CHAPTEE XV 

 PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 



GENERAL FEATURES 



In the preceding chapter we employed geologic facts in the 

 determination of the age of the principal topographic forms. 

 These facts require further discussion in connection with their 

 closest physiographic allies if we wish to show how the topog- 

 raphy of today originated. There are many topographic details 

 that have a fundamental relation to structure; indeed, without a 

 somewhat detailed knowledge of geology only the broader and 

 more general features of the landscape can be interpreted. In this 

 chapter we shall therefore refer not to the scenic features as in a 

 purely topographic description, but to the rock structure and the 

 fossils. A complete and technical geologic discussion is not de- 

 sirable, first, because it should be based upon much more detailed 

 geologic field work, and second because after all our main pur- 

 pose is not to discuss the geologic features per se, but the physio- 

 graphic background which the geologic facts afford. I make this 

 preliminary observation partly to indicate the point of view and 

 partly to emphasize the necessity, in a broad, geographic study, 

 for the reconstruction of the landscapes of the past. 



The two dominating ranges of the Peruvian Andes, called the 

 Maritime Cordillera and the Cordillera Vilcapampa, are com- 

 posed of igneous rock — the one volcanic lava, the other intrusive 

 granite. The chief rock belts of the Andes of southern Peru are 

 shown in Fig. 157. The Maritime Cordillera is bordered on the 

 west by Tertiary strata that rest unconformably upon Palaeozoic 

 quartzites. It is bordered on the east by Cretaceous limestones 

 that grade downward into sandstones, shales, and basal conglom- 

 erates. At some places the Cretaceous deposits rest upon old 

 schists, at others upon Carboniferous limestones and related 



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