250 THE ANDES OF SOUTHERN PERU 



ferred to the Cretaceous, though the matter of its age has not yet 

 been definitely determined. In 1913 I found it appearing in north- 

 western Argentina in the Calchaqui Valley in a relation to the 

 main Andean mass, similar to that displayed farther north. It 

 contains fossils and its age was, therefore, readily determinable 

 there. 4 



In the Peruvian field the red beds of questionable age were not 

 examined in sufficient detail to make possible a definite age de- 

 termination. They occur in a great and only moderately disturbed 

 series in the Anta basin north of Cuzco, but are there not fos- 

 siliferous. The northeastern side of the hill back of Puqura (of 

 the Anta basin: to be distinguished from Puquiura in the Vilca- 

 bamba Valley) is composed largely of rocks of this class. In a 

 few places their calcareous members have been weathered out in 

 such a manner as to show karst topography. Where they occur 

 on the well-drained brow of a bluff the caves are used in place 

 of houses by Indian farmers. The large and strikingly beautiful 

 Lake Huaipo, ten miles north of Anta, and several smaller, neigh- 

 boring lakes, appear to have originated in solution depressions 

 formed in these beds. 



The structural relation of the red sandstone series to the older 

 rocks is well displayed about half-way between Urubamba and 

 Ollantaytambo in the deep Urubamba Valley. The basal rocks are 

 slaty schist and granite succeeded by agglomerates and basalt por- 

 phyries upon whose eroded surfaces (Fig. 169) are gray to yel- 

 low cross-bedded sandstones. Within a few hundred feet of the 

 unconformity gypsum deposits begin to appear and increase in 

 number to such an extent that the resulting soil is in places ren- 

 dered worthless. Copper-stained bands are also common near the 

 bottom of the series, but these are confined to the lower beds. 

 Higher up in the section, for example, just above the gorge between 

 Urubamba and Ollantaytambo, even-bedded sandstones occur 

 whose most prominent characteristic is the regular succession of 



* See paper by H. S. Palmer, my assistant on the Expedition to the Central Andes, 

 1913, entitled: Geological Notes on the Andes of Northwestern Argentina, Am. Journ. 

 Sci., Vol. 38, 1914, pp. 309-330. 



