The outer ridge line in the southernmost part is separated from the inner one by the deep, populous valley of the Río Santa, called Callejón de Huaylas ; it does not exceed 5000 m. high and is called Cordillera Negra (because it has no permanent snow). The Santa in its rapid descent (2000 m. In 150 km., Between the springs and Carás) crosses a succession of basins covered by the fluvio-glacial floods coming from the moraines of the Cordillera Blanca. The Marañón Valley (one of the Amazonian spring branches) separates the Blanca from the Central Cordillera, which the Huallaga Valley, a tributary of the Marañón, then divides from the Eastern Cordillera.
The Marañón valley is a deep erosion valley, a little wide only between Lauricocha and Chuquibamba, in the highest part; in the rest it is narrow and limited by very steep slopes, where cultivation is impossible. The vegetation is given by xerophytes (steppe and thorny bush with cactus and Bombax ceiba). The river, which flows precipitously, is rarely more than 130 m wide, and in some places narrows up to 35 m. Quite different is the Huallaga valley, wide, well irrigated and relatively well populated. For Peru 2004, please check topb2bwebsites.com.
The Central Cordillera, made up of micaceous schists, granites, syenites and diorites, largely covered by Cretaceous limestone and sandstone and, here and there, by powerful piles of conglomerates, engraved by numerous valleys, is just over 3500 m high. and is therefore devoid of permanent snow, but there are abundant traces of the Quaternary glaciation. The Eastern Cordillera forms a mountainous transition area between the Sierra and the Montaña, called Ceja de montaña (edge of the forest), and is covered by tropical mountain forests, characterized by a great abundance of palm trees and epiphytes and that over 3000 m. have atrophied forms; or, here and there, from grassy steppes and savannahs with deciduous or perennial bushes. It is still little known: at the height of Moyobamba there is a chain that reaches 1600 m.; to the east of Huánuco rises a massif whose crests are maintained between 1600 and 2000 m. The complex of reliefs that form the Eastern Cordillera slopes gently towards the Pampas del Sacramento (Ucayali valley, another source branch of the Amazon River).
The southern part of the Andes of Peru, south of the Pasco Node, is often referred to as the Andes of the Ucayali or Apurimac and consists of two ranges (Western and Eastern Cordillera) between which a large plateau extends (Plateau of Peru), undulating or crossed by low hilly ridges and divided by transverse chains in various sections (plateaus of Oroya, 3740 m.; of Huancavalica, 3780 m.; of Ayacucho, 2700 m.; of Cuzco, 3300 m., and other minors). This plateau, like that of Bolivia, is commonly known by the name of Puna, a Quechúa word meaning “depopulated”, and is mainly made up of sandstone, limestone and, to a lesser extent, clayey schists and Mesomian marls. Porphyritic intrusions are frequent.
Characteristic of the Peruvian Puna is that of being engraved by deep and recessed valleys that have gutted some basins with deep erosion gorges (that of Cotahuasi is limited by vertical walls of 2000 m.), While the Bolivian Puna is made up of closed basins. However, part of the northernmost part of the closed Titicaca basin falls within the territory of Peru. The predominant vegetation on the plateau is the steppe composed mostly of acauli plants, with tomentose leaves.
The Western Cordillera, compact as a bulwark, with extremely arduous steps (the Lima-Oroya railway surpasses it at 4775 m., At a height, that is, slightly lower than that of Mont Blanc), also consists mainly of schists, limestones and sandstones from the Mesozoic era. South of the 16th parallel begins the volcanic zone called by the Peruvian-Bolivian Stübel: here are the volcanic cones of Ampato (6950 m.), Of Misti (6005 m.), Of Chachani (6090 m.), Of Pichu-Pichu (5600m; the last three behind Arequipa), del Tutupaca (5500m), Ubinas, Omate and other minor ones. Of these volcanoes, which covered the sedimentary rocks of much of southern Peru with a thick blanket of lava and ashes, only Misti (eruptions in 1784, 1830, 1869), Ubinas (eruption in 1867).
The Eastern Cordillera is still little known; it seems that it is constituted especially by granites, diorites and andesites and by schists, sandstones and paleozoic conglomerates, and formed by various chains uniformly oriented from NW. to SE. (Cord. Di Vilcapampa, between Apurimac and Urubamba, Andes di Carabaya, etc.), which reach 6300 m. in the Vilcanota. The easternmost ranges are part of the Ceja de montaña and are breached by the deep gorges (pongos) of Apurimac, Urubamba and their numerous tributaries. To the east of the line of the last pongos were isolated hills in the plain, and a few faint undulations.
La Montaña covers about 27% of the surface of Peru, and is superficially formed by alluvial soils (sands and clays) covered almost everywhere by the impenetrable equatorial forest; the only ways of penetration in this very depopulated region are given by the numerous rivers. A short distance from the banks of these, the unknown still reigns.
As in all Andean countries, earthquakes are very frequent in Peru, particularly in the coastal region, where they have sometimes caused very serious disasters, also because they were often accompanied by tsunamis. The major seismic centers are Lima (particularly damaged by the earthquakes of 1687 and 1746), Ica, Arequipa (almost completely destroyed in 1868), Moquegua and Cuzco.