286 THE ANDES OF SOUTHERN PERU 



from the terminal moraines, still intact, that cross the valley- 

 floors far below the present limits of the ice. 



In discussing the process of nivation it is necessary to assume 

 a sliding movement on the part of the snow, though it is a condi- 

 tion in Matthes' original problem in which the nivation idea was 

 introduced that the snow masses remain stationary. It is be- 

 lieved, however, that Matthes' valuable observations and conclu- 

 sions really involve but half the problem of nivation; or at the 

 most but one of two phases of it. He has adequately shown the 

 manner in which that phase of nivation is expressed which we find 

 at the border of the snoiv. Of the action beneath the snow he 

 says merely : ' ' Owing to the frequent oscillations of the edge and 

 the successive exposure of the different parts of the site to frost 

 action, the area thus affected will have no well-defined boundaries. 

 The more accentuated slopes will pass insensibly into the flatter 

 ones, and the general tendency will be to give the drift site a cross 

 section of smoothly curved outline and ordinarily concave."* 



From observations on the effects of nivation in valleys, Matthes 

 further concludes that "on a grade of about 12 per cent . . . 

 neve must attain a thickness of at least 125 feet in order that it 

 may have motion, ' ' 5 though as a result of the different line of 

 observations Hobbs concludes ° that a somewhat greater thickness 

 is required. 



The snow cover in tropical mountains offers a number of solid 

 advantages in this connection. Its limits, especially on the Cordil- 

 lera Vilcapampa, on the eastern border of the Andes, are subject 

 to small seasonal oscillations and the edge of the "perpetual" 

 snow is easily determined. Furthermore, it is known from the 

 comparatively "fixed quality of tropical climate," as Humboldt 

 put it, that the variations of the snowline in a period of years do 

 not exceed rather narrow limits. In mid-latitudes on the con- 

 trary there is an extraordinary shifting of the margin of the snow 



* F. E. Matthes, Glacial Sculpture of the Bighorn Mountains, Wyoming, Twentieth 

 Ann. Eept. U. S. Geol. Surv., 1899-1900, Pt. 2, p. 181. 



' Idem, p. 190. 



* W. H. Hobbs, Characteristics of Existing Glaciers, 1911, p. 22. 



