Ancient Egypt can be thought of as an oasis in the desert of northeastern Africa, dependent on the annual inundation of the Nile River to support its agricultural population. The country's chief wealth came from the fertile floodplain of the Nile valley, where the river flows between bands of limestone hills, and the Nile delta, in which it fans into several branches north of present-day Cairo. Between the floodplain and the hills is a variable band of low desert that supported a certain amount of game. The Nile was Egypt's sole transportation artery. The First Cataract at Aswan, where the riverbed is turned into rapids by a belt of granite, was the country's only well-defined boundary within a populated area. To the south lay the far less hospitable area of Nubia, in which the river flowed through low sandstone hills that in most regions left only a very narrow strip of cultivable land. Nubia was significant for Egypt's periodic southward expansion and for access to products from farther south. West of the Nile was the arid Sahara, broken by a chain of oases some 125 to 185 miles (200 to 300 km) from the river and lacking in all other resources except for a few minerals. The eastern desert, between the Nile and the Red Sea, was more important, for it supported a small nomadic population and desert game, contained numerous mineral deposits, including gold, and was the route to the Red Sea.