MARSTERS, PHYSIOGRAPHY OF THE PERUVIAN ANDES 229 



agricultural purposes. The interstream spaces are barely scarred by insig- 

 nificant tributaries, so that we have huge expanses of desert plain extend- 

 ing from valley to valley. As we approach Moquequa and Locumba, how- 

 ever, the elevation has been less, so that the rivers have made broader 

 valley floors on which has been developed considerable grape culture, 

 while, in the Valley of Sama, sugar cane forms the chief agricultural 

 product. 



Geologically considered, the coastal plains of Peru are composed of 

 Tertiary and post-Tertiary sediments and lava flows, laid down on a 

 post-Cretaceous surface, which at the beginning of Tertiary sedimenta- 

 tion had not been worn down to grade. This is proved by the number of 

 half-buried hills now standing above the general horizon of the coastal 

 plain, the so-called "morros" already referred to. In many instances, 

 these hills merge into ridges and form what is known in Peruvian 

 geography as "Cadena de la Costa," or coastal chain. Their distribution 

 may be seen on the accompanying sketch map. 



As might be expected of a coastal plain bordering a mountain range 

 of such tremendous proportions and geologically young, it has under- 

 gone differential elevation to a great degree. In various localities, folding 

 and faulting have taken place on a considerable scale. 

 " In that section to the- north of Paita where I have made some detailed 

 stratigraphical studies, on account of the development of the Zorritos, 

 Lobitos and Nigritos oil fields, it is calculated that not less than 3,000 

 feet of Tertiary sediments enter into the structure of the coastal plain. 

 Some localities are rich in fossil gastropods and nautiloid forms. They 

 are supposed to represent lower and middle Tertiary faunas. 



It is interesting to note the fact that the oil-bearing localities so far 

 developed are associated with the sections of maximum disturbance of 

 the formations. The oil field of Zorritos is located on the eastern flank 

 of a folded section, much of which is located beyond the present shore 

 line. It is also in this section that we find the largest amount and most 

 minute type of surface dissection. The folding of the Zorritos section 

 may be traced to the northeast for some distance. In the interior to the 

 southeast, the folding gradually fades out until we reach the flanks of 

 Amotape Mountain, where its contact with pre-Tertiary formations is 

 encountered. 



Passing to the section of Lobitos, we find the same stratigraphical and 

 structural relationships as noted in Zorritos. The area of maximum fold- 

 ing is near the present shore line. About Lobitos, the original surface 

 of the coastal plain has been completely eroded away. To the north of 

 the Lobitos field, we find the original surface forming an extensive plain 



