of the Cuzco Valley, Peru. 53 



believed not only to represent a former level of the Huatanay 

 but also to mark the shore line at a stage in the life of Lake 

 Morkill. A flight of dissected terraces, three in number, the 

 highest at 11,100 feet, flanks the Huanacauri for a distance of 

 2 miles above its mouth. The largest number of well-developed 

 terraces preserved in the valleys about Cuzco is five ; the small- 

 est, two. A typical group of terraces for the smaller tributaries 

 to the Huatanay is illustrated in fig. 33. 



In the Ayahuaycco Quebrada and certain other steep-walled 

 canyons no terraces were seen, but these exceptions to the com- 

 mon pattern are not significant, because conditions in narrow 

 trenches with very steep bordering slopes are unfavorable for 

 the preservation of terraces. Moreover, many terrace flats have 

 been modified as the result of irrigation and the construction 

 of andenes during the centuries of human occupation. 



Additional proof of climatic fluctuation which resulted in 

 the alternate deposition and removal of alluvium involved in 

 the construction of terraces is afforded by the presence of series 

 of recessional moraines, three to five in number, which cross the 

 floors of glaciated valleys. 22 It is probable that detailed study 

 would result in the correlation of the terraces and moraines 

 and furnish the necessary data for writing the climatic history 

 of the Cuzco Valley from the close of the glacial age to the 

 present. The result of canalization of streams to prevent 

 flooding and the realignment of channels designed to reclaim 

 valley flats must, however, be taken into account. Much to 

 my regret the demands of other work prevented a study of 

 recent physiographic history and its relation to the Inca 

 population. 



III. THE SEDIMENTARY ROCKS. 



Introduction. 



The sedimentary rocks of the Cuzco Valley and adjoining 

 areas between Urubamba and Apurimac rivers consist essen- 

 tially of Mesozoic strata intricately folded and traversed by 

 innumerable faults of large and small displacement. The com- 

 plicated structure of these rocks, which are widely variable in 

 composition and texture, requires detailed study of a high 

 degree of refinement before a satisfactory analysis of the strati- 

 graphic column can be presented. On the basis of the recon- 



22 See p. 41. 



