It is this “topographic” hypothesis that Reiss and Stübel accept for the Ecuadorean volcanoes. Moreover, the volcanoes of Ecuador are practically on the equator—a very critical situation when we wish to use the facts they exhibit in the solution of such large problems as the contemporaneous glaciation of the two hemispheres, or the periodic advance and retreat of the ice over the whole earth. This is not the place to scrutinize either their facts or their hypothesis, but I am under obligations to state very emphatically that the glacial features of the Cordillera Vilcapampa require the climatic and not the topographic hypothesis. Let us see why.
The differences in degree of dissection and the flattening gradient up-valley that we noted in a preceding paragraph leave no doubt that each moraine of the bordering valleys in the Vilcapampa region, represents a prolonged period of stability in the conditions of topography as well as of temperature and precipitation. If change in topographic conditions is invoked to explain retreat from one position to the other there is left no explanation of the periodicity of retreat which has just been established. If a period of cold is inaugurated and glaciers advance to an ultimate position, they can retreat only through change of climate effected either by general causes or by topographic development to the point where the snowfields become restricted in size. In the case of climatic change the ice changes are periodic. In the case of retreat due to topographic change there should be a steady or non-periodic falling back of the ice front as the catchment basins decrease in elevation and the snow-gathering ridges tributary to them are reduced in height.
Further, the matterhorns of the Cordillera Vilcapampa are not bare but snow-covered, vigorous glaciers several miles in length and large snowfields still survive and the divides are not arêtes but broad ridges. In addition, the last two moraines, composed of very loose material, are well preserved. They indicate clearly that the time since their formation has witnessed no wholesale topographic change. If (1) no important topographic changes have taken place, and (2) a vigorous glacier lay for a long period back of a given moraine, and (3) suddenly retreated several miles and again became stable, we are left without confidence in the application of the topographic hypothesis to the glacial features of the Vilcapampa region. Glacial retreat may be suddenly begun in the case of a late stage of topographic development, but it should be an orderly retreat marked by a large number of small moraines, or at least a plentiful strewing of the valley floor with débris.
Fig. 139—Glacial features on the eastern slopes of the Cordillera Vilcapampa.
The number of moraines in the various glaciated valleys of the Cordillera Vilcapampa differ, owing to differences in elevation and to the variable size of the catchment basins. All valleys, however, display the same sudden change from moraine to moraine and the same characteristics of gradient. In all of them the lowermost moraine is always more deeply eroded than the higher moraines, in all of them glacial erosion was sufficiently prolonged greatly to modify the valley walls, scour out lake basins, or broad flat valley floors, develop cirques, arêtes, and pinnacled ridges in limited number. In some, glaciation was carried to the point where only skeleton divides remained, in most places broad massive ridges or mountain knots persist. In spite of all these differences successive moraines were formed, separated by long stretches either thinly covered with till or exposing bare rock.
In examining this group of features it is important to recognize the essential fact that though the number of moraines varies from valley to valley, the differences in character between the moraines at low and at high elevations in a single valley are constant. It is also clear that everywhere the ice retreated and advanced periodically, no matter with what topographic features it was associated, whether those of maturity or of youth in the glacial cycle. We, therefore, conclude that topographic changes had no significant part to play in the glacial variations in the Cordillera Vilcapampa.