The Monti Sibillini (Sibillini Mountains), located in the National Park of the same name, are the fourth highest mountain group of continental Apennines after Gran Sasso, Maiella and Velino-Sirente. It lies along the primary watershed of Central Apennines between Marche and Umbria, in the Provinces of Ascoli Piceno, Fermo, Macerata and Perugia. These mountains are basically made of calcareous rocks, which were formed in warm seas. Their peaks, such as Monte Vettore (2,476 m a.s.l., the highest of the group), Pizzo della Regina (or Monte Priora), Monte Bove and Monte Sibilla, exceed 2,000 m in height. The eastern side of the range features narrow and north-oriented valleys (Aso, Tenna and Ambro Valleys), while the western side features three high-altitude depressions called “Piani di Castelluccio” (Pian perduto, Pian grande and Pian piccolo). Four rivers begin from the Sibillini Mountains: Aso, Tenna, Ambro and Nera. Lake Fiastra (artificial) and Lake Pilato, right below the peak of Monte Vettore, lie within the National Park.