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 Range 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 precipitation upon the higher elevations at the valley head.
A third factor is the size of Coropuna itself. The mountain is not a simple volcano but a composite cone with five main summits reaching well above the snowline, the highest to an elevation of 21,703 feet (6,615 m.). It measures about 20 miles (32 km.) in circumference at the snowline and 45 miles (72 km.) at its base (measuring at the foot of the steeper portion), and stands upon a great tributary lava plateau from 15,000 to 17,000 feet above sea level. Compared with El Misti, at Arequipa, its volume is three times as great, its height two thousand feet more, and its access to ocean winds at least thirty per cent more favorable. El Misti, 19,200 feet (5,855 m.) has snow down as far as 16,000 feet in the wet season and rarely to 14,000 feet, though by sunset a fall of snow may almost disappear whose lower limit at sunrise was 16,000 feet. Snow may accumulate several thousand feet below the summit during the wet season, and in such quantities as to require almost the whole of the ensuing dry season (March to December) for its melting. Northward of El Misti is the massive and extended range, Chachani, 20,000 feet (6,100 m.) high; on the opposite side is the shorter range called Pichu-Pichu. Snow lies throughout the year on both these ranges, but in exceptional seasons it nearly disappears from Chachani and wholly disappears from Pichu-Pichu, so that the snowline then rises to 20,000 feet. It is considered that the mean of a series of years would give a value between 17,000 and 18,000 feet for the snowline on all the great mountains of the Arequipa region.[57] This would, however, include what is known to be temporary snow; the limit of “perpetual” snow, or the true snowline, appears to lie about 19,000 feet on Chachani and above El Misti, say 19,500 feet. It is also above the crest of Pichu-Pichu. The snowline, therefore, appears to rise a thousand feet from Coropuna to El Misti, owing chiefly to the poorer exposure of the latter to the sources of snowy precipitation.
It may also be noted that the effect of the easy access of the ocean winds in the Coropuna region is also seen in the increasing amount of vegetation which appears in the most favorable situations. Thus, along the Salamanca trail only a few miles from the base of Coropuna are a few square kilometers of quenigo woodland generally found in the cloud belt at high altitudes; for example, 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 produce 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 powerful 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 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 condition in Matthes’ original problem in which the nivation idea was introduced that the snow masses remain stationary. It is believed, however, that Matthes’ valuable observations and conclusions 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 snow. 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.”[58]
From observations on the effects of nivation in valleys, Matthes further concludes that “on a grade of about 12 per cent ... névé must attain a thickness of at least 125 feet in order that it may have motion,”[59] though as a result of the different line of observations Hobbs concludes[60] that a somewhat greater thickness is required.