The Taita Hills, with others in the Eastern Arc Mountains, were formed more than one hundred million years ago. About thirty million years ago, the area was covered by extensive rainforest. During a cooler and drier period some ten million years ago, the lowland forests were converted to savanna, leaving the mountain ranges as islands where the tropical forests continued to flourish. The isolation of each mountain range has led to a great deal of endemism, and a very diverse flora and fauna. Some of the other mountain ranges are well-forested, but the Taita Hills retain just of forest. The Taita Hills rise steeply from the Tsavo West National Park.