The depth of the alluvial valley fill due to tributary fan accumulation depends upon both the amount of the material and the form of the valley. Below Urubamba in the Urubamba Valley a fine series is displayed, as shown in [180] . The fans head in valleys extending up to snow-covered summits upon whose flanks living glaciers are at work today. Their heads are now crowned by terminal moraines and both moraines and alluvial fans are in process of dissection. The height and extent of the moraines and the alluvial fans are in rough proportion and in turn reflect the height, elevation, and extent of the valley heads which served as fields of nourishment for the Pleistocene glaciers. Where the fans were deposited in narrow valleys the effect was to increase the thickness of the deposits at the expense of their area, to dam the drainage lines or displace them, and to so load the streams that 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 moraines or bordering alluvial fans at the lower ends of the valleys, they are everywhere important in amount, distinctive in topographic form, and of amazingly 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 accumulated 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 Chinche. A Characteristic feature of the valleys of the Peruvian Andes below the zone of glaciation but within the limits of its aggraditional 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.
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 conditions; so that within a given bed there may be an apparent absence of lamination. Almost identical conditions are exhibited frequently along the railway to Cuzco in the Vilcanota Valley. The material is mixed sand and gravel, here and there running to a bowldery or stony mass where accessions have been received from some source nearby. It is modified along its margin 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, [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.