GLACIAL FEATURES 313 



was aggrading its rock floor below b' and forming a deep valley 

 fill. A return to warmer and drier conditions led to the dis- 

 section of the fill and this is now in progress. The stream has 

 not yet reached its preglacial profile, bnt it has almost reached it. 

 We may, therefore, say that the preglacial valley profile below V 

 fixes the position of the present profile just as surely as if the 

 stream had been magically halted in its work at the beginning of the 

 period of glaciation. There, b'-d-c-b represents the amount of ice 

 erosion. To be sure the line b-c is inference, but it is reasonable in- 

 ference and, whatever position is assigned to it, it cannot be coin- 

 cident with b'-d, nor can it be anywhere near it. The break in the 

 valley profile at b' is always marked by a terminal moraine, re- 

 gardless of the character of the rock. This is not an accidental 

 but a causal association. It proves the power of the ice to erode. 

 In glacial times it eroded the quantity b-c-d-V. This is not an 

 excess of ice over water erosion, but an absolute measure of ice 

 erosion, since a'-b' has remained intact. The only possible error 

 arises from the position assigned b-c, and even if we lower it to 

 b-c (for which we have no warrant but extreme conservatism) we 

 shall still have left b'-c'-d-b as a striking value for rock erosion 

 (plucking and abrasion) by a valley glacier. 



A larger diagram, Fig. 203, represents in fuller detail the 

 topographic history of the Andes of southern Peru and the rela- 

 tive importance of glaciation. The broad spurs with grass- 

 covered tops that end in steep scarps are in wonderful contrast to 

 the serrate profiles and truncated spurs that lie within the zone 

 of past glaciation. In the one case we have minute irregularities 

 on a canyon wall of great dimensions; in the other, more even 

 walls that define a glacial trough with a flat floor. Before glacia- 

 tion on a larger scale had set in the right-hand section of the dia- 

 gram had a greater relief. It was a residual portion of the moun- 

 tain and therefore had greater height also. Glaciers formed upon 

 it in the Ice Age and glaciation intensified the contrast between 

 it and the left-hand section ; not so much by intensifying the relief 

 as by diversifying the topographic forms. 



