GLACIAL FEATURES 291 



marshy tracts scattered throughout the former neve fields, and 

 the many niched hollows. They are developed above Pampaconas 

 in an admirable manner, though their most perfect and general 

 development is in the summit belt of the Cordillera Vilcapampa 

 between Arma and Choquetira, Fig. 135. It is notable in all cases 

 where nivation was associated with the work of valley glaciers 

 that the rounded nivated slopes break rather sharply with the 

 steep slopes that define an inner valley, whose form takes on the 

 flat floor and under-cut marginal walls normal to valley glaciation. 



A classification of numerous observations in the Cordillera 

 Vilcapampa and in the Maritime Cordillera between Lambrama 

 and Antabamba may now be presented as the basis for a tenta- 

 tive expression of the law of variation respecting snow motion. 

 The statement of the law should be prefaced by the remark that 

 thorough checking is required under a wider range of conditions 

 before we accept the law as final. Near the lower border of the 

 snow where rain and hail and alternate freezing and thawing take 

 place, the snow is compacted even though but fifteen to twenty feet 

 thick, and appears to have a down-grade movement and to exer- 

 cise a slight drag upon its floor when the gradient does not fall 

 below 20°. Distinct evidences of nivation were observed on slopes 

 with a declivity of 5° near summit areas of past glacial action, 

 where the snow did not have an opportunity to be alternately 

 frozen and thawed. 



The thickness of the former snow cover could, however, not be 

 accurately determined, but was estimated from the topographic 

 surroundings to have been at least several hundred feet. Upon 

 a 40° slope a snow mass 50 feet thick was observed to be break- 

 ing off at a cliff-face along the entire cross-section as if impelled 

 forward by thrust, and to be carrying a small amount of waste 

 — enough distinctly to discolor the lowermost layers — which was 

 shed upon the snowy masses below. With increase in the degree 

 of compactness of the snow at successively lower elevations along 

 a line of snow discharge, gradients down to 25° were still observed 

 to carry strongly crevassed, waste-laden snow down to the melt- 

 ing border. It appeared from the clear evidences of vigorous 



