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.