Fig. 167—Geologic sketch map and cross-section in the Cotahuasi Canyon at Taurisma, above Cotahuasi. The relations of limestone and lava flows in the center of the map and on a spur top near the canyon floor. Thousands of feet of lava extend upward from the flows that cap the limestone.
From these considerations I think we have a strong suggestion of the geologic date assignable to the development of the great fault that is the most strongly marked structural and physiographic feature of the west coast of South America. Since the development of this fault is so intimately related to the origin of the Pacific Ocean basin its study is of special importance. The points of chief interest may be summarized as follows:
(1) The character of the land Carboniferous implies a much greater extent of the land than is now visible.
(2) The progressive coarsening of the Carboniferous deposits westward and their land derivation, together with the great thickness of the series, point to an elevated land mass in process of erosion west of the series as a whole, that is west of the present coast.
(3) The restricted development of the Cretaceous seas upon the western border of the Carboniferous, and the still more restricted development of the Tertiary deposits between the mountains and the present coast, point to increasing definition of the submarine scarp through the Mesozoic and the Tertiary.
(4) The Tertiary deposits are all clearly derived from the present mountains and have been washed seaward down slopes with geographic relations approximately like those of the present.
(5) From the great width, deep dissection, and subsequent burial of the Tertiary terraces of the coast, it is clear that the greater part of the adjustment of the crust to which the bordering ocean basin is due was accomplished at least by mid-Tertiary time.
