The Andes of southern Peru . efore the difference between a northern or water-formedand a southern or snow-formed profile, Fig. 200. This differenceis also shown in Fig. 201 and from it and the restored initial pro-files we conclude that the rate of water erosion is to that of niva-tion as 1:3. This ratio has been derived from numerous obser-vations on cones so recently formed that the interfluves withoutquestion are still intact. Thus far only those volcanoes have been considered which GLACIAL FEATURES 311 have been modified by nivation. There are, however, many vol-canoes which have been ero
The Andes of southern Peru . efore the difference between a northern or water-formedand a southern or snow-formed profile, Fig. 200. This differenceis also shown in Fig. 201 and from it and the restored initial pro-files we conclude that the rate of water erosion is to that of niva-tion as 1:3. This ratio has been derived from numerous obser-vations on cones so recently formed that the interfluves withoutquestion are still intact. Thus far only those volcanoes have been considered which 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 andwater. It will be seen at once that where a great area of snow istributary to a single valley, the snow becomes compacted intoneve and ice, and that it then erodes at a much faster rate. Alsoa new force—^plucking—^iscalled into action when ice ispresent, and this greatly ac-celerates the rate of it lies outside the limits of my subject to de- j-,^_ 202-Graphic representation. termine quantitatively theratio between water and iceaction, it is worth pointingout that by this method aratio much in excess of 1: 3is determined, which even inthis rough form is of con-siderable interest in view ofthe arguments based on theprotecting influence of bothice and snow. I have, in-deed, avoided the questionof ice erosion up to thispoint and limited myself tothose volcanoes which havebeen modified by nivation only, since the result is more strikingin view of the aU. but general absence of data relating to this formof erosion. If we now turn to the valley profiles of the glaciated portionsof the Peruvian Andes, we shall see the excess of ice over watererosion 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 shouldoccur near the heads of the valleys. T
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Keywords: ., bookcentury1900, bookdecade1910, bookidandeso, booksubjectgeology
