The two dominating ranges of the Peruvian Andes, called the Maritime Cordillera and the Cordillera Vilcapampa, are composed of igneous rock—the one volcanic lava, the other intrusive granite. The chief rock belts of the Andes of southern Peru are shown in [157] . The Maritime Cordillera is bordered on the west by Tertiary strata that rest unconformably upon Palaeozoic quartzites. It is bordered on the east by Cretaceous limestones that grade downward into sandstones, shales, and basal conglomerates. At some places the Cretaceous deposits rest upon old schists, at others upon Carboniferous limestones and related strata, upon small granite intrusives and upon old and greatly altered volcanic rock.
The Cordillera Vilcapampa has an axis of granitic rock which was thrust upward through schists that now border it on the west and slates that now border it on the east. The slate series forms a broad belt which terminates near the eastern border of the Andes, where the mountains break down abruptly to the river plains of the Amazon Basin. The immediate border on the east is formed of vertical Carboniferous limestones. The narrow foothill belt is composed of Tertiary sandstones that grade into loose sands and conglomerates. The inclined Tertiary strata were leveled by erosion and in part overlain by coarse and now dissected river gravels, probably of Pleistocene age. Well east of the main border are low ranges that have never been described. They could not be reached by the present expedition on account of lack of time. On the extreme western border of that portion of the Peruvian Andes herein described, there is a second distinct border chain, the Coast Range. It is composed of granite and once had considerable relief, but erosion has reduced its former bold forms to gentle slopes and graded profiles.
The continued and extreme growth of the Andes in later geologic periods has greatly favored structural and physiographic studies. Successive uplifts have raised earlier deposits once buried on the mountain flanks and erosion has opened canyons on whose walls and floors are the clearly exposed records of the past. In addition there have been igneous intrusions of great extent that have thrust aside and upturned the invaded strata exposing still further the internal structures of the mountains. From sections thus revealed it is possible to outline the chief events in the history of the Peruvian Andes, though the outline is still necessarily broad and general because based on rapid reconnaissance. However, it shows clearly that the landscape of the present represents but a temporary stage in the evolution of a great mountain belt. At the dawn of geologic history there were chains of mountains where the Andes now stand. They were swept away and even their roots deeply submerged under invading seas. Repeated uplifts of the earth’s crust reformed the ancient chains or created new ones out of the rock waste derived from them. Each new set of forms, therefore, exhibits some features transmitted from the past. Indeed, the landscape of today is like the human race—inheriting much of its character from past generations. For this reason the philosophical study of topographic forms requires at least a broad knowledge of related geologic structures.
Fig. 157—Outline sketch showing the principal rock belts of Peru along the seventy-third meridian. They are: 1, Pleistocene and Recent gravels and sands, the former partly indurated and slightly deformed, with the degree of deformation increasing toward the mountain border (south). 2, Tertiary sandstones, inclined from 15° to 30° toward the north and unconformably overlain by Pleistocene gravels. 3, fossil-bearing Carboniferous limestones with vertical dip. 4, non-fossiliferous slates, shales, and slaty schists (Silurian) with great variation in degree of induration and in type of structure. South of the parallel of 13° is a belt of Carboniferous limestones and sandstones bordering (5), the granite axis of the Cordillera Vilcapampa. For its structural relations to the Cordillera see [Figs. 141] and [142]. 6, old and greatly disturbed volcanic agglomerates, tuffs and porphyries, and quartzitic schists and granite-gneiss. 7, principally Carboniferous limestones north of the axis of the Central Ranges and Cretaceous limestones south of it. Local granite batholiths in the axis of the Central Ranges. 8, quartzites and slates predominating with thin limestones locally. South of 8 is a belt of shale, sandstone, and limestone with a basement quartzite appearing on the valley floors. 9, a portion of the great volcanic field of the Central Andes and characteristically developed in the Western or Maritime Cordillera, throughout northern Chile, western Bolivia, and Peru. At Cotahuasi (see also [Fig. 20]) Cretaceous limestones appear beneath the lavas. 10, Tertiary sandstones of the coastal desert with a basement of old volcanics and quartzites appearing on the valley walls. The valley floor is aggraded with Pleistocene and Recent alluvium. 11, granite-gneiss of the Coast Range. 12, late Tertiary or Pleistocene sands and gravels deposited on broad coastal terraces. For rock structure and character see the other figures in this chapter. For a brief designation of index fossils and related forms see [Appendix B]. For the names of the drainage lines and the locations of the principal towns see Figs. [20] and [204].
SCHISTS AND SILURIAN SLATES [50]
The oldest series of rocks along the seventy-third meridian of Peru extends eastward from the Vilcapampa batholith nearly to the border of the Cordillera, [157] . It consists of (1) a great mass of slates and shales with remarkable uniformity of composition and structure over great areas, and (2) older schists and siliceous members in restricted belts. They are everywhere thoroughly jointed; near the batholith they are also mineralized and altered from their original condition; in a few places they have been intruded with dikes and other form of igneous rock.