We shall now see what further ground there is for the determination of the age of these sediments. Just below Chuquibamba, where they first appear, the sediments rest upon a floor of volcanic and older rock belonging to the great field now known from evidence in many localities to have been formed in the early Tertiary, and here known to be post-Cretaceous from the relations between Cretaceous limestones and volcanics in the Cotahuasi Valley (see p. 247). Although volcanic flows were noted interbedded with the desert deposits, these are few in number, insignificant in volume, and belong to the top of the volcanic series. The same may be said of the volcanic flows that locally overlie the desert deposits. We have then definite proof that the sandstones, conglomerates, and related formations of the Majes Valley and bordering uplands are older than the Pliocene or early Pleistocene and younger than the Cretaceous and the older Tertiary lavas. Hence it can scarcely be doubted that they represent a considerable part of the Tertiary period, especially in view of the long periods of accumulation which the thick sediments represent, and the additional long periods represented by the two well-marked unconformities between the three principal groups of strata.
If we now trace the physical history of the region we have first of all a deep depression between the granite range along the coast and the western flank of the Andes. Here and there, as in the Vitor, the Majes, and other valleys, there were gaps through the Coast Range. Nowhere did the relief of the coastal chain exceed 5,000 feet. The depression had been partly filled in early geologic (probably early Paleozoic) time by sediments later deformed and metamorphosed so that they are now quartzites and shales. The greater resistance of the granite of the Coast Range resulted in superior relief, while the older deformed sedimentaries were deeply eroded, with the result that by the beginning of the Tertiary the basin quality of the depression was again emphasized. All these facts are expressed graphically in Fig. 171. On the western flanks of the granite range no corresponding sedimentary deposits are found in this latitude. The sea thus appears to have stood farther west of the Coast Range in Paleozoic times than at present.
