GLACIAL FEATURES 285 



tions. Thus, along the Salamanca trail only a few miles from the 

 base of Coropuna are a few square kilometers of quenigo wood- 

 land generally found in the cloud belt at high altitudes ; for ex- 

 ample, at 14,000 feet above Lambrama and at 9,000 feet on the 

 slope below Incahuasi, east of Pasaje. The greater part of the 

 growth is disposed over hill slopes and on low ridges and valley 

 walls. It is, therefore, clearly unrelated as a whole to the greater 

 amount of ground- water with which a part is associated, as along 

 the valley floors of the streams that head in the belt of perpetual 

 snow. The appearance of this growth is striking after days of 

 travel over the barren, clinkery lava plateau to eastward that has 

 a less favorable exposure. The quenigo forest, so-called, is of 

 the greatest economic value in a land so desolate as the vast arid 

 and semi-arid mountain of western Peru. Every passing traveler 

 lays in a stock of fire-wood as he rests his beasts at noonday; and 

 long journeys are made to these curious woodlands from both 

 Salamanca and Chuquibamba to gather fuel for the people of the 

 towns. 



NIVATION 



The process of nivation, or snow erosion, does not always pro- 

 duce visible effects. It may be so feeble as to make no impression 

 upon very resistant rock where the snow-fall is light and the 

 declivity low. Ablation may in such a case account for almost the 

 whole of the snow removed. On strong and topographically 

 varied slopes where the snow is concentrated in headwater alcoves, 

 there is a more pronounced downward movement of the snow 

 masses with more prominent effects both of erosion beneath the 

 snow and of accumulation at the border of the snow. In such 

 cases the limit of perpetual snow may be almost as definitely 

 known as the limit of a glacier. Like glaciers these more power- 

 ful snow masses change their limits in • response to regional 

 changes in precipitation, temperature, or both. It would at first 

 sight appear impossible to distinguish between these changes 

 through the results of nivation. Yet in at least a few cases it may 

 be as readily determined as the past limits of glaciers are inferred 



