262 THE ANDES OF SOUTHERN PERU 



that succeed the unconformity we have evidence of accumulation 

 under conditions of renewed uplift in the mountains and of full 

 streams competent to remove the increasing load. 



It is in the character of the sediments toward the top of the 

 Tertiary that we have the clearest evidence of progressive desic- 

 cation of the climate of the region. The amount of wind-blown 

 material steadily increases and the uppermost five hundred feet is 

 composed predominantly, and in places exclusively, of this ma- 

 terial. The evidences of wind action lie chiefly in the fine (in 

 places fluffy) nature of the deposits, their uniform character, and 

 in the tangency of the layers with respect to the surface on which 

 they were deposited. There are three diagnostic structural fea- 

 tures of great importance : the very steep dip of the fine laminae ; 

 the peculiar and harmonious blending of their contacts ; the man- 

 ner in which the highly inclined laminae cut off and succeed each 

 other, whereby quite bewildering changes in the direction of dip 

 of the inclined beds are brought about on any exposed plane. 

 Some of these features require further discussion. 



It is well known that the front of a sand dune generally con- 

 sists of sand deposited on a slope inclined at the angle of repose, 

 say between 30° and 35°, and rolled into place up the long back 

 slope of the dune by the wind. It has not, however, been gener- 

 ally recognized that the angle of repose may be exceeded (a) when 

 there exists a strong back eddy or (b) when the wind blows vio- 

 lently and for a short time in the opposite direction. In either case 

 sand is carried up the short steep slope of the dune front and 

 accumulated at an angle not infrequently running up to 43° and 

 48° and locally, and under the most favorable circumstances, in 

 excess of 50°. The conditions under which these steep angles are 

 attained are undoubtedly not universal, but they can be found in 

 some parts of almost any desert in the world. They appear not 

 to be present where the sand grains are of uniform size through- 

 out, since that leads to rolling. They are found rather where there 

 is a certain limited variation in size that promotes packing. 

 Packing and the development of steep slopes are also facilitated 

 in parts of the coastal desert of Peru by a cloud canopy that hangs 



