PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 249 



erosion west of the series as a whole, that is west of the present 

 coast. 



(3) The restricted development of the Cretaceous seas upon 

 the western border of the Carboniferous, and the still more Re- 

 stricted development of the Tertiary deposits between the moun- 

 tains and the present coast, point to increasing definition of the 

 submarine scarp through the Mesozoic and the Tertiary. 



(4) The Tertiary deposits are all clearly derived from the 

 present mountains and have been washed seaward down slopes 

 with geographic relations approximately like those of the present. 



(5) From the great width, deep dissection, and subsequent 



Clays and sands. 



Red sandstone and shale. 



Gray and yellow sandstone and shale. 



Basal sandstone. 

 Volcanic agglomerate. 



Volcanic flows. 



Slaty schist. 

 Granite. 



Fig. 168 — Composite structure section representing the succession of rocks in the 

 Urubamba Valley from Urubamba to Torontoy. 



burial of the Tertiary terraces of the coast, it is clear that the 

 greater part of the adjustment of the crust to which the bordering 

 ocean basin is due was accomplished at least by mid-Tertiary 

 time. 



Aside from the fossiliferous limestones of known Cretaceous 

 age there have been referred to the Cretaceous certain red sand- 

 stones and shales marked, especially in the central portions of the 

 Cordillera, by the presence of large amounts of salt and gypsum. 

 These beds were at first considered Permian, but Steinmann has 

 since found at Potosi related and similar formations with Creta- 

 ceous fossils. In this connection it is also necessary to add that 

 the great red sandstone series forming the eastern border of the 

 Andes in Bolivia is of uncertain age and has likewise been re- 



