The physical environment

The mountain topography of the central Zagros is one of the two outstanding and striking features of the natural environment in which the nomads make their home. (2) The mountain system is composed of a series of parallel ridges of declining altitude; they run from the northwest to the southeast. Folding, rather than faulting, has been the major mechanism in the formation of these ridges, and as a result, they constitute a classic series of long anticlinal ridges separated by deep synclinal valleys. The ridges are densely packed everywhere, but in the north they are generally broad and high, whereas in the south they are small and fine. Outstanding dominant peaks are seldom found. Instead, there is a gradual increase in the altitude of the individual ridges from the south to the north, those in the south averaging about 600 m, while occasional ridge domes can be found in the north that are as high as 4,000-4,500 m.

Erosion, compounded by overgrazing and by overcutting of the forest, has resulted in deep, incised, and narrow river valleys. These rivers flow parallel to the mountain ridges in a grid pattern, except where they are able to break through the mountain barriers in a series of spectacular gaps and gorges called tangs. Through the tangs run the principal avenues into the interior. In certain locations streams are unable to pierce the anticline ridges and interior drainage basins, and associated swamps and shifting saline lakes are the result. Because of somewhat heavier precipitation, the mountain slopes in the northern areas are apt to have an oak forest cover, but farther south, where the rainfall is less abundant and where deforestation is more advanced, slopes of exposed rock in striking colors of red, yellow, white, gray, green, or black are common. (3) Valley floors are deep, may range from 1,200 to 2,500 m above sea level, and are covered by a park-like grass and shrub vegetation that is sharply contrasted with the denuded slopes above. It is upon these valley floors that the nomads of the region find the principal supply of grass along their seasonal migration routes.

Falling almost entirely in the winter months, precipitation is scanty; the lowest totals, generally around 500 mm, fall in Fars province. Precipitation amounts increase markedly as one goes up-slope and northward into the higher elevations. (4) Yet, except for these higher elevations and favored sites along perennial streams draining the higher mountains, aridity is the rule. This paucity of precipitation is due to the dominance of dry, hot, Indian monsoon air in the summer, and dry, cold, stable air flowing out of the Siberian anticyclone in the winter. Rain that does fall originates in the Mediterranean region in the form of cyclonic disturbances and is confined almost entirely to the winter season. At irregular intervals, the dominance of the Siberian anticyclone breaks down and Mediterranean cyclones advance across the Fertile Crescent towards the Zagros mountains. Most of the moisture is deposited in Syria and Iraq as rain, and upon the higher ridges of the northern Zagros as snow. Occasional cyclones are turned south by the Zagros barrier and retain enough moisture to support a rather scanty vegetation on the low ridges and hills along the Persian Gulf. Thus, precipitation decreases in quantity from the north to the south in rough correlation with the elevation of the mountain ridges.

Iran in general, and Fars in particular, experience a marked continentality in temperature regime, with high summer and cold winter temperatures being characteristic. This is an important feature of the climate since, when coupled with the marked seasonality in and variable geographic distribution of rainfall, it contributes in an important way to the seasonal availability or scarcity of pasturage.