258 THE ANDES OF SOUTHERN PERU 



posits on the terraces may similarly be correlated with alluvial 

 deposits in the valley. 



We shall now see what further ground there is for the de- 

 termination of the age of these sediments. Just below Chuqui- 

 bamba, 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 rela- 

 tions between Cretaceous limestones and volcanics in the Cota- 

 huasi Valley (see p. 247). Although volcanic flows were noted 

 interbedded with the desert deposits, these are few in number, in- 

 significant 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 sand- 

 stones, conglomerates, and related formations of the Majes Val- 

 ley and bordering uplands are older than the Pliocene or early 

 Pleistocene and younger than the Cretaceous and the older Ter- 

 tiary 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 rep- 

 resent, 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 ex- 

 ceed 5,000 feet. The depression had been partly filled in early 

 geologic (probably early Paleozoic) time by sediments later de- 

 formed 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 empha- 

 sized. All these facts are expressed graphically in Fig. 171. On 



