Encyclopedia of the Incas

Southward the chains converge in a knot around Cerro de Pasco (10°45’S), out
of which two main cordilleras align in a north-south direction. Volcanic
landscapes dominate the western chain with lava 2,500 meters (8,202 feet) thick,
and featuring the iconic peaks of Coropuna (6,425 meters [21,079 feet]) and
Misti (5,822 meters [19,101 feet]). The eastern cordilleras have glaciers on many
peaks above 5,200 meters (17,060 feet) whose meltwaters feed streams tapped
for irrigation. Southeast of Cuzco, the Cordillera de Vilcanota holds the fabled
peak of Ausangate (6,384 meters [20,945 feet]) and, beyond that, Quelccaya
(5,470 meters [17,946 feet]), which is covered by the largest expanse of glacial
ice in the Central Andes.
Two major rivers, the Urubamba and the Apurimac, flow northward more or
less parallel until they join in the eastern forest to become the Ucayali River.
Whereas the Urubamba valley contained an Inca road and was an important zone
of food surplus, the even deeper Apurimac is narrow, dry, sparsely endowed with
flat alluvial spaces, and has no easy route for human movement through it.
The western and eastern cordilleras converge in the Nudo de Vilcanota
(14°30’S) and southward from it two mountain chains again follow parallel but
more distant paths. The western chain, studded with volcanic cones, forms the
border between Bolivia and Chile. About 400 kilometers to the east is the eastern
chain, where at 16°38’S in northern Bolivia, the glaciers begin at 5,486 meters
(17,999 feet). In comparison, on Huascarán, 1,350 kilometers (839 miles) to the
north in Peru at latitude 9°07'S, the permanent snowline starts at 3,962 meters
(12,999 feet), a difference explained by shorter days and thus lower rates of
snowmelt on Huascarán. Covering the 400 kilometers between the two
cordilleras is the Altiplano, a plateau 3,700–4,000 meters (12,139–13,123 feet)
above sea level, whose northern portion is dominated by Lake Titicaca, the
largest body of fresh water in the Andes. High evaporation and low rainfall in
the southern portion of the Altiplano have created not just desert, but also a stark
landscape of salt pans and salt lakes.
Differences in exposure explain many of the rainfall patterns in the Central
Andes. On the coast of Peru and northern Chile, low cloud banks hang over the
west side of the western cordillera. Air cooled at its base by the cold, northward-
flowing Humboldt (or Peru) Current keeps the daily high temperatures below
25°C (77°F) for most of the year and generates frequent mist but virtually no
rain. In this 2,200-kilometer-long (1,367-mile-long) desert strip, the natural
vegetation response varies from a thorn tree woodland and shrubby plant
formation known as lomas, to the sheer absence in southern Peru and northern