The Alborz mountain range forms a barrier between the south Caspian and the
Iranian plateau. It is only 60–130 km wide and consists of sedimentary series dating from Upper
Devonian to
Oligocene, prevalently
Jurassic limestone over a
granite core. Continental conditions regarding
sedimentation are reflected by thick Devonian
sandstones and by
Jurassic shales containing coal seams. Marine conditions are reflected by
Carboniferous and
Permian strata that are composed mainly of limestones. In the Eastern Alborz Range, the far eastern section is formed by
Mesozoic (chiefly
Triassic and Jurassic) rocks, while the western part of the Eastern Alborz Range is made primarily of
Paleozoic rocks.
Precambrian rocks can be found chiefly south of the city of
Gorgan situated in the southeast of the
Caspian Sea and, in much smaller amounts, in the central and western parts of the Central Alborz Range. The central part of the Central Alborz Range is formed mainly of
Triassic and
Jurassic rocks, while the northwestern section of the range is mainly composed of Jurassic rocks. Very thick beds of
Tertiary (mostly
Eocene) green
volcanic tuffs and
lavas are found mainly in the southwestern and south-central parts of the range. The far northwestern part of the Alborz, that constitutes the Western Alborz Range or the
Talish Mountains, is mainly made up of
Upper Cretaceous volcano-sedimentary deposits, with a strip of Paleozoic rocks and a band of Triassic and Jurassic rocks in the southern parts, both in a northwest–southeast direction. As the
Tethys Sea was closed and the
Arabian plate collided with the
Iranian plate, and was pushed against it, and with the clockwise movement of the
Eurasian plate towards the
Iranian plate and their final collision, the Iranian plate was pressed from both sides. The collisions finally caused the folding of the Upper Paleozoic,
Mesozoic, and
Paleogene rocks and the
Cenozoic (chiefly the
Eocene)
volcanism, to form the Alborz Mountains, primarily during the
Miocene. The
Alpine orogeny began, therefore, with Eocene volcanism in southwestern and south-central parts of the Alborz, and continued with the uplift and folding of the older sedimentary rocks in the northwestern, central and eastern parts of the range, during the most important orogenic phases, which date from the Miocene and
Pliocene epochs. ==Ecoregions, flora and fauna==