Whymper spent most of his time in exploring recent volcanoes or those recently in eruption, hence did not have the most favorable opportunities for gathering significant data. Reiss was carried off his feet by the attractiveness of the hypothesis[48] relating to the effect of glacial denudation on the elevation of the snowline. Gregory appeared not to have recognized the work of Hettner on the Cordillera of Bogotá and of Sievers[49] and Acosta on the Sierra Nevada de Santa Marta in northern Colombia.
The importance of the glacial features of the Cordillera Vilcapampa developed on a great scale in very low latitudes in the southern hemisphere is twofold: first, it bears on the still unsettled problem of the universality of a colder climate in the Pleistocene, and, second, it supplies additional data on the relative depression of the snowline in glacial times in the tropics. Snow-clad mountains near the equator are really quite rare. Mount Kenia rising from a great jungle on the equator, Kilimandjaro with its two peaks, Kibo and Mawenzi, two hundred miles farther south, and Ingomwimbi in the Ruwenzori group thirty miles north of the equator, are the chief African examples. A few mountains from the East Indies, such as Kinibalu in Borneo, latitude 6° north, have been found glaciated, though now without a snow cover. In higher latitudes evidences of an earlier extensive glaciation have been gathered chiefly from South America, whose extension 13° north and 56° south of the equator, combined with the great height of its dominating Cordillera, give it unrivaled distinction in the study of mountain glaciation in the tropics.
Furthermore, mountain summits in tropical lands are delicate climatic registers. In this respect they compare favorably with the inclosed basins of arid regions, where changes in climate are clearly recorded in shoreline phenomena of a familiar kind. Lofty mountains in the tropics are in a sense inverted basins, the lower snowline of the past is like the higher shoreline of an interior basin; the terminal moraines and the alluvial fans in front of them are like the alluvial fans above the highest strandline; the present snow cover is restricted to mountain summits of small areal extent, just as the present water bodies are restricted to the lowest portions of the interior basin; and successive retreatal stages are marked by terminal moraines in the one case as they are marked in the other by flights of terraces and beach ridges.
I made only a rapid reconnaissance across the Cordillera Vilcapampa in the winter season, and cannot pretend from my limited observations to solve many of the problems of the field. The data are incorporated chiefly in the chapter on Glacial Features. In this place it is proposed to describe only the more prominent glacial features, leaving to later expeditions the detailed descriptions upon which the solution of some of the larger problems must depend.
At Choquetira three prominent stages in the retreat of the ice are recorded. The lowermost stage is represented by the great fill of morainic and outwash material at the junction of the Choquetira, and an unnamed valley farther south at an elevation of 11,500 feet (3,500 m.). A mile below Choquetira a second moraine appears, elevation 12,000 feet (3,658 m.), and immediately above the village a third at 12,800 (3,900 m.). The lowermost moraine is well dissected, the second is ravined and broken but topographically distinct, the third is sharp-crested and regular. A fourth though minor stage is represented by the moraine at the snout of the living glacier and still less important phases are represented in some valleys—possibly the record of post-glacial changes of climate. Each main moraine is marked by an important amount of outwash, the first and third moraines being associated with the greatest masses. The material in the moraines represents only a part of that removed to form the successive steps in the valley profile. The lowermost one has an enormous volume, since it is the oldest and was built at a time when the valley was full of waste. It is fronted by a deep fill, over the dissected edge of which one may descend 800 feet in half an hour. It is chiefly alluvial in character, whereas the next higher one is composed chiefly of bowlders and is fronted by a pronounced bowlder train, which includes a remarkable perched bowlder of huge size. Once the valley became cleaned out the ice would derive its material chiefly by the slower process of plucking and abrasion, hence would build much smaller moraines during later recessional stages, even though the stages were of equivalent length.
Fig. 136—Glacial sculpture on the southwestern flank of the Cordillera Vilcapampa. Flat-floored valleys and looped terminal moraines below and glacial steps and hanging valleys are characteristic. The present snowfields and glaciers are shown by dotted contours.