Sections of the sierra may be briefly summarized as f ollows: In southern Peru the puna, or altiplano, is overshadowed on the east by the Cordillera Oriental (Real) and on the west by the Cordillera Occidental. It is a vast intermontane basin, roughly soo by 1 oo m., sloping gently to the south, very high, cold and dry. Upon it, ranges of low hills alternate with alluvial tracts, salt swamps and many lakes, the largest of which is Titicaca. At an altitude of 12,466 ft., Lake Titicaca is 130 by 45 m., with an average depth of 1 oo ft. (maximum about i,000 ft.). Well defined on the east, the bordering scarps of this great tectonic depression are concealed on the west by thick lava-flows. Near the lake, crops of barley, quinoa and potatoes are grown, while the western part of the plain is pastoral, covered with grass and occasional stretches of bushy growth. Puno is the only large town. Seen from the western slopes of the Cordillera Occidental, the even sky line of the lava plateau is broken by a series of volcanic peaks, some eroded, some symmetrical. They are isolated, snow capped, dormant or extinct, 19,000 to 20,000 f t. in height (Coro puna, 21,703 ft.). Earthquakes are frequent throughout this area. The bordering scarps of the altiplano converge on the north at the Nudo de Vilcanota (4° 26' S.). In the valley of the Huatanay, ft. high, is the ancient city of Cuzco. In this beautiful, luxuriant valley enclosed by mountains the Incas founded their great empire. Between 12° and 14° S. the heart of the sierra is a wilderness of gorges with cliffed sides, some over 6,000 ft. deep. Into these it is usually impossible to descend. The valley of the Apurimac is spectacular, its highly-coloured walls with tufts of cactus reaching from fever-ridden floor to snowy peaks. Between o° and 12° S. the puna is called the Pampa of Junin. It has many lakes, the largest of which is Lake Junin (Chinchaicocha), 20 by 7 m., altitude 13,415 ft. From it the Mantaro flows south across the plain—a deep, silent, swift-flowing stream. The chief activities on this plain are mining and pastoral, the only crops, potatoes and quinoa.
North of the Pampa of Junin is the Cordillera de Huayhuash, as the Nudo de Cerro de Pasco is now called, a desolate min ing region. North of this, the western chain is divided in two, Cordillera Negra, toward the west, and Cordillera Blanca, so called because of its snow-caps. The latter (8° 43' and 9° 53' S.) is about 7o m. long, its average height 15,000 ft., and its highest peak, Huascaran (22,180 ft.), a laccolith of granodiorite. Inhab itants are few, Indians living on slopes and valley moraines in small groups of huts like animal burrows. The Cordillera Ne gra (8° 43' S. and io° i 1' S.) is about ioo m. long and about ft. high (maximum 16,000 ft.). The bare, dark-grey rocks and even skyline contrast with the sharp peaks of the Cordillera Blanca. It is an unbroken wall without practicable passes. Between the two ranges lies the fertile valley of the Huaraz, averaging 25 m. in width, one of the most thickly settled parts of the sierra. The river flows north through a series of basins each with its town—Recuay, Huaraz, Yungay, Caraz, Huaylas, strung like beads upon a thread, responding to the presence of good soil, water and favourable climate Temperate zone crops are raised in upper reaches of the valley and tropical in the lower, known as the Callejon de Huaylas. At 8° 4o' S., the Huaraz, now called Santa, cuts the Cordillera Negra at right angles and crosses the desert to the Pacific. Two great rivers. Maranon and Huallaga, rise on the northern slopes of the Cor dillera de Huayhuash. The valley of the Maranon, 500 m. long, 30'–io° 20' S.) and for most of that distance more than a mile deep, is east of the Cordillera Blanca and parallel to it. In the rainy season the Maranon is impassable. It cuts the Cordillera Oriental at the Pongo de Manseriche 25' S.) and flows toward the Atlantic. The valley of the Huallaga, farther east than the Maranon, is more open and populated. Huanuco, in the upper valley, communicates by a good road with Cerro de Pasco. The Huallaga cuts the eastern chain at the Pongo de Aguirre (6° 3o' S.). North of 8° S. the cordilleras gradually subside. There are no more snowy peaks. The jalca, 7,000 to 9,00o ft. high, is covered by dense steppe vegetation with little seasonal variation, and much dissected by youthful valleys, some being canyons with vertical walls and narrow floors. The ancient town of Cajamarca is the agricultural centre of the northern sierra.