of the Volga is the large Caspian Depression, parts of which lie more than 90 feet (25 metres) below sea level. The Russian Plain also extends southward through the Azov-Caspian isthmus (in the North Caucasus region) to the foot of the Caucasus Mountains, the crest line of which forms the boundary between Russia and the Transcaucasian states of Georgia and Azerbaijan; just inside this border is Mount Elbrus, which at 18,510 feet (5,642 metres) is the highest point in Russia. The large Kuban and Kuma plains of the North Caucasus are separated by the Stavropol Upland at elevations of 1,000 to 2,000 feet (300 to 600 metres).