Fig. 191—The “pocked” surface characteristically developed in the zone of light nivation. Compare with Fig. 194, showing the effects of heavy nivation.
Fig. 192—Steep cirque walls and valleys head in the Central Ranges between Lambrama and Chuquibambilla. The snow is here a vigorous agent in transporting talus material and soil from all the upper slopes down to the foot of the cirque wall.
The snow cover in tropical mountains offers a number of solid advantages in this connection. Its limits, especially on the Cordillera Vilcapampa, on the eastern border of the Andes, are subject to small seasonal oscillations and the edge of the “perpetual” snow is easily determined. Furthermore, it is known from the comparatively “fixed quality of tropical climate,” as Humboldt put it, that the variations of the snowline in a period of years do not exceed rather narrow limits. In mid-latitudes on the contrary there is an extraordinary shifting of the margin of the snow cover, and a correspondingly wide distribution of the feeble effects of nivation.