of the Cuzco Valley, Peru. 



47 



study. The Cuzco Basin is bordered by an almost continuous 

 terrace of gravel overlying and interbedded with adobe, sand, 

 and limestone — the cut edges of fans and quiet-water deposits, 

 now elaborately dissected (fig. 29). The Oropesa and Lucre 

 basins contain accumulations of gravel, and beyond the limits 

 of the Cuzco Valley dissected fans and gravel terraces are 

 prominent along the Urubamba and Apurimac drainage 

 systems. 



Two compound fans of unusual dimensions — Cuzco and San 

 Greronimo — are shown on the topographic map. The Cuzco 



Fig. 29. 



Fig. 29. Dissected terraces south side of Cuzco Basin, looking across 

 the village of San Geronimo from the base of Cerro Picol. A. H. Bumstead, 

 photo. 



fan, on whose lower slopes the ancient Inca city was built, 

 has a length of 2% miles and a maximum width exceeding 1 

 mile. The surface of the upper portion is nearly flat; the 

 lower portion has a slope of 200 feet to the mile. Canyons 

 exceeding 150 feet in depth are sunk into the gravel without 

 reaching the base of the deposits. The accumulation of gravel 

 at this locality is favored by the topography, for a narrow 

 preglacial valley at elevations of 11,000 to 11,500 feet is bor- 

 dered by walls which attain heights exceeding 14,000 feet 

 in a distance of less than 3 miles. The structure, composition, 



