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a mountain range in the Andes, part of the Cordillera Occidental of Peru, rising in Mount Yerupajá to 6,632 m. About 150 km long, the range is composed chiefly of Mesozoic sedimentary rock; effusive rock predominates in the northwest and south. Alpine relief forms are characteristic, and there is significant contemporary glaciation in the north. There are many lakes. The Marañon, Huallaga, and Mantaro rivers of the Amazon basin rise on its eastern slopes and the rivers of the Pacific Ocean basin on its western slopes.