THE BORDER VALLEYS OF THE EASTERN ANDES 



79 



The one is forested, the other grass-cov- 

 ered. Slopes that receive the noon and 

 afternoon sun the greater part of the year 

 are hottest and therefore driest. For 

 places in 11° south latitude the sun is well 

 to the north six months of the year, nearly 

 overhead for about two months, and to the 

 south four months. Northwesterly as- 

 pects are therefore driest and warmest, 

 hence also grass-covered. In many places 

 the line between grass and forest is de- 

 veloped so sharply that it seems to be the 

 artificial edge of a cut-over tract. This is 

 true especially if the relief is steep and 

 the hill or ridge-crests sharp. 8 



At Santa Ana this feature is developed 

 in an amazingly clear manner, and it is 

 also combined with the dry timber line and 

 with productivity in a way I have never 

 seen equaled elsewhere. The diagram will 

 explain the relation. It will be seen that 

 the front range of the mountains is high 

 enough to shut off a great deal of rainfall. 

 The lower hills and ridges just within the 

 front range are relatively dry. The deep 

 valleys are much drier. Each broad ex- 

 pansion of a deep valley is therefore a dry 

 pocket. Into it the sun pours even when 



8 Marcoy who traveled in Peru in the middle of the 

 last century was greatly impressed by the sympathetic 

 changes of aspect and topography and vegetation in the 

 eastern valleys. He thus describes a sudden change of 

 scene in the Occobamba valley: "... the trees had dis- 

 appeared, the birds had taken wing, and great sandy 

 spaces, covered with the latest deposits of the river, al- 

 ternated with stretches of yellow grass and masses of 

 rock half-buried in the ground." (Travels in South 

 America, translated by Elihu Rich, 2 vols. New York, 

 1875, Vol. 1, p. 326.) 



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