Fig. 164—Geologic sketch map and section, Antabamba region. The Antabamba River has cut through almost the entire series of bedded strata.
Fig. 165—The upper diagram (A) represents the hypothetical distribution of land and sea during the Carboniferous Period, as inferred from the present distribution and character of Carboniferous limestones and slates. The lower diagram (B) represents the present relief. The dotted line at the left of the two diagrams connects identical points. The fragmentation of the former continental border is believed to have left only a small portion of a former coastal chain and to have been contemporaneous with the development of ocean abysses near the present shore.
The wide distribution of the Carboniferous sediments and especially the limestones, together with the uniformity of the fossil faunas, makes it certain that the sea extended entirely across the region now occupied by the Andes. However, from the relation of the Carboniferous to the basal schists, and the most conservative extension of the known Carboniferous, it may be inferred that the Carboniferous sea did not completely cover the entire area but was broken here and there by island masses in the form of an elongated archipelago. The presence of land plants in the Carboniferous of Pisco warrants the conclusion that a second island mass, possibly an island chain parallel to the first, extended along and west of the present shore.
CRETACEOUS
The Cretaceous formations are of very limited extent in the belt of country under consideration, in spite of their generally wide distribution in Peru. They are exposed distinctly only on the western border of the Cordillera and in special relations. In the gorge of Cotahuasi, over seven thousand feet deep, about two thousand feet of Cretaceous limestones are exposed. The series includes only a very resistant blue limestone and terminates abruptly along a well-marked and highly irregular erosion surface covered by almost a mile of volcanic material, chiefly lava flows. The character of the bottom of the section is likewise unknown, since it lies apparently far below the present level of erosion.