270 THE ANDES OF SOUTHERN PERU 



was well charged with debris, an overriding not marked by 

 morainal accumulations, chiefly because the ice did not maintain 

 an extreme position for a long period. 



In the vicinity of the terminal moraines the alluvial valley fill 

 is often so coarse and so unorganized as to look like till in the cut 

 banks along the streams, though its alluvial origin is always 

 shown by the topographic form. This characteristic is of special 

 geologic interest since the form may be concealed through deposi- 

 tion or destroyed by erosion, and no condition but the structure 

 remain to indicate the manner of origin of the deposit. In such 

 an event it would not be possible to distinguish between alluvium 

 and till. The gravity of the distinction appears when it is known 

 that such apparently unsorted alluvium may extend for several 

 miles forward of a terminal moraine, in the shape of a wide- 

 spreading alluvial fan apparently formed under conditions of ex- 

 tremely rapid aggradation. I suppose it would not be doubted in 

 general that a section of such stony, bowldery, unsorted material 

 two miles long would have other than a glacial origin, yet such 

 may be the case. Indeed, if, as in the Urubamba Valley, a future 

 section should run parallel to the valley across the heads of a 

 great series of fans of similar composition, topographic form, and 

 origin, it would be possible to see many miles of such material. 



The depth of the alluvial valley fill due to tributary fan ac- 

 cumulation 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 Fig. 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 



