Physically, Iran consists of a complex of mountain chains enclosing a series of interior basins that lie at altitudes of 1,000 to 4,000 feet above sea-level. In terms of physical geography, Iran comprises the western and larger portion of a more extensive mountain zone that extends from eastern Asia Minor and the Caucasus as far as the plains of the Punjab. There are four major divisions of the country: the Zagros system, including small outer plains (chiefly the Khūzistān region), which are part of the Mesopotamian and Persian Gulf lowlands; the Alburz and Tālish systems and associated Caspian plain; the eastern and south-eastern upland rim; and the interior central desert basins. Within this broad framework more local and subregional contrasts can be drawn. Whilst in a few instances precise demarcation between the major units is far from straightforward, for the most part the scheme allows easy breakdown into units of distinct geographical significance.