The Central Caucasus between Mount Elbrus and Mount Kazbek
The Eastern Caucasus between Mount Kazbek and the Caspian Sea
In the wetter Western Caucasus, the mountains are heavily forested (deciduous forest up to 1,500 metres (4,900 ft), coniferous forest up to 2,500 metres (8,200 ft) and alpine meadows above the tree line). In the drier Eastern Caucasus, the mountains are mostly treeless.
The watershed of the Caucasus was the border between the Caucasia province of the Russian Empire in the north and the Ottoman Empire and Persia in the south (1801) until the Russian victory in 1813 and the Treaty of Gulistan which moved the border of the Russian Empire well within Transcaucasia.[2]
The border between Georgia and Russia still follows the watershed almost exactly (except for Georgia's western border, which extends south of the watershed, and a narrow strip of territory in northwestern Kakheti and northern Mtskheta-Mtianeti where Georgia extends north of the watershed), while Azerbaijan is south of the watershed except that its northeastern corner has five districts north of the watershed (Khachmaz, Quba, Qusar, Shabran, and Siazan).
^18th-century definitions drew the boundary north of the Caucasus, across the Kuma–Manych Depression. This definition remained in use in the Soviet Union during the 20th century.
In western literature, the continental boundary has been drawn along the Caucasus watershed since at least the mid-19th century.
See e.g. Baron von Haxthausen, "Transcaucasia" (1854); review Dublin university magazine
Douglas W. Freshfield, "Journey in the Caucasus", Proceedings of the Royal Geographical Society, Volumes 13–14, 1869.