Fig. 168—Composite structure section representing the succession of rocks in the Urubamba Valley from Urubamba to Torontoy.
Aside from the fossiliferous limestones of known Cretaceous age there have been referred to the Cretaceous certain red sandstones 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 Potosí related and similar formations with Cretaceous 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 referred to the Cretaceous, though the matter of its age has not yet been definitely determined. In 1913 I found it appearing in northwestern Argentina in the Calchaquí 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.[53]
In the Peruvian field the red beds of questionable age were not examined in sufficient detail to make possible a definite age determination. They occur in a great and only moderately disturbed series in the Anta basin north of Cuzco, but are there not fossiliferous. The northeastern side of the hill back of Puqura (of the Anta basin: to be distinguished from Puquiura in the Vilcabamba 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, neighboring lakes, appear to have originated in solution depressions formed in these beds.
Fig. 169—The line of unconformity between the igneous basement rocks (agglomerates at this point) and the quartzites and sandstones of the Urubamba Valley, between the town of Urubamba and Ollantaytambo.