The country sits astride the Equator and is bisected from north to South by the Great Rift Valley, a natural phenomenon that runs through most of the length of Africa. Kenya is a land of great physical contrasts divided into roughly five ecological zones. To the north and northeast, the country ranges from semi-arid to the desert. About two-thirds of the country are in this zone, whose most important feature is Lake Turkana on whose shores at Koobi Kora.