240 ANNALS NEW YORK ACADEMY OF SCIENCES 



Cretaceous or older sediments and voleanies. The latest formations, at 

 least, are not younger than Cretaceous. We also have good reason to 

 believe that the oldest formations entering into the coastal plain are not 

 older than early Tertiary. It therefore follows that the topography of 

 the West Slope must have been developed in post-Cretaceous time, or 

 more accurately, between the period of uplift of the Cretaceous sediments 

 and their associated voleanies and that of the initiation of Tertiary sedi- 

 mentation. It was during this interval that the present topographic 

 detail in its large phases as now expressed on the West Slope was deline- 

 ated. 



From the data at hand, the successive steps seem to have been as fol- 

 lows : At the close of Cretaceous sedimentation and volcanic activity the 

 West Eange of the Andes was elevated ; the west side of the uplifted sec- 

 tion was subsequently worn down to a poorly graded surface, at least in 

 the Huacho-Oyon section; this stage was followed by a strong uplift and 

 subsequent deep dissection, or erosion. This was followed, in turn, by 

 the depression of at least the shoreward edge of the post-Cretaceous land- 

 surface and the initiation of Tertiary deposition. In other words, Ter- 

 tiary deposition took place on the partly drowned edge of the West Slope. 



Ocona-Cora Puno Section. — This section has been chosen for the rea- 

 son that the observed facts show considerable variation from the one 

 already described. The town of Ocona is situated on the coast some 

 seventy miles to the northwest of Mollendo. The point known as Cora 

 Puno is located near the western edge of the West Eange. It is one of 

 the highest collection of volcanic peaks on the west of the Cordillera. 

 Starting at Ocona, we shall run our section across the immense plain or 

 desert of Cuno cuno to the valley of the Chorunga, thence up the Andaray 

 cuesta to Cora Puno and the West Range. 



The formational and topographical facts are roughly shown in the 

 accompanying section. At Ocona, we have the pre-Tertiary coastal ridge 

 facing the present shore line. On its west slope may be seen patches 

 of Tertiarjr and post-Tertiary still inviting the attack of the Pacific 

 waves. Passing inland, however, we find that, as we reach the level of 

 the pampas, pre-Tertiary formations may be seen at many places sticking 

 through the thin sheet of late sediments at elevations of 3,000 or more- 

 feet above sea level. As we pass towards the north we soon realize the 

 fact that the surface of the Cuno cuno plain has once been covered in part 

 at least b}^ a sheet of mud-like lava. Only in the deeper side-valleys 

 leading to the Ocona can}^on can we see the sediments as recognized near 

 the coast. This light-colored sheet now persists until we reach the steep 

 slope that passes into the Chorunga Valley. Here the Chorunga as well 



