The Semien Mountains were formed prior to the creation of the Rift Valley, from lava outpourings between 40 and 25 million years ago during the Oligocene period. The volcano is believed to have spread over more than 5000 m2 and resulted in a thick sequence of basaltic lava some 3,000-3,500 m thick that was deposited on Precambrian crystalline basement. The major part of the Semien Mountains consists of remnants of a Hawaiian-type shield volcano. The Kidus Yared peak is situated near the center of the shield volcano. Ras Dejen, Bwahit and Silki were formed from the outer core of this ancient volcano. The extreme escarpment in Semien appears to be a precondition for the formation of the extended uplift of the whole mountain massif 75 million years ago. The dramatic views are due to this volcanic activity. Especially of note is the 2,000 m high escarpment extending in a southwest-northeast direction. There are different types of soils as a result of the difference in geological formation, glaciations, topography, and climate. The Humic Andosol is the dominant soil type which is mainly found at an altitude of 3,000 m. The other types of soil are shallow Andosols, Lithosols, and Haplic Phaeozems that are mainly common in the area between 2,500 and 3,500 m. The Semien Mountains are highly eroded as a result of human land use practices and as a result of the topography of the area.
Human history
Although the wordSemien means "north" in Amharic, according to Richard Pankhurst the ancestral form of the word actually meant "south" in Ge'ez, because the mountains lay to the south of Aksum, which was at the time the center of Ethiopian civilization. But as over the following centuries the center of Ethiopian civilization itself moved to the south, these mountains came to be thought of as lying to the north, and the meaning of the word likewise changed. The Semiens are remarkable as being one of the few spots in Africa where snow regularly falls. First mentioned in the Monumentum Adulitanum of the 4th century AD, the presence of snow was undeniably witnessed by the 17th century Jesuit priest Jerónimo Lobo. Although the later traveler James Bruce claims that he had never witnessed snow in the Semien Mountains, the 19th century explorer Henry Salt not only recorded that he saw snow there, but explained the reason for Bruce's failure to see snow in these mountains – Bruce had ventured no further than the foothills into the Semiens. Despite their ruggedness and altitude, the mountains are dotted with villages linked by tracks. Historically they were inhabited by Ethiopian Jews, who after repeated attacks by the zealous Christian Emperors in the 15th century withdrew from the province of Dembiya into the more defensible Semien mountains. Towards the end of the Zemene Mesafint, DejazmachWube Haile Mariam maintained his arsenal and treasury on Mount Hai.
