Africa's geography is very diverse. It is composed of deserts, grasslands, rain forests, and mountains. Much of Sub-Saharan Africa is a plateau. The plateau is dimpiled by river basins and valleys. Because of the steep shore line it has few natural harbors. Most of the rivers are blocked by rapids, therefore, boats couldn't go far upstream from the ocean. This protected the interior from oceanic invasions, it made trading and communication difficult however. The Sahara Desert covers a quarter of the continent of Africa. It used to be fertile and well watered but over time the changing weather patterns and winds had made the land increasingly dry and inhabitable.