Page:Isaiah Bowman - Desert Trails of Atacama (1924).pdf/337

 strongly emphasized that the slight changes in climate and water supply that we have noted have produced such im- portant and clearly defined effects that the greater changes must have produced still greater effects. If such effects were felt during the period of human occupation they would form the objects of one of the most important stuclies of climate and relief and vegetation on the one hand and of man on the other that the continent of South America affords today. ‘The region is ideally situated for producing such effects and for making the studies just described. In contrast with the climatic belts farther north, which are more sharply defined upon the borders of the mountains and which have a relatively fixed quality, the climatic zones of the border of the Puna de Atacama have a wider range of action. The seasonal extremes are much farther apart here than farther north toward lower latitudes, and the seasonal rainfall is a much more clearly marked thing than in the rest of the Central Andes northward to central Peru. Any disturbance in the level of the climatic zones, in short any change in climate, would be felt over a wider extent of country, over a greater expanse of mountain slope, through a wider range of altitudes.

Such climatic changes as we have described must not be thought of as purely speculative guesses. Everywhere through- out the Central Andes are the marks of past glaciation which represents a major climatic change of the general type that preceded and followed the Ice Age itself. In 1911 I discovered that glaciers of great length had come down through all the valleys of the high Cordillera Vilcapampa. One of them was at least fifteen miles long. These were the ancestors of gla- ciers that now inhabit only the heads of the valleys, where they are nourished by a permanent glacial cap of amazing extent and climatic significance in latitude 12° $., almost overlooking the border of the Amazonian plains. | have estimated the height of the snow line of the glacial period to have been 2000 feet lower than the snow line of today. The relationships of the upper and lower limits of the zone of