Trending generally from northwest to southeast, the Caucasus Mountains consist of two ranges-the Greater Caucasus in the north and the Lesser Caucasus in the south. The watershed of the Greater Caucasus, the backbone of the system, traditionally has been part of the line dividing Europe and Asia, but the whole region has been so subject to Asian influences that there is now general agreement in assigning the ranges to Asia.
The Greater Caucasus marks the northern boundary of Transcaucasia and extends for approximately 750 miles (1,200 kilometres) south-eastward across the Caucasian isthmus from the Taman Peninsula, which separates the Black Sea from the Sea of Azov, to the Apsheron Peninsula, which juts into the Caspian Sea east of the oil-rich port of Baku.
The southern slopes of the Greater Caucasus are steeper than the northern. The middle of the system is comparatively narrow, but its western and eastern ends have widths of 100 miles or more. The main axis of the system contains Mount Elbrus, which at 18,510 feet (5,642 metres) is the range's tallest peak; Mount Dombay-Ulgen (13,274 feet) in the west; Mounts Shkhara, Dykhtau, and Kazbek, all more than 16,000 feet, in the central region; and Mounts Tebulosmta and Bazardyuzyu, both more than 14,600 feet, in the east. Spurs tonguing north and south from the main axis occasionally reach elevations approaching 10,000 feet. The highest parts carry permanent mountain glaciers, the shrunken remains of Quaternary ice fields and glaciers.
South of the Greater Caucasus, on the Black Sea coast, lies the Kolkhida alluvial plain, the site of ancient Colchis. South of the range on the Caspian side the Shirak Steppe, between the Greater and Lesser Caucasus ranges, falls sharply into the Kura-Araks Lowland, an extensive depression in the centre of which the Kura River receives its major right-bank tributary, the Araks River. To the northeast the hills of southeastern Kobustan separate the Kura-Araks Lowland from the Apsheron Peninsula; and to the extreme southeast the narrow Lankaran Lowland extends to the south between the Caspian Sea and the Talish Mountains, which reach elevations exceeding 8,000 feet.
West of the Kura-Araks Lowland rises the Lesser Caucasus Range, which is extended southward by the Dzhavakhet Range and the Armenian Highland, the latter straddling the frontier with Turkey. East of Lake Sevan in the eastern Lesser Caucasus, the highest peaks rise above 12,000 feet, while Mount Aragats the highest peak in the range, rises west of the lake to 13,418 feet. From their western sources in the Armenian Highland, the Kura and Araks rivers both flow around the Lesser Caucasus-the Kura to the north of the range and the Araks to the south-before their confluence in the east.
The greater part of Caucasia originated in the vast structural downwarp in the Earth's crust known as the Alpine geosyncline, dating from the Late Oligocene Epoch (about 30 to 23.7 million years ago), and the region thus reflects some of the same structural characteristics as the younger mountains of Europe.
Structurally the Greater Caucasus represents a great anticline (upfold) uplifted at the margin of the Alpine geosyncline about 25 million years ago and subsequently altered by fresh cycles of erosion and uplift. Hard, crystalline, metamorphosed rocks such as schists and gneisses, as well as granites that predate the Jurassic Period (i.e., older than 208 million years), have been exposed at the core of the western sector, while softer, clayey schists and sandstones of Early and Middle Jurassic origin (from 208 to 163 million years ago) have emerged in the east. The spurs of the Greater Caucasus are composed of younger limestones, sandstones, and marls. The Greater Caucasus is a zone of crustal instability, as evidenced by several extinct volcanoes (e.g., Mount Elbrus) and the earthquakes, often locally disastrous, that disturb the area.
The Kolkhida and Kura-Araks lowlands are both structural depressions linked to the Alpine geosyncline; the former is related to the formation of the Black Sea, the latter to that of the Caspian. In Kolkhida the overall surface of deposits laid down less than 25 million years ago is broken, at the foot of the mountains, by the protrusion of slightly older sedimentary rocks. Younger rocks also underlie the Kura-Araks Lowland.
