Bolivia is essentially made up of two parts: one high, to the west, and a low, mostly flat, east. The western part consists of a section of the Andean system and includes a central depressed area, still partially covered by large lake basins without drainage to the sea (Titicaca, Poopó) and two higher marginal areas, surmounted by peaks often exceeding 6000 m. The central depressed zone, directed from NO. to the SE., it is 3800 m high on average. to the north and 3700 to the south, and was partly leveled by floods. Between Titicaca and 10 ° S. it is divided into various smaller basins by some series of andesitic hills, generally directed by E.-SE. to W.-NO., which protrude from the clayey soil (minute clays, black clays, clays with saline efflorescence) that covers it, which is partly of lake origin, salares of Coipasa and Uyuni are nothing but residues. For Bolivia 2000, please check neovideogames.com.
Ancient beach lines and lake terraces have been recognized in various locations, and testify to how Lake Minchin stretched from north to south for 400 km. and was separated from Titicaca, which was then also much larger (Lake Ballivián), by a low threshold. It is assumed that during the Glacial the Illimani glaciers blocked the road to the Río de La Paz, which conveys part of the waters of the plateau to the Beni, and therefore there was a rise in level and a consequent greater extension of the lake Minchin. Then when the glaciers retreated. some of the waters resumed their normal course towards the Amazons. Not all the plain is covered by lake sedimentation: at its edges there are coarse floods, there deposited by the streams that descend from the mountain ranges during a period in which the climate of the region was similar to the current one, that is essentially dry and irregular. The depression is closed to the west by the Western Cordillera, formed by a basement of ancient eruptive or intrusive rocks and of Mesozoic and Cenozoic sedimentary rocks, which bear recent eruptive rocks on large tracts. On this base, which forms a penepiano, sometimes almost perfectly horizontal, various volcanic peaks rise, mostly extinct or in the state of solfatara, including those of Tacora (5950 m.), Of Sahama (6520 m.), Huallatiri (6693 m.), Isluga (5530 m.: last eruption in 1913), dell’Ollagüe (5870 m.) and Licancahur (5930 m.), which support some small glaciers. The volcanic forms in the southern part of the cordillera are better preserved than in the northern one, where there is a greater amount of precipitation and therefore a more developed hydrographic network.
To the east, the central depression is limited by the Eastern Cordillera, constituted, from the Peruvian border up to the 17th century, by a large chain that connects to the north with the Andes of Carabaya (which close the Titicaca basin to the north) and which is said Cordillera Real to the furrow dug by the Río de la Paz, then taking the name of Cordillera de Quimza Cruz. The base is made up of granite rocks, on which black Siluric schists and Devonico sandstones rest. The highest peaks are granite, such as the Illimani (6450 m.) And the Illampu or Nevado de Sorata (6615 m.). The chain in the higher areas has truly alpine forms and is covered by important glaciers; it rapidly descends to the lowland of Beni-Mamoré with a slope which, abundantly watered by the rains, is covered by dense forests and deeply engraved by a large number of precipitous rivers. It is faced, towards the plain, by a series of minor chains (formed of perhaps Devonic and Cretaceous sandstones), whose height decreases towards the NE. To S. of the 17th southern parallel there is no longer a real chain, but a plateau (whose average height varies between 3600 and 4100 m.), consisting of folded Paleozoic and Mesozoic sediments, with here and there, in the western part, andesitic laccoliths. Above this base there are some peaks that exceed 5000 m. to the north, 5500 in the center, and reach almost 6000 m. in the southern part, which has the name of the Lípez mountain range.
The morphology of the Eastern Cordillera, even more than that of the Western Cordillera, is affected by climate change. In fact, in the northern and central part, with more abundant rains, the erosion was very intense, and the rivers cut deep valleys. In the southern part, however, hydrography is not very developed and forms typical of arid climates prevail. Furthermore, there are considerable differences in the morphology of the Eastern Cordillera between the external, very humid, and the internal side, dry. In both the cordillera there was, in the Quaternary, a much more intense glaciation than the current one, as evidenced by notable glacial traces, numerous especially in the Eastern Cordillera, which descend to below 4000 m. (For more information on the Andean area, seeandes).
The eastern plain region includes the lowlands of Beni and Mamoré to the north, the Chiquitos and Velasco plateau in the center and the Bolivian Chaco lowland to the south. The lowlands of Beni and Mamoré are a strip of the great Amazonian lowland. In the southernmost part there are some series of dunes, which the winds form with the sands deposited by the Río Grande and its tributaries, which they draw from the sandstones of Santa Cruz. Proceeding southwards, no further than the 15th southern parallel, these alluvial plains, formed essentially of clay, are almost perfectly horizontal and flooded periodically for several months. The Llanos de Mojos are covered by the waters from December to May, or even in June, on an area that is estimated to be 120,000 sq km. Between Rio Grande and San Miguel, also it tributary of the Guaporé, there is the so-called Monte Grande, a higher area, covered with dense forests. Immense forests extend north of the 12th parallel, and west of the Beni.
The Chiquitos and Velasco plateau rises between the Beni and Mamoré lowlands and the Bolivian Chaco. It has an irregular surface, formed of gneiss, with humpbacks and small reliefs that reach 500-600 m. In the depressed parts the products of the decay of the gneiss are accumulated. Since the soil is not very permeable, here and there there are temporary ponds and marshes, called curiches. On this gneissic base there are some sierras of sandstone perhaps devonic (Sierra de Ricardo Franco, Sierra de San José etc.), which reach 1200 m. about and which are covered with forests, because they receive abundant rains: the platform that supports them is covered by the savannah.
To the south of this plateau area lies the Bolivian Chaco, a region still very little known, especially in the southern part, formed by an immense alluvial lowland sloping towards the E., undulated by rows of dunes, very poor in surface waters, absorbed by the sand, largely covered by a low scrub of xerophilous plants or by sparse forests of mimosas, partly by grasslands.