The mountains of Bhutan define its three main geographic zones: the Great Himalaya, the Lower Himalayan Range (or Inner Himalaya), and the Sub-Himalayan Range. The snowcapped Great Himalaya in the north ranges from about to peaks of over above sea level, extending along the Bhutan-China border. The northern region consists of an arc of glaciated mountain peaks with an arctic climate at the highest elevations. Watered by snow-fed rivers, alpine valleys in this region provide pasturage for livestock tended by a sparse population of migratory shepherds. Spur-like mountain ranges of the Lower Himalaya, between and , run northwest to southeast in western Bhutan, and northeast to southwest in eastern Bhutan. These mountains, and especially their western valleys, make up the economic and cultural heart of the kingdom, including most of its dzongs. These mountainous areas are contrasted with the hilly Sub-Himalaya, with elevations of up to , and the lower Duars. Many lower mountain ranges are composed of coarse granite sandstone, while rocks at the highest elevations consist of gneiss among upheaved strata of mica and talcose slate. Many ranges are abundant in limestone.