260 THE ANDES OF SOUTHERN PERU 



prolonged erosion on surrounding highlands that served as the 

 feeding grounds of the streams. The alternations in the charac- 

 ter of the deposits, alternations which, in a general view, give a 

 banded appearance to the rock, are produced by successions of 

 beds of fine and coarse material, though all of it is sandstone. 

 Such successions are probably to be correlated with seasonal 

 changes in the volume and load of the depositing streams. 



To gain an idea of the conditions of deposition we may take 

 the character of the sediments as described above, and from them 

 draw deductions as to the agents concerned and the manner of 

 their action. 



We may also apply to the area the conclusions drawn from 

 the study of similar deposits now in process of formation. We 

 have between the coast ranges of northern Chile and the western 

 flanks of the Cordillera Sillilica, probably the best example of 

 piedmont accumulation in a dry climate that the west coast of 

 South America affords. 



Along the inner edge of the Desert of Tarapaca, roughly be- 

 tween the towns of Tarapaca and Quillagua, Chile, the piedmont 

 gravels, sands, silts, and muds extend for over a hundred miles, 

 flanking the western Andes and forming a transition belt between 

 these mountains and the interior basins of the coast desert. The 

 silts and muds constitute the outer fringe of the piedmont and 

 are interrupted here and there where sands are blown upon them 

 from the higher portions of the piedmont, or from the desert 

 mountains and plains on the seaward side. Practically no rain 

 falls upon the greater part of the desert and the only water it re- 

 ceives is that borne to it by the piedmont streams in the early 

 summer, from the rains and melted snows of the high plateau and 

 mountains to the eastward. These temporary streams spread 

 upon the outer edge of the piedmont a wide sheet of mud and silt 

 which then dries and becomes cracked, the curled and warped 

 plates retaining their character until the next wet season or until 

 covered with wind-blown sand. The wind-driven sand fills the 

 cracks in the muds and is even drifted under the edges of the up- 

 curled plates, filling the spaces completely. Over this combined 



