The Great Rift Valley is part of an intra-continental ridge system that runs through Kenya from north to south. It is part of the Gregory Rift, the eastern branch of the East African Rift, which starts in Tanzania to the south and continues northward into Ethiopia. It was formed on the “Kenyan Dome” a geographical upwelling created by the interactions of three major tectonics: the Arabian, Nubian, and Somalian plates. In the past, it was seen as part of a “Great Rift Valley” that ran from Madagascar to Syria. Most of the valley falls within the former Rift Valley Province. The valley contains the Cherangani Hills and a chain of volcanoes, some of which are still active. The climate is mild, with temperatures usually below 28 °C (82 °F). The most rain falls during the March–June, and October–November periods. The Tugen Hills to the west of Lake Baringo contain fossils preserved in lava flows from the period 14 to 4 million years ago. The relics of many hominids, ancestors of humans, were found here.