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, and some are under government reservations. It is home to one of the world's largest remaining low-altitude rainforests, with numerous unknown endemic species of flora and fauna and biological diversity in the protected area known as the Northern Sierra Madre Natural Park. Isabela has of Cagayan Valley’s of forest cover.