of the Cuzco Valley, Peru. 7 



For the spelling of geographic names, the expedition is 

 indebted to the Centro Geografico del Cnzco, and its President, 

 Dr. A. A. Giesecke, Rector of the Cugo University. 



Note on the geologic map. — The geologic map of the Cuzco 

 Valley (Plate II) represents the results of reconnaissance, not 

 detailed work. At the time the geologic work was done only 

 an incomplete print of the central portion of the topographic 

 map was available, and it has not been possible to avoid errors 

 in transferring to the completed map observations obtained 

 by compass traverse. JSTo boundaries of formations have been 

 traced throughout their extent; and the directions of fault 

 traces are only approximately known. The altitude of the 

 strata is indicated by dip and strike symbols ; but it has not 

 seemed wise to record my incomplete observations regarding 

 trends of the intricately interlocking synclines and anticlines 

 or to undertake at this time a systematic discussion of local 

 and regional structure. 



Though obviously subject to radical revision, the map repre- 

 sents substantially the present state of knowledge regarding the 

 geology of the Cuzco Valley, and I trust it will be found 

 helpful to those who undertake detailed work in the little 

 cultivated but fertile field of Peruvian geology. 



I. TOPOGRAPHY. 



Topographic setting. — The geographic position of the Cuzco 

 Valley is shown in fig. 2. Though possessing unusual scientific 

 interest, it is relatively a minor topographic feature. Viewed 

 broadly it is one of many small valleys tributary to a group of 

 canyons whose waters supply the Amazon. A surface profile of 

 southern Peru drawn through Cuzco reveals the presence of an 

 extensive area of upland sharply bounded on the southwest by 

 the Peruvian coastal plain and on the northeast by slopes 

 leading to the Amazon (fig. 3). This surface, which may be 

 called the Peruvian Plateau, is part of the Andean Plateau — 

 one of the greatest of the world's highlands. Above this 

 plateau surface, which stands at 13,000 to 15,000 feet above 

 the sea, rise snow-clad peaks — volcanic on the Pacific side and 

 erosion remnants on the Amazon side — and below it are sunk 

 profound canyons. The upward and downward departures 

 from a level at approximately 14,000 feet are about equal in 

 amount and involve areas of about equal extent. On the 

 southern border of Peru the upland surface is broken into a 



