of the Cuzco Valley, Peru. 31 



subject to intermittent uplift is indicated by a succession of 

 unlike slopes, and that erosion by the Huatanay at this point 

 has had a predominant vertical component is shown by its 

 narrow valley. It is possible that at certain periods in its his- 

 tory the narrows marked the position of a divide, but further 

 study is necessary before the relation of the narrows to the 

 development of the Cuzco Valley can be interpreted. 



Contrasted portions of valleys. — The features — peneplain 

 remnants, mature slopes, and truncated spurs — indicating 

 three cycles of erosion, are matched by three unlike portions 

 into which most streams in the Cuzco Valley are divided. For 

 example, the Llampuhuaycco, rising in glacial debris, flows 

 for a mile through an old-age valley, marked by a wide, flat, 

 grass-covered floor in which the stream is neither aggrading 

 nor degrading to an appreciable degree and is carrying little 

 sediment. The middle portion of the stream begins abruptly 

 with waterfalls and rapids and descends precipitously, carry- 

 ing much sediment along the bed of a narrow rock-walled 

 canyon. Its tributaries drop rills of water from the sloping 

 mouths of hanging valleys. This middle torrent portion is fol- 

 lowed by a canyon intrenched in a gravel fan of Pleistocene 

 age. These contrasted portions of the Llampuhuaycco Valley 

 are duplicated in stream channels throughout the Cuzco region. 



Relation of valleys to structure. — Most of the streams enter- 

 ing the Cuzco Valley are consequents and owe their position to 

 the topographic slopes. In general they cross the strata nearly 

 at right angles, and their spacing and length are results of 

 initial topographic conditions which determined the shape and 

 extent of their drainage basins. Of the larger consequent 

 streams the Huaccoto is typical. It rises on the Urubamba 

 divide by gathering waters from numerous springs in glacial 

 debris; follows the dip slope to the Huaccoto volcano, which 

 it avoids by a circuitous route; traverses a canyon cut across 

 the strata to Huayllabamba Finca; and reaches the Huatanay 

 by following a depression at the side of the San Geronimo 

 fan. It is everywhere conformable to pre-existing topography 

 and presents no evidence of structural control. 



Subsequent streams are represented by many tributaries to 

 northward-flowing consequents on the south side of the Cuzco 

 Basin. The' Chchocco branch of the Huancaro has selected 

 for its course the more calcareous beds of a syncline (fig. 22) ; 

 the upper Huilcarpay and the east branch of Rio Pillau exhibit 

 the same relations. The Hspha is walled for a distance of 2 



