The Andes of southern Peru, geographical reconnaissance along the seventy-third meridian . d by water. But theseassumptions are contrary to fact. The slopes underneath thesnow are deeply recessed; so deeply eroded indeed, that they arebordered by steep cliffs or cirque walls. The products of erosionalso are to some extent displayed about the border of the snowcover. In strong contrast the snow-free slopes are so slightlymodified that little of their original symmetry is lost—only a fewlow hills and shallow valleys have been formed. The measure of the excess of snow erosion over water erosionis
The Andes of southern Peru, geographical reconnaissance along the seventy-third meridian . d by water. But theseassumptions are contrary to fact. The slopes underneath thesnow are deeply recessed; so deeply eroded indeed, that they arebordered by steep cliffs or cirque walls. The products of erosionalso are to some extent displayed about the border of the snowcover. In strong contrast the snow-free slopes are so slightlymodified that little of their original symmetry is lost—only a fewlow hills and shallow valleys have been formed. The measure of the excess of snow erosion over water erosionis therefore 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 thelimits of my subject to de-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
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Keywords: ., bookauthorbo, bookcentury1900, bookdecade1910, booksubjectgeology
