MARSTERS, PHYSIOGRAPHY OF THE PERUVIAN ANDES 219 



Topographic Expression as Eelated to the Geology of the 



Peruvian Andes 



plains and sections 



Zorritos-Lambeyaque Plain. — The formations of this division of the 

 coastal plain are, so far as known, Tertiary in age. They are entirely 

 sedimentary and are composed in the main of clays and sands with occa- 

 sional pebble and conglomerate beds. Erom a section prepared by me, 

 between Fernandez and the shore line, something like 3,000 feet of sedi- 

 ments can be calculated. More than 2,000 feet have been penetrated by 

 the drill in search of new petroleum-bearing horizons both in Zorritos 

 and Lobitos. I wish here to emphasize the point that the only localities 

 where a topographic expression resulting from uplift and subsequent 

 erosion may be found are confined to the three localities, Zorritos, Lobi- 

 tos and Mgritos, where petroleum in large quantities has been obtained. 

 Each one of these places is located on the east limb of a somewhat broken 

 or locally faulted anticline. The formations as seen in the Fernandez 

 section are regarded as lower and middle Tertiary. In the Lobitos, it is 

 believed that we have middle and upper Tertiary. 



In the Paita section, reference has already been made to the occurrence 

 of a series of sands and conglomerates resting unconformably upon a 

 mass of red clays. These deposits contain some fossils, apparently the 

 same as those living at the shore line to-day. They are now 250 to 300 

 feet above sea level. As we pass in the direction of Piura, we can see 

 little mesa-like elevations. These were found to be composed of the red 

 clays seen in the lower part of the Piata section. It is thus evident that 

 the red-clay deposits were elevated and eroded before the deposition of 

 the conglomerates referred to above; and, further, the erosion must have 

 been largely confined to the outer half of the Paita section. 



It is important to note another fact at this point. At a number of 

 places in the Paita-Piura Pampa or Despoblado, as the natives are accus- 

 tomed to call these plains, you cannot fail to see small areas strewn with 

 shells, all of which appear to be specifically the same as those living on 

 the present shore line. This would suggest that, in very late geological 

 time, the Paita-Piura Plain was beneath sea level, and that it was subse- 

 quently elevated to its present position. 



As we pass to the south, the despoblado has been barely scratched by 

 the Piura Eiver and the surface subsequently littered up with wind-blown 

 sands. That is to say, topographically, the plain is so young that it yet 

 has the same expression as when it emerged from sea level. This con- 



