30 



II. E. Gregory — A 'Geologic Reconnaissance 



Since its drainage was directed eastward the Cuzco Valley 

 has passed through several episodes whose sequence and 

 significance have not been satisfactorily interpreted. Some 

 of them, however, may be read in the light of physiography. 

 For example, the broad floor of the Oropesa Basin has been cut 

 and again filled by the meandering Huatanay, whose course 

 was temporarily arrested by a lava flow. In a similar manner 

 the ancient outlet of the Huatanay was permanently blocked 



Fig. 21. 



^.*'*L-^i*~k 



Fig. 21. The Cuzco Valley at the mouth of the Huatanay looking south 

 across the Urubamba River. Huambutio in middle distance. 



by the volcanic flows of Rumiccolca, and the stream was 

 turned abruptly northward over the low pass at Huambutio 

 to join the Urubamba at Sierra-Bella (fig. 21). The Cuzco 

 Basin has experienced successive stages of downwarp and at 

 one period was entirely or at least partly separated from the 

 lower Cuzco Valley and held the waters of an ancient lake. 12 I 

 have been unable satisfactorily to account for the narrows at 

 Angostura. That the region bordering the narrows has been 

 12 See p. 34. 



