The Sierra is the commanding feature of Peru's territory, reaching heights up to 6,768 meters. Hundreds of permanently glaciated and snowcapped peaks tower over the valleys. The steep, desiccated Pacific flank of the Andes supports only a sparse population in villages located at infrequent springs and seepages. In contrast, tropical forests blanket the eastern side of the Andes as high as 2,100 meters. Between these extremes, in the shadows of the great snowpeaks, lie the most populous highland ecological zones: the intermontane valleys (kichwa) and the higher uplands and grassy puna or Altiplano plateaus. Approximately 36 percent of the population lives in thousands of small villages and hamlets that constitute the rural hinterland for the regional capitals and trading centers. Over 15 percent of Peruvians live at altitudes between 2,000 and 3,000 meters, 20 percent live between 3,000 and 4,000 meters, and 1 percent regularly reside at altitudes over 4,000 meters.

Although rich in mineral resources, such as copper, lead, silver, iron, and zinc, which are mined at altitudes as high as 5,152 meters, the Andes are endowed with limited usable land. The highlands encompass 34 percent of the national territory, or 437,000 square kilometers, but only 4.5 percent of the highlands, or 19,665 square kilometers, is arable and cultivated. Nevertheless, this area constitutes more than half the nation's productive land. About 93,120 square kilometers of the Sierra is natural pasture over 4,000 meters in altitude, too high for agriculture. The 4.5 percent of arable land, therefore, has fairly dense populations, particularly in Puno, Cajamarca, and in valleys such as the Mantaro in Junín Department and Callejón de Huaylas in Ancash Department. The highland provinces have a population density of 460 persons per square kilometer of habitable, arable land.

The best areas for cultivation are the valleys, which range from 2,000 to 3,500 meters in altitude. Although many valleys have limited water supplies, others, due to glacial runoffs, enjoy abundant water for irrigation. In the protected valleys, the dry climate is temperate, with no frost or great heat. In the high plateau or puna regions above 3,939 meters, the climate is cold and severe, often going below freezing at night and seldom rising above 16° C by day. A myriad of native tubers thrives at altitudes from 2,800 meters to almost 4,000 meters, including over 4,000 known varieties of the potato, oca, and olluco, as well as grains such as quinoa. The hardy native llamas and alpacas thrive on the tough ichu grass of the punas European sheep and cattle, when adapted, do well at lesser altitudes.

For the Peruvians, there are two basic Andean seasons, the rainy winter from October through April and the dry summer in the remaining months. Crops are harvested according to type throughout the year, with potatoes and other native tubers brought in during the middle to late winter and grains during the dry season. The torrential rains of the winter months frequently cause severe landslides and avalanches, called 81d m>huaycos, throughout the Andean region, damaging irrigation canals, roads, and even destroying villages and cities. In the valley of Callejón de Huaylas, the city of Huaraz (Huarás) was partially destroyed in 1941 by just such a catastrophe, an event repeated a few kilometers away in 1962, when the town of Ranrahirca was annihilated by a huayco that killed about 3,000 people.

The formidable terrain of the Andes, where the land may fall away from 4,848 meters to 545 meters and then rise to 6,666 meters in a space of 48 kilometers as the condor flies, poses a ubiquitous challenge to any modern means of transport. Thus, the Andean region was not penetrated by wheeled vehicles until railroads were built in the latter half of the nineteenth century. Moreover, most of the nation did not see wheels until the dirt road system was under construction in the 1920s. To do this, President Augusto B. Leguía y Salcedo (1908-12, 1919-30) revived a national system of draft labor harkening back to the Inca's conscripted labor force, or mita (see Glossary), used for road and bridge building in ancient times.

Data as of September 1992