The province is divided into three physiographic areas. The eastern area, straddled by the Sierra Madre mountain range, is rugged and thickly forested. A substantial portion is uncharted. These unexplored hinterlands are home to a rich variety of flora and fauna, while others are government reservations. The western area is a sprawling fertile valley hemmed by the Central Cordillera. It is criss-crossed by the mighty Cagayan River, Siffu river, and Magat River.
