316 Bowman — Geologic Relations of the Cuzco Remains. 



cordances ; for they would imply a repetition of accidents of 

 both time and place. There are, to be sure, certain differences 

 between the deposits, but they are differences of detail. The 

 deposits at the mouths of the larger valleys are thicker ; while 

 both classes are coarse, those from the glaciated valley heads 

 are coarser ; though both classes are eroded and are being 

 eroded, the thicker deposits are eroded to a greater depth, 

 though as a rule at a higher absolute elevation. The principal 

 characteristics of the belt of alluvium are brought out in figs. 

 1 and 10. 



That the deposits of these two classes are contemporaneous 

 points conclusively to some climatic condition which affected 

 both alike, whether or not that condition led, as in some cases, 

 to glaciation. It is inferred from the former extension of gla- 

 cial systems that the climate of the Pleistocene was colder : it 

 has not always been necessary from the conditions of a given 

 place also to postulate greater precipitation, though such a 

 postulate is imperative in many cases, including the one under 

 consideration. The basis for the inference is relatively simple. 

 Before the Pleistocene, waste supply and waste removal main- 

 tained a certain balance whereby large quantities of waste 

 clung to the upper slopes, while another part of the waste 

 was transported down valley without important aggradation. 

 With the advent of the Pleistocene, waste was shed more 

 rapidly from the upper slopes than it could be removed from 

 the lower slopes and a fringe of alluvium was formed. That 

 this should result in valleys whose heads were unglaciated 

 means that a greater amount of water and snow (water by later 

 periodic melting) fell on these valleys, and if on these valleys 

 on all valleys tributary to the basin. 



The conditions governing the actual formation of glaciers 

 on the mountains about Cuzco are relatively few in number. 

 Glaciers formed (1) in those valleys that reached above 12,500 

 feet, (2) in valleys with headwater tributaries able to supply 

 important masses of snow or ice. Pig. 11 represents a glaci- 

 ated valley at 13,000 feet. It has an extensive system of 

 tributary slopes and minor valleys extending up to summits 

 above 14,000 feet, where snow now collects in important 

 amounts during the southern winter. Neighboring unglaci- 

 ated valleys have few contributing slopes and a lower summit 

 altitude. 



The topographic characters of the glaciated portions of the 

 valleys about Cuzco are strongly marked. Glacial striae occur, 

 though these are relatively few in number since the rock in so 

 many instances is soft and upon exposure to weathering the 

 striae are soon destroyed. The lowest marks of glaciation are 

 at 12,250 feet (aneroid), in the valley of the Chacimayu south- 



