272 



THE ANDES OF SOUTHERN PERU 



The fill is composed of both, fine and coarse material laid down 

 by water in steep valley floors to a depth of many feet. It breaks 

 the steep slope of each valley, forming terraces with pronounced 

 frontal scarps facing the river. On the raw bluffs at the scarps 

 made by the encroaching stream good exposures are afforded. 

 At Chinche in the Urubamba Valley above Santa Ana, the material 

 is both sand and clay with an important amount of gravel laid 

 down with steep valleyward inclination and under torrential con- 

 ditions; so that within a 

 given bed there may be an 

 apparent absence of lamina- 

 tion. Almost identical con- 

 ditions are exhibited fre- 

 t quently along the railway to 

 Cuzco in the Vilcanota Val- 

 ley. The material is mixed 

 sand and gravel, here and 

 there running to a bowldery 

 or stony mass where acces- 

 sions have been received 

 from some source nearby. 

 It is modified along its mar- 

 gin not only in topographic 

 form but also in composition 

 by small tributary alluvial 

 fans, though these in general 

 constitute but a small part of the total mass. At Cotahuasi, Fig. 

 29, there is a remarkable fill at least four hundred feet deep in 

 many places where the river has exposed fine sections. The 

 depth of the fill is, however, not determined by the height of the 

 erosion bluffs cut into it, since the bed of the river is made of the 

 same material. The rock floor of the valley is probably at least 

 an additional hundred feet below the present level of the river. 



Similar conditions are well displayed at Huadquina, where a 

 fine series of terraces at the lower end of the Torontoy Canyon 

 break the descent of the environing slopes ; also in the Urubamba 



Fig. 183 — Two-cycle slopes and alluvial fill 

 between Huichihua and Chuquibambilla. The 

 steep slopes on the inner valley border are in 

 many places vertical and rock cliffs are every- 

 where abundant. Mature slopes have their 

 greatest development here between 13,500 and 

 15,000 feet (4,110 to 4,570 m.). Steepest ma- 

 ture slopes run from 15° to 21°. Least steep 

 are the almost level spur summits. The depths 

 of the valley fill must be at least 300, and may 

 possibly be 500 feet. The break between valley 

 fill and steep slopes is most pronounced where 

 the river runs along the valley wall or under- 

 cuts it; least pronounced where alluvial fans 

 spread out from the head of some ravine. It 

 is a bowldery, stony fill almost everywhere 

 terraced and cultivated. 



