of the Cuzco Valley, Peru. 21 



water in the lowlands are normal for June and July. At 

 elevations of 12,000 feet the quarrying action of freezing water 

 is probably felt for five months in the year, and above 13,000 

 feet frost may occur on any night the year round. Snow 

 usually lies on Pachatusca from March to November. However, 

 frost work, as indicated by the amount of talus, is not con- 

 spicuous compared with that of colder climates. 



Ground water is an active erosional agent in the Cuzco 

 region. Springs and seeps are common along canyon walls 

 and within shallow quebradas. Landslides are very frequent. 

 Within the gravel-walled quebradas adjoining the city of Cuzco 

 eight occurred in a single season. The rock and gravel dis- 

 placed by slides vary in amount from a few cubic yards to 

 the enormous mass of material represented by the prominent 

 scar on Cerro Picol, which is visible for a distance of 10 miles. 



The relief of the Cuzco Valley is favorable for rapid erosion. 

 With the exception of the Huatanay few streams have gradi- 

 ents less than 600 feet to the mile, and considerable stretches 

 of many streams traverse slopes whose inclinations exceed 

 1,000 feet to the mile. (See fig. 15-20.) In such channels, 

 aided by fluctuating run-off and patchy cover of vegetation, 

 disintegrated materials are rapidly manufactured and easily 

 removed. The erosional work of streams is further facilitated 

 by the calcareous cement of certain strata, by the almost 

 universal presence of open joints, and by the wide extent of 

 partly consolidated glacial and lacustrine deposits. 



Mountains. 



With the exception of the low volcanic dome of Huaccoto 

 none of the mountain ranges or individual peaks of the Cuzco 

 region retain their original form. They clearly belong to a 

 generation long postdating the great earth movements that 

 gave to the Andes their initial elevation, but they have experi- 

 enced several cycles of erosion and uplift. The present moun- 

 tain summits have been carved from truncated edges of folds in 

 which sedimentary strata, chiefly thin-bedded sandstone, have 

 been overturned along sinuous axes. Erosion on the limbs of 

 these complex folds has etched the higher lands into a series 

 of cuestas with wide variety of trend and inclination. The 

 crest of the ridge forming the Huatanay-Apurimac divide is 

 in places serate, and the dominance of the Apurimac drainage 

 is indicated by the contrast between the steep bare slopes cut 



