On the basis of geologic structure and relief, Russia can be divided into two main parts—western and eastern—roughly along the line of the Yenisey River. In the western section, which occupies some two-fifths of Russia's total area, lowland plains predominate over vast areas broken only by low hills and plateaus. In the eastern section the bulk of the terrain is mountainous, although there are some extensive lowlands. Given these topological factors, Russia may be subdivided into six main relief regions: the Kola-Karelian region, the Russian Plain, the Ural Mountains, the West Siberian Plain, the Central Siberian Plateau, and the mountains of the south and east.