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Most of the great mountain systems now in existence were developed fairly late in geologic history. The greatest mountain masses are in North and South America, including the Andes, Rockies, Sierra Nevada, and Coast Ranges of the United States, Canada, and Alaska; and the Eurasian mountain belt, in which lie the Pyrenees, Atlas, Alps, Balkans, Caucasus, Hindu Kush, Himalayas, and other ranges. Among notable single peaks are Everest, K2 (Godwin-Austen), and Kanchenjunga in Asia; Aconcagua, Chimborazo, and Cotopaxi in South America; McKinley, Logan, and Popocatepetl in North America; Mont Blanc and Elbrus in Europe; Kilimanjaro, Kenya, and Ruwenzori in Africa.
Mountains have important effects upon the climate, population, economy, and state of civilization of the regions in which they occur. By intercepting prevailing winds they cause precipitation; regions on the windward side of a great range thus have plentiful rainfall, while those on its lee side are arid. Mountains are in general thinly populated, not only because the cold climate and rarefied atmosphere of high regions are unfavorable to human life, but also because the higher reaches of mountains are unfit for agriculture. Mountains frequently contain valuable mineral ores, deposited out of solution by water or by gases. Mountains act as natural barriers between countries and peoples; they determine the routes followed by traders, migrants, and invading armies. The difficulties of travel and communication in mountain regions tend to favor political disunity.
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