GLACIAL FEATURES 



311 



have been modified by nivation. There are, however, many vol- 

 canoes which have been eroded by ice as well as by snow and 

 water. It will be seen at once that where a great area of snow is 

 tributary to a single valley, the snow becomes compacted into 

 neve and ice, and that it then erodes at a much faster rate. Also 

 a new force — plucking — is 



called into action when ice is 

 present, and this greatly ac- 

 celerates the rate of erosion. 

 While it lies outside the 

 limits of my subject to de- 

 termine quantitatively the" 

 ratio between water and ice 

 action, it is worth pointing 

 out that by this method a 

 ratio much in excess of 1 : 3 

 is determined, which even in 

 this rough form is of con- 

 siderable interest in view of 

 the arguments based on the 

 protecting influence of both 

 ice and snow. I have, in- 

 deed, avoided the question 

 of ice erosion up to this 

 point and limited myself to 



Fig. 202 — Graphic representation of 

 amount of glacial erosion during the glacial 

 period. In the background are mature slopes 

 surmounted by recessed asymmetrical peaks. 

 The river entrenched itself below the mature 

 slopes before it began to aggrade, and, when ag- 

 gradation set in, had cut its valley floor to 

 a'-b'-c. By aggradation the valley floor was 

 raised to a-b while ice occupied the valley head. 

 By degradation the river has again barely 

 lowered its channel to a'-b', the ice has disap- 

 peared, and the depression of the profile repre- 

 sents the amount of glacial erosion. 



a'-b'-c = preglncial profile. 



a-b-d-c = present profile. 



b'-d-c-b = total ice erosion in the glacial 

 period. 



a-b = surface of an alluvial valley fill 

 due to excessive erosion at valley 

 head. 



b-b' = terminal moraine. 



d-c = cirque wall. 



e, e' e" = asymmetrical summits. 



those volcanoes which have 

 been modified by nivation only, since the result is more striking 

 in view of the all but general absence of data relating to this form 

 of erosion. 



If we now turn to the valley profiles of the glaciated portions 

 of the Peruvian Andes, we shall see the excess of ice over water 

 erosion expressed in a manner equally convincing. To a thought- 

 ful person it is one of the most remarkable features of any gla- 

 ciated region that the flattest profiles, the marshiest valley flats, 

 and the most strongly meandering stretches of the streams should 

 occur near the heads of the valleys. The mountain shepherds 



