PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 271 



they have not yet cleared their beds after thousands of years of 

 work under torrential conditions. 



Below Urubamba the alluvial fans entering the main valley 

 from the east have pushed the river against its western valley 

 wall, so that the river flows on one side against rock and on the 

 other against a hundred feet of stratified material. In places, as 

 at the head of the narrows on the valley trail to Ollantaytambo, 

 a flood plain has been formed in front of the scarp cut into the 

 alluvium, while the edge of the dissected alluvial fans has been 

 sculptured into erosion forms resembling bad-lands topography. 

 On the western side of the valley the alluvial fans are very small, 

 since they are due to purely local accumulations of waste from 

 the edge of the plateau. Glaciation has here displaced the river. 

 Its effects will long be felt in the disproportionate erosion of the 

 western wall of the valley. 



By far the most interesting of the deposits of glacial time are 

 those laid down on the valley floors in the form of an alluvial fill. 

 Though such deposits have 



greater thickness as a rule 

 near the nourishing mo : 

 raines or bordering allu- 

 vial fans at the lower ends 

 of the valleys, they are 

 everywhere important in 

 amount, distinctive in topo- 

 graphic form, and of amaz- 

 ingly wide extent. They 

 reach far into and possibly 

 across the Amazon basin, 

 they form a distinct though 

 small piedmont fringe along the eastern base of the Andes, and 

 they are universal throughout the Andean valleys. That a deposit 

 of such volume — many times greater than all the material accumu- 

 lated in the form of high-level alluvial fans or terminal moraines 

 — should originate in a tropical land in a region that suffered but 

 limited Alpine glaciation vastly increases its importance. 



Fig. 182 — Dissected alluvial fans on the 

 border of the Urubamba Valley near Hacienda 

 Chinehe. A characteristic feature of the 

 valleys of the Peruvian Andes below the zone 

 of glaciation but within the limits of its ag- 

 graditional effects. Through alluviation the 

 valleys and basins of the Andean Cordillera, 

 and vast areas of the great Amazon plains east 

 of it, felt the effects of the glacial conditions 

 of a past age. 



