266 THE ANDES OF SOUTHERN PERU 



not only in the scale of the features but also in the direction and 

 amount of the dips. 



Finally, we may note that a section of dune deposits has a dis- 

 tinctive feature not exhibited by water deposits. If the foreset 

 beds of a cross-bedded water deposit be exposed in a plane 

 parallel to the strike of the beds, the beds will appear to be hori- 

 zontal. They could not then be distinguished from the truly 

 horizontal beds above and below them. But the conditions of wind 

 deposition we have just noted, and chiefly the facts expressed by 

 Fig. 178, make it impossible to select a position in which both 

 tangency and irregular dips are not well developed in a wind de- 

 posit. I believe that we have in the foregoing facts and inferences 

 a means for the definite separation of these two classes of de- 

 posits. Difficulties will arise only when there is a quick succession 

 of wind and water action in time, or where the wind produces 

 powerful and persistent effects without the actual formation of 

 dunes. 



The latest known deposits in the coastal region are found sur- 

 mounting the terrace tops along the coast between Camana and 

 Quilca, where they form deposits several hundred feet thick in 

 places. The age of these deposits is determined by fossil evidence, 

 and is of extraordinary interest in the determination of the age of 

 the great terraces upon which they lie. They consist of alternating 

 beds of coarse and fine material, the coarser increasing in thick- 

 ness and frequency toward the bottom of the section. It is also 

 near the bottom of the section that fossils are now found; the 

 higher members are locally saline and throughout there is a 

 marked inclination of the beds toward the present shore. The de- 

 posits appear not to have been derived from the underlying gran- 

 ite-gneiss. They are distributed most abundantly near the mouths 

 of the larger streams, as near the Vitor at Quilca, and the Majes 

 at Camana. Elsewhere the terrace summit is swept clean of 

 waste, except where local clay deposits lie in the ravines, as back 

 of Mollendo and where "tierras blancas" have been accumulated 

 by the wind. 



These coastal deposits were laid down upon a dissected ter- 



