378 I. Bowman — Physiography of the Central Andes. 



The wide distribution of Cretaceous strata in Bolivia, north- 

 ern Peru, and Ecuador indicates how late was the formation of 

 the western Andes. Yon Ihering* assigns the uplift of the 

 western ranges to the point of continuous extent from Colom- 

 bia to Bolivia to the same period in which the Antarctic region 

 sank and the bridge between Australia and South America 

 was destroyed, that is, to the Eocene. However, the absence 

 of the Upper Cretaceous and the exceedingly limited occurrence 

 of the Eocene, and probable Miocene, upon the western fringe 

 of the Maritime Andes would seem to be sufficient ground for 

 concluding that the formation of the western ranges was well 

 begun in the Cretaceous, though undoubtedly completed only 

 in the early Tertiary. 



The occurrence of a widely developed baseleveled surface 

 now uplifted to a great height furnishes an interesting means 

 for the determination of recent geologic history. The 

 common physiographic history of the eastern and western 

 Andes, as indicated by the equally well-developed peneplain 

 formed upon both, is sufficient basis for the conclusion that the 

 mountain-making movements were completed with the formation 

 of the western Andes. From that time on, the history of both 

 orographic systems is written in terms of erosion cycles whose 

 development to different stages — the first to completion, the 

 second to maturity of form, and the third but fairly begun — 

 supplies us at once with the full means for topographic corre- 

 lation. The drainage features of southeastern Bolivia unite 

 with the topographic features to indicate that, whatever the 

 nature of the deformations that accompanied the regional 

 uplift of the peneplain, the peneplain was only developed after 

 the orogenic movements had occurred which involved the red 

 Cretaceous sandstones, for these are indicated by Hoek to " lie 

 concordantly " with respect to the older series of rocks, and 

 are involved with them in the same series of folds. The drain- 

 age relations support this view fully. The superposed drain- 

 age which cuts across hard and soft, across Silurian and Creta- 

 ceous alike, and is directed regardless of the structural axes, 

 clearly supports the fact already established by the topographic 

 features, that the peneplain was developed subsequent to that 

 orogenic movement which closed the Mesozoic era and finally 

 excluded the sea from that part of the continent now known 

 as the central Andes. We are therefore assured that pene- 

 planation followed the Mesozoic era. It must have occurred 

 some time during the Tertiary, although profound erosion and 

 an approach to peneplanation must have occurred in the 

 eastern Andes during the deposition of the Cretaceous and 

 Jurassic sediments that form the western Andes. This 



* Archhelenis und Archinotis, 1907, pp. 118-119. 



