Structurally, the Peruvian territory has its fundamental element in the Andean system, here formed by a series of folds from which originated the chains that, in a longitudinal sense, follow one another from E to W: the Cordillera Oriental, the Cordillera Central and the Cordillera Occidental. They are separated from each other by depression basins of Mesozoic rocks covered by Neozoic layers, which hydrographically open towards the Amazon region, but which are sometimes endorheic, such as the one that hosts Lake Titicaca. The three mountain ranges have different structure and grandeur; they are the result of bending occurred in various periods and therefore also differ geologically. The Cordillera Oriental, mainly made up of rocks paleozoic, does not have a unitary trend or aspect: imposing to the S, where, among others, the Ausangate (6384 m) and Salcantay (6271 m) mountains rise, dominating the Río Urubamba valley, towards the N it is divided into a series of minor backbones decreasing in grandeur. Even the Cordillera Central is not very unitary, except in the northern section, and is in any case the least powerful of the three alignments. The Cordillera Occidental is instead a mighty bastion that hinders communications between the interior and the coast; the passes are very high: that of Ticlio, reached by the Lima-La Oroya railway, the highest in the world, is at 4829 m of altitude. Orographically, the Cordillera Occidental looks like a succession of sierras mainly made up of granite materials. Geologically it is the result of Cenozoic, with the general lifting of the Andean area, the definitive settlement of the large intrusive masses formed after the folds that had given rise to the Cordillera Oriental and the Central one. A great granite Horst is for example the Cordillera Blanca, dominated by Huascarán (6768 m), the highest peak in Peru and one of the main elevations of the Andes, the second on the continent. The orogenetic process that established the Peruvian relief was accompanied, as elsewhere in the Andean lands, by vast volcanic phenomena and also in Peru there are numerous eruptive systems, although not as frequent as in other Andean sections. The largest volcanic area is found in the southern part of the country, where the cone of the Mixed. The seismic activity is still relevant and denotes the youth and instability of the Peruvian Andean region, where catastrophes are frequent and disastrous, such as that of 1970 which saw a huge landslide detach from Huascarán, causing several thousand deaths. The conformation of the Andean highlands in Peru is quite different from the rest of the Andes: here the great South American chain reaches its maximum width (500 km at the height of Lake Titicaca), which explains how the Andes have and have had so much importance in the organization territorial of the country. Towards the Pacific, the Cordillera Occidental has a steep slope that ends on a mountainous region, largely of Mesozoic rocks, which depresses towards narrow coastal desert plains of recent formation.
Hydrographically, the Peruvian territory pays for approx. 3/4 to the Atlantic through the Amazon River: this is the result of the same conformation of the Andean chain, which with the younger and more steep Cordillera Occidental closes the other lands to the Pacific. The hydrographic development has a longitudinal trend in the Andes; then, through lateral passages, where the Cordillera Oriental lowers and breaks, the rivers find their way to the eastern lowlands. The rivers that drain the elevated Andean basins run towards the E very sunken and eventually flow into the major rivers, the Ucayali and the Marañón, both approx. 1800 km, which merge just upstream of Iquitos, originating the Amazon River. The Marañón, which draws its waters from the internal slopes of the Cordillera Blanca, is enriched by the contributions of the Huallaga, which drains the vast basin between the Oriental and Central mountain ranges; Ucayali is formed from the union of the Apurímac and the Urubamba, which arise respectively from the Cordillera Occidental and Oriental. Although the Andean rivers have a glacial supply at their sources and flow encased in gigantic gorges, they are considerably enriched with water along the eastern side, which is very rainy, and when they reach the Amazonian plain they are slow, wide and navigable. In any case, they have a well-marked seasonal regime: in the months of the austral winter (from June to September) they are in a lean phase and their level drops by several meters compared to the period of full summer. The rivers of the Pacific, on the other hand, have a relatively short course (less than 400 km), as they drain the steep slope of the Cordillera Occidental, and are torrential. They flow along precipitous valleys and extend only towards the coastal strip, where their waters are used to irrigate the numerous oases of foothills that follow one another in the region. Among the endorheic basins the main one is that of Titicaca (8300 km², about half in Bolivian territory); the great lake has an important place in the geography of Peru, a country of South America defined by ezinereligion, com, both for its central position in the other lands, and because its waters determine a certain climatic mitigation – the lake, located at 3810 m, is among the highest in the globe – which influences the population of the area.