MARSTERS, PHYSIOGRAPHY OF THE PERUVIAN ANDES 239 

 WEST SLOPE AND WEST RANGE OP THE ANDES 



We now come to the geographical province which I have designated as 

 the West Slope and West Eange or Cordillera of the Andes. So far as 

 I am aware, no one has attempted to outline the physiographic history of 

 this section, nor is it my intention to try to solve this problem in all its 

 details ; nevertheless, I wish to present a few of the larger physiographic 

 features and their variations as observed in various sections on the West 

 Slope, with the hope at least that these may lead in the direction of a 

 correct and final analysis. 



As a basis upon which to formulate our views, I shall describe what 

 to me appear to be two fairly typical cross sections of the West Slope in 

 central and southern Peru. These I shall designate as the Huacho-Oyon 

 section and the Ocona-Cora Puno section. 



Huacho-Oyon Section. — The town of Huacho lies about sixty miles to 

 the northwest of Callao, on' the coast, in the valley of the Huari. You 

 may ascend from Huacho up to the Huari Valley and its important 

 branches to Oyon. Oyon stands at the base of the more prominent peaks 

 of the West Eange, hence the choice of the two names to locate the section 

 to be described. The distance from Huacho to the east side of the West 

 Eange is approximately 35 leagues, 175 kilometers, or 105 miles. Begin- 

 ning at Huacho, we find that the valley of the Huari has a broad flat 

 floor at the present shore line. It extends inland some eighteen to twenty 

 miles. It is the seat of important agricultural industries. At the apex 

 of the Huari floor we find the river running in a deeply incised valley. 

 Its tributaries have likewise cut canyon-like side valleys. On either side 

 of the Huari Valley, at the coast line, we find that the foothills of the 

 West Eange come practically to the present coast line. Should we ascend 

 to a point in the foothills from which a long-distance view of the upland 

 surface may be clearly seen, we shall at once note a moderately even but 

 highly inclined surface descending in the direction of the sea and ascend- 

 ing in the direction of the culminating points of the West Eange. As we 

 pass in the direction of the snow-capped prominences to the northeast, 

 the inclined surface gives way to prominences standing out in clear relief 

 above the upland surface. These finally culminate in the peaks of the 

 West Eange of the Andes. In the Lima-Oroya section, the above physi- 

 ographic features can be discerned, but they are by no means so clearly 

 defined. If we look into the details of topographic expression on the 

 West Slope, we find that it has been most minutely cut by steep, narrow 

 valleys leading to the master transverse lines of drainage from the in- 

 terior. The formations entering into the structure of the West Slope are 



