Africa is a continent of great size, almost 12 million square miles or about three times the size of the United States. Most of it lies in the tropics and, although we often think of Africa in terms of its rain forests, less than ten percent of the continent is covered by tropical forests, and those are mostly in West Africa. Much of the African surface is covered by savannas, or open grasslands, and by arid plains and deserts. In geological terms, the continent is really formed by a series of high plateaus broken in the east by the Great Rift valley and the mountains that surround it. Large rivers - the Congo, the Nile, the Zambezi, and the Niger - begin in the interior of the continent and flow to the sea over great falls and cataracts that mark the passage from the plateau to the coast. These falls have historically made movement from the coast to the interior difficult, but the great river systems have also provided the interior of Africa with routes of communication.
