GLACIAL FEATURES 283 



snow extended 500 feet lower on the southern slope. The degree 

 of canting is extraordinary at this point, single volcanoes only 

 1,500 to 2,000 feet above the general level and with bases but a 

 few miles in circumference exhibit a thousand feet of difference 

 in the snowline upon northern and southern aspects. This 

 is to be attributed no less to the extreme elevation of the snow 

 (and, therefore, stronger contrasts of shade and sun tempera- 

 tures) than to the extreme aridity of the region and the high day- 

 time temperatures. The aridity is a factor, since heavy snowfall 

 means a lengthening of the period of precipitation in which a 

 cloud cover shuts out the sun and a shortening of the period of 

 insolation and melting. 



Contrasts between shade and sun temperatures increase with 

 altitude but their effects also increase in time. Of two volcanoes 

 of equal size and both 20,000 feet above sea level, that one will 

 show the greater degree of canting that is longer exposed to the 

 sun. The high daytime temperature is a factor, since it tends to 

 remove the thinnest snow, which also falls in this case on the side 

 receiving the greatest amount of heat from the sun. The high 

 daytime temperature is phenomenal in this region, and is owing 

 to the great extent of snow-free land at high elevations and yet 

 below the snowline, and to the general absence of clouds and the 

 thinness of vegetation. 



On approach to the western coast the snowline descends again 

 to 17,500 feet on Coropuna. There are three chief reasons for 

 this condition. First, the well-watered Majes Valley is deeply 

 incised almost to the foot of Coropuna, above Chuquibamba, and 

 gives the daily strong sea breeze easy access to the mountain. 

 Second, the Coast Eange is not only low at the mouth of the Majes 

 Valley, but also is cut squarely across by the valley itself, so that 

 heavy fogs and cloud sweep inland nightly and at times completely 

 cover both valley and desert for an hour after sunrise. Although 

 these yield no moisture to the desert or the valley floor except 

 such as is mechanically collected, yet they do increase the precipi- 

 tation upon the higher elevations at the valley head. 



A third factor is the size of Coropuna itself. The mountain 



