Vallès Occidental borders the comarques of Bages (to the north), Vallès Oriental (to the north and east), Baix Llobregat (to the west), and Barcelonès (to the south). It centers on the Catalan prelittoral depression, limited to the west by the River Llobregat and to the east by the River Caldes. The northern part of the comarca is mountainous. From west to east, it contains part of the Obac range (924m altitude at Castellsapera); the massifs of Sant Llorenç de Munt (1095m at La Mola and 1053m at Montcau) and Puig de la Creu (664m); and the Sant Sadurní (954m) and Farell cliffs (789m). At the south of the comarca is the Collserola range (512m), which forms part of the Catalan Littoral. Between these various ranges is the valley that gives its name to the comarca.