114 THE ANDES OF SOUTHERN PERU 



The greater part of the coastal region is occupied by the des- 

 ert. Its outer border is the low, dry, gentle, eastward-facing slope 

 of the Coast Range. Its inner border is the foot of the steep 

 descent that marks the edge of the lava plateau. This descent is 

 a fairly well-marked line, here and there broken by a venturesome 

 lava flow that extends far out from the main plateau. Within 

 these definite borders the desert extends continuously northwest- 

 ward for hundreds of miles along the coast of Peru from far be- 

 yond the Chilean frontier almost to the border of Ecuador. It is 

 broken up by deep tranverse valleys and canyons into so-called 

 "pampas," each of which has a separate name; thus west of 

 Arequipa between the Vitor and Majes valleys are the "Parapa 

 de Vitor" and the "Pampa de Sihuas," and south of the Vitor 

 is the "Pampa de Islay." 



The pampa surfaces are inclined in general toward the sea. 

 They were built up to their present level chiefly by mountain 

 streams before the present deep valleys were cut, that is to say, 

 when the land was more than a half-mile lower. Some of their 

 material is wind-blown and on the walls of the valleys are alter- 

 nating belts of wind-blown and water-laid strata from one hun- 

 dred to four hundred feet thick as if in past ages long dry and 

 long wet periods had succeeded each other. The wind has blown 

 sand and dust from the desert down into the valleys, but its chief 

 work has been to drive the lighter desert waste up partly into the 

 mountains and along their margins, partly so high as to carry it 

 into the realm of the lofty terrestrial winds, whence it falls upon 

 surfaces far distant from the fields of origin. There are left 

 behind the heavier sand which the wind rolls along on the sur- 

 faces and builds into crescentic dunes called medanos, and the peb- 

 bles that it can sandpaper but cannot remove bodily. Thus there 

 are belts of dunes, belts of irregular sand drifts, and belts of true 

 desert "pavement" (a residual mantle of faceted pebbles and 

 irregular stones). 



Yet another feature of the desert pampa are the "dry" val- 

 leys that join the through-flowing streams at irregular intervals, 

 as shown in the accompanying regional diagram. If one follow 



