The country west of the Cordillera Vilcapampa had been reduced to early topographic maturity before the Ice Age, and then uplifted with only moderate erosion of the masses of the interfluves. That on the east had passed through the same sequence of events, but erosion had been carried much farther. The reason for this is found in a strong climatic contrast. The eastern is the windward aspect and receives much more rain than the western. Therefore, it has more streams and more rapid dissection. The result was that the eastern slopes were cut to pieces rapidly after the last great regional uplift; the broad interfluves were narrowed to ridges. The region eastward from the crest of the Cordillera to the Pongo de Mainique looks very much like the western half of the Cascade Mountains in Oregon—the summit tracts of moderate declivity are almost all consumed.
The effect of these climatic and topographic contrasts is manifested in strong contrasts in the position and character of the glacial forms on the opposite slopes of the range. At Pampaconas on the east the lowermost terminal moraine is at least a thousand feet below timber line. Between Vilcabamba pueblo and Puquiura the terminal moraine lies at 11,200 feet (3,414 m.). By contrast the largest Pleistocene glacier on the western slope, nearly twelve miles long, and the largest along the traverse, ended several miles below Choquetira at 11,500 feet (3,504 m.) elevation, or just at the timber line. Thus, the steeper descents of the eastern side of the range appear to have carried short glaciers to levels far lower than those attained by the glaciers of the western slope.
It seems at first strange that the largest glaciers were west of the divide between the Urubamba and the Apurimac, that is, on the relatively dry side of the range. The reason lies in a striking combination of topographic and climatic conditions. Snow is a mobile form of precipitation that is shifted about by the wind like a sand dune in the desert. It is not required, like water, to begin a downhill movement as soon as it strikes the earth. Thus, it is a noteworthy fact that snow drifting across the divides may ultimately cause the largest snowfields to lie where the least snow actually falls. This is illustrated in the Bighorns of Wyoming and others of our western ranges. It is, however, not the wet snow near the snowline, but chiefly the dry snow of higher altitudes that is affected. What is now the dry or leeward side of the Cordillera appears in glacial times to have actually received more snow than the wet windward side.
Fig. 140—Glacial sculpture in the heart of the Cordillera Vilcapampa. In places the topography has so high a relief that the glaciers seem almost to overhang the valleys. See Figs. [96] and [179] for photographs.
The topography conspired to increase this contrast. In place of many streams, direct descents, a dispersion of snow in many valleys, as on the east, the western slopes had indirect descents, gentler valley profiles, and that higher degree of concentration of drainage which naturally goes with topographic maturity. For example, there is nothing in the east to compare with the big spurless valley near the pass above Arma. The side walls were so extensively trimmed that the valley was turned into a trough. The floor was smoothed and deepened and all the tributary glaciers were either left high up on the bordering slopes or entered the main valley with very steep profiles; their lateral and terminal moraines now hang in festoons on the steep side walls. Moreover, the range crest is trimmed from the west so that the serrate skyline is a feature rarely seen from eastern viewpoints. This may not hold true for more than a small part of the Cordillera. It was probably emphasized here less by the contrasts already noted than by the geologic structure. The eastward-flowing glaciers descended over dip slopes on highly inclined sandstones, as at Pampaconas. Those flowing westward worked either in a jointed granite or on the outcropping edges of the sandstones, where the quarrying process known as glacial plucking permitted the development of excessively steep slopes.
There are few glacial steps in the eastern valleys. The western valleys have a marvelous display of this striking glacial feature. The accompanying hachure maps show them so well that little description is needed. They are from 50 to 200 feet high. Each one has a lake at its foot into which the divided stream trickles over charming waterfalls. All of them are clearly associated with a change in the volume of the glacier that carved the valley. Wherever a tributary glacier entered, or the side slopes increased notably in area, a step was formed. By retreat some of them became divided, for the process once begun would push the step far up valley after the manner of an extinguishing waterfall.