The structures of the Lesser Caucasus, of the Talish Mountains, and of the Dzhavakhet-Armenian ranges likewise originated from folds uplifted from the Alpine geosyncline. While the western sector of the Lesser Caucasus and the Talish in the far southeast are formed chiefly from deposits laid down about 50 million years ago in the downwarp episode of the geosyncline, the central and eastern sectors of the Lesser Caucasus consist of sedimentary strata intruded in places with volcanic rock that are at least twice as old. Geologically recent volcanism and contact metamorphism (the intrusion of molten material and its effects on pre-existing strata) have everywhere played a great role in shaping the landscape. The folded base of the Dzhavakhet Range and of the Armenian Highland, for example, is masked by volcanic debris from eruptions that occurred in the Tertiary and Quaternary periods (i.e., the past 66.4 million years), but to the east much older rocks emerge between the middle course of the Araks and the latitude of Lake Sevan.
The Kura (and Araks) and Kuma rivers flow into the Caspian Sea, and the Rioni and the Inguri flow into the Black Sea. In the spring, when snow and ice begin to melt, rivers of the Greater Caucasus and some of those of the Lesser Caucasus begin a flood cycle that may last six months. Other Transcaucasian rivers are characterised by shorter-term spring flooding, while the rivers of the southern slopes of the Greater Caucasus generally have summer floods as well. The karst regions along some spurs of the Greater Caucasus contain rivers that intermittently plunge beneath the earth into caverns within the soluble limestone bedrock.
|The Aragvi River flowing through the central sector of the Greater Caucasus north of Tbilisi, Georgia|
Lake Sevan in the eastern Lesser Caucasus is the largest lake of Transcaucasia; its overflow drains into the Hrazdan River, a tributary of the Araks. The higher elevations of the Greater Caucasus contain numerous small mountain lakes, while a number of saltwater lakes occur in the arid regions of northeastern Transcaucasia.
The Greater Caucasus has more than 2,000 glaciers, which occupy only a tiny fraction of its total area. Some seven-tenths of them occur on the cooler northern face, with a concentration on the higher central slopes. The largest-notably Dykhsu, Bezengi, and Karaugom glaciers on the northern face and Lekzyr and Tsanner glaciers in western Georgia-are often seven miles or so long. The desolate flanks of Mount Elbrus are streaked by many glaciers.
Standing on the border between the temperate mid-latitude and the subtropical climatic zones, the Greater Caucasus accentuates this climatic difference by impeding the movement of cold air masses from the north into Transcaucasia. Average temperatures in January range from 39° to 43° F (4° to 6° C) in Kolkhida and from 34° to 37° F (1° to 3° C) in eastern Transcaucasia. In summer the temperature differences between north and south are slight, while there is a contrast between the west (average temperatures 73° to 79° F [23° to 26° C]), with its cooler maritime climate, and the more continental east (77° to 84° F [25° to 29° C]).
Kolkhida has a humid subtropical climate with mild winters, hot and humid summers, and a relatively large annual rainfall of 47 to 71 inches (1,200 to 1,800 millimetres). In the southeast the climate of the Lankaran Lowland is also humid subtropical but with a dry season at the start of the summer; and the Kura-Araks Lowland has a dry subtropical climate (annual rainfall of 8 to 16 inches that is lower in the east) with mild winters and hot summers. The Middle Araks Trough in the Armenian Highland has a climate similar to that of the lowland downstream but not as warm.
In the Greater Caucasus, temperatures decrease and the growing season becomes correspondingly shorter with an increase in altitude, and more total precipitation falls on the mountain slopes than on the neighbouring plains. Above an altitude of about 6,500 feet, a westerly air current prevails, strengthening maritime influences and greatly moderating climatic conditions; average air temperatures reach 18° F (-8° C) in January and 55° F (13° C) in August.
