South Tyrol lies in the heart of the Alpine arch, encircled by the primitive rock of the central Alps to the north, to the east by the pale Dolomite range and to the west by the Ortler massif which at 3,905 metres (13,175 ft) is South Tyrol's highest mountain. The Bozen porphyry plate dominates the central part of the region. The rivers Eisack, Etsch and Rienz flow through the province and long ago formed the main valleys. The Etsch (Adige), the most important of the three springs at the Reschen pass in the north-west and runs through the Vinschgau and the Etsch valleys, south to Salurn and beyond to the Adriatic. The Eisack river originates at Brenner and gives its name to the valley which extends from there to Bozen, where it flows into the Adige. Finally the Rienz flows from the Austrian part of Tyrol in the north east, past Toblach and through the Pustertal valley as far as Brixen, where it runs into the Eisack.