Sudan is mainly composed of vast plains and plateaus that are drained by the Nile River and its tributaries. This river system runs from south to north across the entire length of the east-central part of the country. The immense plain of which Sudan is composed is bounded on the west by the Nile-Congo watershed and the highlands of Darfur and on the east by the Ethiopian Plateau and the Red Sea Hills (ʿAtbāy). This plain can be divided into a northern area of rock desert that is part of the Sahara; the western Qawz, an area of undulating sand dunes that merges northward into the rock desert; and a central-southern clay plain.