As the Greater Caucasus range stands at an angle to the westerly air currents, the heaviest precipitation, reaching more than 160 inches, accumulates on the south- and southwest-facing slopes. In the higher elevations, a cold, alpine climate with high humidity prevails, and perennial snow cover shrouds the highest crests. Along the northern Black Sea coast, the climate is Mediterranean, with mild, rainy winters and dry summers.
The northern slopes of the Lesser Caucasus facing toward the southern slopes of the Greater Caucasus have a climate similar to that of the latter at corresponding altitudes, with rainfall concentrated in the west. On the slopes of the Talish Mountains in the southeast, the climate is humid, with annual precipitation reaching 67 inches. The Armenian Highland, despite its proximity to the Black Sea, has a more continental climate than that of the Greater Caucasus at corresponding altitudes; at 6,500 feet, for example, the average monthly temperature in the Armenian Highland is 10° F (-12° C) in January and 64° F (18° C) in July. Snow cover throughout the southern highlands lasts for four to five months, while annual precipitation averages about 20 inches, with a spring maximum. Overall, the climate of the upland plateaus is moderately cold and continental, giving rise to semiarid steppe landscapes, whereas the climate of the heights is more humid and alpine, with cool summers and cold, prolonged winters.
Wormwood (Artemisia), saltworts, and ephemeral species characterise the arid Kura-Araks Lowland, and similar vegetation occurs in the Middle Araks Trough. In Kolkhida and in the Lankaran Lowland, the subtropical broad-leaved forests once covering these areas have given way to cultivation.
In the lower elevations of the mountains themselves, at altitudes between 2,000 and 3,000 feet, arboreal brushwood and broad-leaved forest predominate. On the Black Sea slopes of the Greater Caucasus and in the mountainous hinterland of Kolkhida, there are mixed forests of beech, oak, hornbeam, chestnut, and alder, with lianas and an evergreen undergrowth on terra rossa and yellow soils. In the Talish Mountains, forests of chestnut-leaf oak and ironwood (Parrotia persica) flourish on yellow soils, while farther north on the heights backing the dry Kura-Araks Lowland grow forests and brushwood of xerophytic (drought-resistant) species.
At higher altitudes, up to 6,500 feet, both the Greater and the Lesser Caucasus support forests of oak, hornbeam, and beech on brown soils; these are superseded at higher elevations by forests of Caucasian elm and Nordmann fir in the west and southwest and, occasionally, by pine forests farther east. Feather grass and needle grass cover the black soil of the steppes on the lava plateaus and plains of the Armenian Highland.
Above elevations of 6,500 feet in the Greater Caucasus and in the Transcaucasian ranges, mountain meadow vegetation covers three successive belts: subalpine, alpine, and subnivean. A zone of glaciers and perpetual snow begins at about 10,000 feet.
The fauna of the Greater and Lesser Caucasus includes certain endemic species-the West Caucasian and the Dagestanian mountain goat, ortur, the Caucasian black grouse, and the Caucasian mountain turkey, or ular-and even some endemic genera, such as the long-clawed mole vole (Prometheomys schaposchnikowi). Other common mammals include the chamois, red deer, bear, lynx, and fox. The Kura-Araks Lowland is home to the Persian gazelle (Gazella subgutturosa), long-eared hedgehog, jerboa, and jungle cat (Felis chaus). The fauna of the Talish Mountains, which includes leopard and porcupine, is related to that of more southerly territories, while that of the Armenian Highland is related tothat of Anatolia, with its ground squirrel (Citellus), and mountain jerboa (Allactaga williamsi).
The rural population of Transcaucasia is unevenly distributed. The most densely populated part of the region is along the Black Sea coast; the Rioni River valley and several smaller valleys are intensively cultivated and support large farm populations; and the foothills of the mountains are thickly settled. The alpine regions of the Caucasus and the arid steppes and lowlands of the Caspian littoral, however, are sparsely populated. Urban dwellers account for nearly three-fifths of the entire population of Transcaucasia, and in Armenia the proportion is even greater. Three cities - Baku, Tbilisi, and Yerevan - each have populations well in excess of one million.