separated Africa from Arabia. Around 30 million years ago, a flood basalt plateau began to form, piling layers upon layers of voluminous fissure-fed basaltic lava flows. Most of the flows were tholeiitic, save for a thin layer of alkali basalts and minor amounts of felsic (high-silica) volcanic rocks, such as rhyolite. In the waning stages of the flood basalt episode, large explosive caldera-forming eruptions also occurred. The Ethiopian Highlands were eventually bisected by the Great Rift Valley as the African continental crust pulled apart. This rifting gave rise to large alkaline basalt shield volcanoes beginning about 30–31 million years ago. The northern Ethiopian Highlands contain four discernible planation surfaces, the oldest one being formed not later than in the Ordovician Period. The youngest surface formed in the Cenozoic, being partly covered by the Ethiopia-Yemen Continental Flood Basalts. Contrary to what has been suggested for much of Africa, planation surfaces in northern Ethiopia do not appear to be pediplains nor etchplains. Climate The predominant climate of the Ethiopian Highlands is the Alpine climate. Because the highlands elevate Ethiopia, located close to the equator, this has resulted in giving this country an unexpectedly temperate climate. Further, these mountains catch the precipitation of the monsoon winds of the Indian Ocean, resulting in a rainy season that lasts from June until mid-September. These heavy rains cause the Nile to flood in the summer, a phenomenon that puzzled the ancient Greeks, as the summer is the driest season in the Mediterranean climate that they knew. Ecology The Ethiopian Highlands share a similar flora and fauna of other mountainous regions of Africa; this distinctive flora and fauna is known as Afromontane but from the time of the last ice age has been populated with some Eurasian (palearctic) flora. The habitats are somewhat different on either side of the Great Rift Valley that splits the highlands. At lower elevations, the highlands are surrounded by tropical savannas and grasslands, including the Sahelian Acacia savanna to the northwest and the East Sudanian savanna to the west. . The highlands themselves are divided into three distinct ecoregions, distinguished by elevation. The Ethiopian montane forests lie between 1,100 and 1,800 meters elevation, above the lowland grasslands and savannas, and extends to areas of similar habitat in Eritrea, Sudan, and Djibouti. This woodland belt has several natural plant communities, but has mostly been heavily grazed and converted to agricultural use now. Kolla,