. The Andes of southern Peru; geographical reconnaissance along the seventy-third meridian. Yale Peruvian Expedition (1911); Physical geography; Geology. 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 a
. The Andes of southern Peru; geographical reconnaissance along the seventy-third meridian. Yale Peruvian Expedition (1911); Physical geography; Geology. 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 a
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