Fig. 190—To illustrate the difference in the degree of canting of the snowline on large and on small mountain masses.
The advantages of the proposed method of indicating the degree of canting of the snowline lie in the possibility thus afforded of ultimately separating and expressing quantitatively the various factors that affect the position of the line. In the Cordillera Vilcapampa, for example, the dominant canting force is the difference between sun and shade temperatures, while in the volcanoes of Ecuador, where symmetrical volcanoes, almost on the equator, have equal insolation on all aspects and the temperature contrasts are reduced to a minimum—the differences are owing chiefly to varying exposure to the winds. The elusive factors in the comparison are related to the differences in area and in elevation.
The value of arriving finally at close snowline analyses grows out of (1) the possibility of snowline changes in short cycles and (2) uncertainty of arriving by existing methods at the snowline of the glacial period, whose importance is fundamental in refined physiographic studies in glaciated regions with a complex topography. To show the application of the latter point we shall now attempt to determine the snowline of the glacial period in the belt of country along the route of the Expedition.
In the group of peaks shown in [188] between Lambrama and Antabamba, the elevation of the snowline varies from 16,000 to 17,000 feet (4,880-5,180 m.), depending on the topography and the exposure. The determination of the limit of perpetual snow was here, as elsewhere along the seventy-third meridian, based upon evidences of nivation. It will be observed in [191] that just under the snow banks to the left of the center are streams of rock waste which head in the snow. Their size is roughly proportional to the size of the snow banks, and, furthermore, they are not found on snow-free slopes. From these facts it is concluded that they represent the waste products of snow erosion or nivation, just as the hollows in which the snow lies represent the topographic products of nivation. On account of the seasonal and annual variation in precipitation and temperature—hence in the elevation of the snowline—it is often difficult to make a correct snowline observation based upon depth and apparent permanence. Different observers report great changes in the snowline in short intervals, changes not explained by instrumental variations, since they are referred to topographic features. It appears to be impossible to rely upon present records for small changes possibly related to minor climatic cycles because of a lack of standardization of observations.
Nothing in the world seems simpler at first sight than an observation on the elevation of the snowline. Yet it can be demonstrated that large numbers of observers have merely noted the position of temporary snow. It is strongly urged that evidences of nivation serve henceforth as proof of permanent snow and that photographic records be kept for comparison. In this way measurements of changes in the level of the snowline may be accurately made and the snow cover used as a climatic gauge.
Farther west in the Maritime Cordillera, the snowline rises to 18,000 feet on the northern slopes of the mountains and to 17,000 feet on the southern slopes. The top of the pass above Cotahuasi, 17,600 feet (5,360 m.), was snow-free in October, 1911, but the 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 temperatures) than to the extreme aridity of the region and the high daytime 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.