III. THE GEOLOGY AND PHYSIOGRAPHY OF THE ALTIPLANO OF PERU AND BOLIVIA

Abstract

What has been said concerning the Tertiary history of the Altiplano may be summarised briefly as follows. At the end of the Cretaceous, or in very early Eocene times, the only mountains in the Altiplano region were the worn stumps of the ancient Eastern Cordillera. With the onset of the Andean phase of earth movements the Western Cordillera appeared and the height of the ancient Eastern Cordillera was increased. These movements were completed by Miocene times, and the whole area peneplaned. Any lakes or elevated arms of the sea were probably filled in and obliterated during this phase of erosion. In Miocene times there began the Incaic phase of earth-movements accompanied by block-faulting which resulted in the formation of the Altiplano. This initiated the internal system of drainage prevailing at the present day, and the first of a long series of lakes appeared. In the Pliocene the Altiplano was probably only a few hundred metres high and had a semi-tropical vegetation, supporting a fauna dominated by large herbivores. Through-out Pliocene and post-Pliocene times, however, the last and grèatest uplift of the Andes was taking place. As these movements were epeirogenetic, the whole region was elevated without undue disturbance from lateral thrusting. The increasing aridity which resulted from this elevation brought about the diminution of the luxuriant vegetation and the extinction of the animals dependent on it for food. The Altiplano thus assumed its present-day aspect, but owing to the increasing dryness of the climate the lakes may have become greatly diminished. In Pleistocene times there were two great lakes in the Altiplano region. One of these, Lake Ballivian, included the present-day Poopó and Titicaca. The other, slightly more recent, is referred to as Lake Minchin, and included only the present Poopó basin, although Titicaca was probably in existence. These lakes were probably glacial, resulting from the melting ice-fields during inter-glacial phases of the Pleistocene glaciation of the area. With the retreat of the glaciers the aridity of the country increased again, and Lakes Poopó and Titicaca are gradually shrinking. Poopó is nearly dry, while Titicaca will sink below the outlet sill of the Rio Desaguadero and become increasingly salt. Later, however, its waters will freshen as new outlets are produced when the Titicaca basin is tapped by the cutting back of the Rio Sorata and the La Paz river.